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Modeling deep ocean water and sediment dynamics in the eastern Pacific Ocean using actinium-227 and other naturally occurring radioisotopes

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Modeling deep ocean water and sediment dynamics in the eastern Pacific Ocean using actinium-227 and other naturally occurring radioisotopes

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Content MODELING DEEP OCEAN WATER AND SEDIMENT DYNAMICS IN THE EASTERN PACIFIC OCEAN USING ACTINIUM-227 AND OTHER NATURALLY OCCURRING RADIOISOTOPES by Nathaniel James Kemnitz A Thesis Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (GEOLOGICAL SCIENCES) December 2022 Copyright 2022 Nathaniel James Kemnitz ii ACKNOWLEDGMENTS I want to first thank my advisor, Doug Hammond. Doug has mentored me for almost my entire career at USC: beginning with advising me in my undergraduate courses, to my senior and Master thesis, and now my PhD dissertation. Throughout my time at USC, Doug has always been available for me to reach out to and has never turned me away when I come knocking at his door with questions. His dedication to helping students learn about geochemistry is indeed inspiring. Doug has helped me grow into the well-rounded scientist I am today. I want to thank my family for all their support they have given me throughout the last 10 years I have been in college. I want to thank my wife, Brittney for her sacrifice and dedication to our family. Without her I would not be where I am today. I want to thank my Mom and Dad for their relentless support to my health. I would also like to thank my brother, Ryan Kemnitz. Ryan has helped guide me in my academic endeavors. I want to thank all my professors at USC and peers that have helped me along the way. They all have made the USC Earth Sciences department a great place to learn and grow. I want to especially thank my PhD committee members, Will Berelson, Jim Moffett, and Seth John. I also want to thank Nick Rollins, Marty Fleisher, Emily Le Roy, Henrietta Dulai, Matt Charette, Willard Moore, and Paul Henderson for all their support and guidance with lab work and fieldwork. The assistance of the chief scientists Phoebe Lam, Karen Casciotti, and Greg Cutter during the GEOTRACES program is also much appreciated. Also, I want to thank all the friends, lab mates, and cruise mates that have helped me in my academic career: Yi Hou, Audra Bardsley, Ben Melechin, Karen Vo, and Abby Lunstrum. iii I would also like to thank Will Berelson and Jess Adkins for allowing me to attend two sea cruises, C-Disk-IV and C-DISP in 2017 and 2020. Those cruises were an experience of a life-time, and I will never forget the memories and friends that I made during those cruises. Also, I want to thank Will Berelson for his guidance for helping me publish my first manuscript in 2020. Lastly, I want to thank National Science Foundation (NSF) and USC Earth Sciences department for financial support through my academic career. iv TABLE OF CONTENTS Acknowledgments .......................................................................................................................... ii List of Tables ................................................................................................................................. vi List of Figures ............................................................................................................................... vii Abstract .......................................................................................................................................... ix Chapter 1: Introduction ................................................................................................................... 1 1.1 Previous work with naturally-occurring U- and Th-decay series isotopes as mixing tracers in the Deep Sea ....................................................................................................... 1 1.2 Bioturbation and accumulation rates in sediments based on 210 Pb ............................... 5 1.3 References .................................................................................................................. 10 Chapter 2: Actinium & Radium Fluxes from the Seabed in the Northwest Pacific Basin ........... 14 2.1 Introduction. ............................................................................................................... 16 2.2 Theory ......................................................................................................................... 18 2.3 Study Area .................................................................................................................. 23 2.4 Materials and Methods ............................................................................................... 25 2.5 Results and Discussion ............................................................................................... 38 2.6 Conclusions ................................................................................................................ 65 2.7 References .................................................................................................................. 68 Chapter 3: Actinium-227 Distribution along the GEOTRACES Meridional Transect as an Indicator of Solute Transport ........................................................................................................ 73 3.1 Introduction ................................................................................................................ 76 3.2 Geochemistry of 227 Ac ................................................................................................ 78 3.3 Theory relating 227 Ac profiles to mixing processes .................................................... 79 3.4 Study Area .................................................................................................................. 81 3.5 Materials and Methods ............................................................................................... 85 3.6 Results and Discussion ............................................................................................... 91 3.7 Conclusions .............................................................................................................. 113 3.8 References ................................................................................................................ 116 Chapter 4: Evidence of Changes in Sedimentation Rate and Sediment Fabric in a Low Oxygen Setting: Santa Monica Basin, CA ............................................................................................... 120 4.1 Introduction .............................................................................................................. 122 4.2 Study Area ................................................................................................................ 125 4.3 Methods .................................................................................................................... 127 4.4 Results ...................................................................................................................... 131 4.5 Discussion ................................................................................................................. 141 4.6 Conclusions .............................................................................................................. 152 4.7 References ................................................................................................................ 155 Chapter 5: Summary ................................................................................................................... 159 v 5.1 Dissertation Summary .............................................................................................. 159 5.2 Future Work .............................................................................................................. 164 5.3 References ................................................................................................................ 167 Appendix: Supplementary Information ...................................................................................... 168 A.1 Radiochemical procedures for 210 Pb, 227 Ac, 230 Th, and 232 Th for deep-sea sediment samples ........................................................................................................................... 168 A.2 Rapid measurement of 227 Ac Using a High Purity Germanium Well-Type Detector .... 185 A.3 Intercalibration of Lead-210 in Marine Sediments .................................................. 190 A.4 Calibration of 229 Th Solution: USC #89B ............................................................... 205 A.5 McLane in-situ pump setup for Ra and Ac extraction from seawater ..................... 211 A.6 Reaction-Transport Model MATLAB Code ........................................................... 215 A.7 Numerical Model MATLAB Code .......................................................................... 221 A.8 Santa Monica Basin Supplementary Information .................................................... 225 A.9 Total dissolved 231 Pa, 227 Ac, 227 Acex along GP15 transect ...................................... 227 vi LIST OF TABLES Table 2.1: Station information for C-Disk-IV cruise ................................................................... 24 Table 2.2: Information for radioisotopes measured on HPGe well-type detector ........................ 36 Table 2.3: 228 Ra kd results using 232 Th spike solution .................................................................. 40 Table 2.4: 225 Ac kd results using 229 Th spike solution .................................................................. 41 Table 2.5: Activities of 238 U, 230 Th, 226 Ra, 210 Pb, 232 Th, 228 Ra, 231 Pa, and 227 Ac .......................... 48 Table 2.6: Model parameters for best fit solid phase 227 Ac profiles ............................................. 50 Table 2.7: Model parameters for best fit solid phase 226 Ra profiles ............................................. 50 Table 2.8: Model parameters for solid phase 228 Ra profiles ......................................................... 50 Table 2.9: Mixed layer depth, 230 Th activity, mass accumulation rates, sedimentation rate, inventory of 230 Th, and ages for C-Disk-IV profiles .................................................................... 53 Table 2.10: Observed deficiencies in sediments for 226 Ra, 228 Ra, and 227 Ac ............................... 54 Table 2.11: Numerical model results for 227 Ac flux ..................................................................... 56 Table 2.12: Bateman equation fit results for 227 Ac, 223 Ra, & 228 Ra from core incubation ........... 57 Table 2.13: 227 Ac, 226 Ra, and 228 Ra fluxes for water column and sediments ................................ 61 Table 3.1: b, Kz, bottom and isopycnal slopes, alpha, and Kp values .......................................... 98 Table 4.1: Macrofauna for selected SMB 2016 MUC cores ...................................................... 133 Table 4.2: Station ID, year collected, mass flux, depth, inventory, and 210 Pbex for all cores greater than 800 meters depth in the Santa Monica Basin ...................................................................... 142 Table A.1.1: Energies, branching ratio, half-lives for radionuclides used in this study ............. 182 Table A.2.1: Information for radioisotopes measured on HPGe well-type detector .................. 186 Table A.3.1: 210 Pb activity for NIST #4337 solution on USC detectors .................................... 191 Table A.3.2: Self-absorption effects for marine sediments in small PP tubes ........................... 196 Table A.3.3: 210 Pb Needle experiment for sediment samples and DI water ............................... 199 Table A.3.4: Activities for deep-sea and coastal sediments ....................................................... 202 Table A.3.5: HPGe and Alpha Spectroscopy ratios for coastal and deep-sea sediments ........... 202 Table A.3.6: Summary for HPGe and alpha spectroscopy results ............................................. 203 Table A.4.1: Energies, branching ratio, half-lives for radionuclides used in this study ............. 208 Table A.4.2 Summary of calibration for #89B spike solution .................................................... 210 Table A.9.1 Total dissolved 231 Pa, 227 Ac, and 227 Acex along GP15 transect ............................... 227 vii LIST OF FIGURES Figure 2.1: Study Area of the Northeast Pacific Basin (NEPB) ................................................... 23 Figure 2.2: Decay series of 238 U, 237 Np, 235 U, and 232 Th .............................................................. 28 Figure 2.3: Flow diagram for separation of Ac from DGA resin ................................................. 32 Figure 2.4: Ac and Ra kd vs. %Mn for all C-Disk-IV stations ..................................................... 42 Figure 2.5: 225 Ac vs. 228 Ra kd values ............................................................................................ 43 Figure 2.6: 231 Pa activity in surface sediments vs. latitude in the Eastern Pacific ....................... 44 Figure 2.7: Solid phase 231 Pa, 227 Ac, and 210 Pbex profiles for all 5 C-Disk-IV stations ................ 45 Figure 2.8: Solid phase 230 Th, 226 Ra, 232 Th, & 228 Ra profiles for all 5 C-Disk-IV stations .......... 45 Figure 2.9: Bateman equation fits for 223 Ra, 224 Ra, 228 Ra, and 227 Ac benthic fluxes .................... 58 Figure 2.10: Method comparison for 227 Ac, 226 Ra, and 228 Ra benthic flux .................................. 60 Figure 2.11: 227 Ac Flux vs. F, kd, and 231 Pa for C-Disk-IV sediments ......................................... 62 Figure 2.12: : 226 Ra, 228 Ra, and 227 Ac Flux vs. Station # for C-Disk-IV sediments ..................... 63 Figure 3.1: Schematic of the geochemical behavior of the 235 U series in the ocean .................... 79 Figure 3.2: Study Area of C-Disk-IV and GP15 cruise ................................................................ 83 Figure 3.3: Abyssal Circulation Pathways in the Pacific Ocean .................................................. 85 Figure 3.4: Inverse model using hydrography data in the NEPB ................................................. 86 Figure 3.5: Total dissolved 227 Ac vs. 231 Pa for GP15 stations ...................................................... 88 Figure 3.6: Total dissolved 227 Ac vs. depth for cross-over stations of GP15 and GP16 .............. 89 Figure 3.7: All profiles of 227 Acex vs. depth along GP15 transect ................................................ 94 Figure 3.8: ODV plot of 227 Ac activity vs. depth along the GP15 transect .................................. 94 Figure 3.9: 227 Ac flux vs. latitude along the GP15 transect .......................................................... 96 Figure 3.10: : Inverse model using 227 Acex data in the NEPB at 4600 dbar ................................. 97 Figure 3.11: 227 Acex vs. DAB for GP15 stations 8, 10, 14, and 16 ............................................... 99 Figure 3.12: Neutral density vs. depth juxtaposed with 227 Acex vs. depth .................................. 101 Figure 3.13: ODV plot of Neutral Density vs. depth along the GP15 transect .......................... 102 Figure 3.14: GP15 station 21 topography and weighting function model .................................. 106 Figure 3.15: 227 Acex vs. DAB juxtaposed with topography for different size models ................ 106 Figure 3.16: GP15 stations 21, 33, and 37 227 Acex vs. DAB juxtaposed with topography for different transport parameters ..................................................................................................... 110 Figure 3.17: 227 Acex vs. depth juxtaposed with d 3 He vs. depth for GP15 Sta. 35 and 37 .......... 112 Figure 4.1: Coring locations for Santa Monica Basin ................................................................ 123 Figure 4.2: Oxygen and T-S plot for SMB obtained in spring 2016 .......................................... 127 Figure 4.3: Porosity profiles for SMB 2016 MUC cores ........................................................... 132 viii Figure 4.4: %Corg content for 0-1 cm intervals from MUC cores and Gorsline data ................. 133 Figure 4.5: Photographs of selected 2016 MUC cores ............................................................... 134 Figure 4.6: X-radiographs of cores MUC 9 and 10 .................................................................... 135 Figure 4.7: Eight multi-cores sampled for 210 Pbex in the Santa Monica Basin ........................... 137 Figure 4.8: Eight multi-cores sampled for 137 Cs in the Santa Monica Basin ............................. 138 Figure 4.9: ∆ 14 C vs. Integrated Mass for SMB cores MUC 9 and MUC 10 .............................. 139 Figure 4.10: : ∆ 14 C vs. Integrated Mass, including designation of turbidite and bomb carbon region and corresponding d 13 C data ........................................................................................... 140 Figure 4.11: Semi-log plot of 210 Pbex activity vs. integrated mass ............................................. 143 Figure 4.12: Same as Fig. 4.11 but for 8 cores obtained from depths between 870-900 m ....... 144 Figure 4.13: Spreading of the laminated sediments area in Santa Monica Basin ...................... 146 Figure A.1.1: k’ for Ac in HNO3 and HCl for DGA resin ......................................................... 174 Figure A.1.2: DGA resin capacity factor (k’) for Ac, Th, Fe, and earth alkaline metals ........... 174 Figure A.1.3: Vacuum setup for DGA cartridge and funnels ..................................................... 175 Figure A.1.4: Flow diagram for separation of Ac from DGA cartridge ..................................... 176 Figure A.1.5: Alpha-spectrum of 225 Ac and its daughters on Day 1 .......................................... 179 Figure A.1.6: Alpha-spectrum of 225 Ac and its daughters on Day 120 ...................................... 180 Figure A.1.7: Ingrowth factors for 223 Ra and 227 Th for the determination of 227 Ac ................... 180 Figure A.1.8: Alpha-spectrum of 230 Th, 232 Th, 228 Th, and 229 Th ................................................ 181 Figure A.2.9: Gamma vs. alpha spectroscopy results for 227 Ac measurements ......................... 189 Figure A.3.10: 210 Pb efficiency vs. height for HPGe det. MCB1, MCB2, and MCB3 .............. 194 Figure A.3.2: Volume (cc) vs. height for Big and Small PP tubes ............................................. 198 Figure A.3.3: Activities determined from gamma and alpha spectroscopy for deep-sea and coastal marine sediments ............................................................................................................ 203 Figure A.4.1: Alpha-spectrum of 225 Ac and its daughters on Day 1 .......................................... 206 Figure A.4.2: Alpha-spectrum of 225 Ac and its daughters on Day 120 ...................................... 207 Figure A.4.3: Alpha spectrum for #89B and Harwell #32C solutions ....................................... 207 Figure A.4.4: Alpha spectrum for #89B and Thorium Ore (IAEA-RGTh-1) solutions ............. 209 Figure A.5.1: : McLane in-situ pump (ISP) setup for Ra and Ac extraction .............................. 213 Figure A.5.2: Schematic for seawater flow through McLane in-situ pump (ISP) system ......... 214 Figure A.6.1: Sum-Square (SS) vs. Fraction released (F) for C-Disk-IV station 1 .................... 217 Figure A.8.1: 226 Ra values for SMB cores MUC 9, DOE 65, and DOE 25 ............................... 225 Figure A.8.2: Mass Flux vs. Collection Year for cores > 850 meters in SMB .......................... 226 Figure A.8.3: Timeline of Los Angeles basin land usage in 1800 until present ........................ 226 ix Abstract The disequilibrium that occurs within the U and Th-decay series in marine environments can be exploited to give rates of transport of dynamic processes. To provide the most accurate transport rates, the radiotracer should have the following characteristics: (1) the radiotracer should be easily sampled and measured; (2) the source function and geochemical behavior of the radiotracer should be well understood; (3) the radiotracer’s half-life should be on the same order as the process being investigated. This dissertation will utilize the distribution of 227 Ac on the basin-scale range in the North- and Southeast Pacific in order to provide rates of transport of other solutes. The half-life of 227 Ac (t1/2=21.8 y) is well suited for this scale and its geochemical behavior has been well-studied throughout the last few decades. The GEOTRACES program protocol for sampling and measuring Ac and Ra in the water column has led to multiple 227 Ac profiles throughout the world’s oceans. This dissertation will utilize U and Th-decay series equilibria to establish 227 Ac bottom fluxes that can provide insight into transport rates and circulation pathways in the Northeastern Pacific Ocean. 227 Ac is directly produced by decay of 231 Pa (t1/2=32.8 ky). 231 Pa is scavenged from the water column by falling particulates that carry it to sediments, where it decays to its more soluble daughter. A fraction of this 227 Ac diffuses out of deep-sea sediments and is transported vertically and horizontally as it decays in the water column. The water column distribution of excess 227 Ac ( 227 Acex) was measured along the U.S. GEOTRACES Meridional Transect (GP15) from Alaska to Tahiti in fall 2018. To constrain its benthic input, cores from 5 stations near the northern half of the GP15 transect were collected in the summer of 2017 (C-Disk-IV transect stations 23- x 50°N). The GP15 transect parallels the C-Disk-IV cruise track in the Northeast Pacific, offset by a few hundred km. Five sediment cores along the C-Disk-IV transect were measured and modeled with the objective of characterizing the behavior of 227 Ac, 228 Ra, and 226 Ra and their fluxes into the overlying water column. Solid phase profiles of these isotopes were measured, and reaction- transport models were applied that incorporate effects of molecular diffusion, bioturbation, sedimentation, distribution coefficient (kd), and fraction of each isotope released to pore water by parent decay (called F). Good fits to the 226 Ra profiles showed F values of 57-83% and sedimentation rates of 0.10 – 0.40 cm kyr -1 for C-Disk-IV sediments. The 228 Ra profiles were difficult to measure due to high counting uncertainties, but F values obtained from 228 Ra profiles were similar to 226 Ra values. Most solid phase 226 Ra profiles showed a large deficiency compared to 230 Th in the upper 15cm of sediments, while the 228 Ra profiles showed a modest deficiency relative to its 232 Th parent in the top 3cm of sediments. F values for 227 Ac had a much larger range than the Ra isotopes, ranging from 5 - 94% for C-Disk-IV sediments. About half of the 227 Ac profiles showed a large deficiency relative to 231 Pa in the upper few cm of sediments, while the other half showed a very small deficiency. Sediment composition, loss of surficial material, non-steady state behavior, and non-local bioturbation transport of sediments might explain the discrepancy between the two types of 227 Ac profiles. It is noteworthy that 230 Th budgets indicate significant sediment winnowing at sites with low F values, perhaps indicating that exhumation of formerly buried sediment plays some role. xi Two independent approaches were used to quantify the source function of 227 Ac and 228 Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter-parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agree within uncertainty, and the average 227 Ac and 228 Ra sediment fluxes for the Northeast Pacific are 90 ± 20 and 600 ± 200 dpm m -2 yr -1 . The 226 Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific. The average sediment flux for 226 Ra along the C-Disk-IV cruise is 1300 ± 10 dpm m -2 -yr -1 , which is over 2x higher than the water column inventory of 226 Ra in this region (600 dpm m -2 -yr -1 ). 227 Ac fluxes for the southern half of the GP15 transect were calculated by estimating F and using 231 Pa measurements in the upper few cm of sediments. Profiles of 227 Ac and 231 Pa were measured and modeled in the water column along the GP15 transect. Along the GP15 transect, 227 Ac and 231 Pa are typically near equilibrium between 0-3000m depths, and below this horizon, 227 Ac is often in excess over its parent. Excess 227 Ac ( 227 Acex) generally increases in activity with increasing depth and the highest concentrations of 227 Acex are contained within the bottom 1000m. The highest concentrations of 227 Acex in the Eastern Pacific are found near the center of the Northeast Pacific Basin (NEPB) and south of 10˚S. These areas are dominated by low sedimentation and high 231 Pa activity in sediments. Along the southern leg of the GP15 transect (Sta.19 - 37), some elevated activities of 227 Acex are found at mid-depths (~2600m). These areas appear to be influenced by hydrothermal activity from the East Pacific Rise (EPR), due to the proximity of d 3 He anomalies and 227 Acex activities at those depths, although maxima in the two tracers are not perfectly coincident. xii Three types of 227 Acex profiles were found in the water column along the GP15 transect: (1) an expected, exponential decrease of 227 Acex away from the seafloor; (2) a well-mixed 500m thick bottom layer with very little 227 Acex above; (3) and lastly, an unexpected, erratic distribution of 227 Acex that has local maxima in 227 Acex overlying bottom waters of lower concentration. The first type of 227 Acex profile is found toward the northern end of the GP15 transect (Sta.6 – 10). These profiles can be generated if water circulation is flowing along a constant depth, where the 227 Ac bottom source is constant. If vertical diffusion of 227 Acex is constant, it should produce an 227 Acex distribution that is exponentially decreasing away from the seafloor due to the combination of diffusion and radioactive decay. This is the most ideal profile and can be used to find apparent vertical eddy diffusivity rates (Kz), if the bottom is flat, or inclined with a constant slope. The apparent vertical diffusivity will be affected by both diapycnal and isopycnal transport, in situations where the isopycnals and/or the bottom topography are inclined. The second type of 227 Acex profile found along the GP15 transect shows nearly constant 227 Acex activities within the bottom few hundred meters. These profiles are found in the middle of the Northeast Pacific (30˚ - 40˚N) and indicate that the bottom ~500 meters are rapidly mixed, reflecting the density structure. Above this benthic layer, little 227 Acex is found, indicating low vertical transport. Profiles in the southern half of the GP15 transect show the 3 rd type of 227 Acex profile: an erratic distribution of 227 Acex in the bottom few hundred meters that correlates with the depth distribution of regional topographic features. The irregularity appears to reflect high density stratification, coupled with inputs from irregular topography in the Southeast Pacific that xiii produces localized maxima in the 227 Acex source function at multiple depths, which then travels horizontally along isopycnals, mimicking the complex source function. Horizontal advection of 227 Acex is significant in some parts of the transect, as shown by comparing the integrated decay of 227 Acex in the water column to the benthic source: between 40˚ - 30˚N and 10˚ - 0˚N, water column decay is smaller than benthic input and south of 10˚S, it is larger than benthic input. Areas where horizontal advection does not produce a significant effect are: north of 40°N, between 30˚ - 10˚N, and near the equator. In these areas, water column decay is comparable to benthic input. This pattern is consistent with predictions from an inverse model in the Northeast Pacific (Hautala, 2018) that indicates deep-water advection is strongest between 40˚ - 30˚N and circulation is moving in a west-east direction (along 150˚W). North of 40˚N, the model suggests that circulation is moving in a north-south direction. The last chapter of thesis focuses on sedimentary dynamics in the Santa Monica Basin (SMB) during the last 250 years, with an emphasis on the last 40 years. Mass accumulation rates (MAR) for the deepest and lowest oxygen-containing parts of the SMB basin have been remarkably consistent during the past century, averaging 17.1 ± 0.6 mg cm -2 yr -1 . However, MAR were slower prior to ~ 1900 CE (~10.5 mg/cm 2 -yr). The increase in sedimentation rate towards the recent occurs at about the time previous studies predicted an increase in siltation and the demise of a shelly shelf benthic fauna on the SMB shelf. The post-1900 CE constancy of sedimentation through a period of massive urbanization in Los Angeles is surprising. 1 Chapter 1: Introduction 1.1 Previous work with naturally-occurring U- and Th-decay series isotopes as mixing tracers in the Deep Sea Previous research has determined that direct measurements of circulation and mixing near the seafloor can be a challenge without the aid of radionuclides (Ku and Luo, 2009). Radionuclides are useful due to their chemical properties and radioactive decay, which can be used to infer mixing rates, depending on their half-lives and how they behave in the water column. Another reason radionuclides are such useful tracers is that very small quantities can be accurately measured. For over a half-century now, various types of radioisotopes have been used to determine rates of chemical and physical processes occurring in the ocean (e.g., Broecker et al., 1967, 1968; Berelson et al., 1982; Hammond et al., 1990; Geibert, 2002; Shaw and Moore, 2002; Ku and Luo, 2009). These radionuclides have helped oceanographers understand fluxes and transport rates of other solutes in the ocean, facilitating ocean modeling. In past studies, 222 Rn has been used to study diapycnal mixing in the oceans (Broecker et al., 1967, 1968; Chung and Craig, 1972; Chung and Kim, 1980; Sarmiento and Rooth, 1980; Berelson et al., 1982). Due to its half-life of 3.8 days, it is an excellent tracer for studying vertical mixing rates because it may not experience large lateral transport. However, due to its short half-life, it could only be used to model vertical mixing near the sea floor, often within 100 meters of the bottom, and measurements usually need to be performed quickly after retrieval. 2 Radium-228 (t1/2=5.75 y) is another deep-sea tracer that has been used to study diapycnal mixing (Sarmiento et al., 1976; Sarmiento et al., 1982; Moore, 1976). 228 Ra is produced from the decay of 232 Th (t1/2=1.41x10 10 y), which enters the oceans in detrital form and sinks to bottom sediments. 228 Ra is much more soluble than 232 Th, and thus it enters sediment pore waters and diffuses across the sediment-water interface (SWI) where it mixes in the overlying water column. Continental margins and hydrothermal system also contribute 228 Ra into the oceans, but the former is the major source of input into the oceans (Ku and Luo, 2009). Deep-sea profiles of 228 Ra in the North Atlantic Ocean showed a two-layered structure, similar to the 222 Rn profiles (Sarmiento et al., 1976). Vertical mixing rates were determined from a two-layered model: 100 cm 2 s -1 for the bottom 900 meters and an order of magnitude lower for the top half of the structure (~10 cm 2 s - ). However, these rates were much too large and were inconsistent with other tracers and models. Thus, it was determined that 228 Ra is largely transported along isopycnals rather than across density gradients due to abyssal topographic highs in the North Atlantic (Sarmiento et al., 1982; Broecker and Peng, 1982). Thus, the importance of horizontal transport on 228 Ra distribution in the North Atlantic means that a 1-D model cannot resolve vertical mixing rates. Other radiotracers, such as 227 Ac, must be used in conjunction with 228 Ra and more sophisticated 2-D or 3-D models should likely be developed in order to resolve vertical mixing rates near the ocean bottom (Charette et al., 2015). Actinium-227 (t1/2=21.77 y) has shown potential as a tracer for vertical mixing on the basin scale (Nozaki, 1984; Nozaki et al., 1990; Nozaki et al., 1993; Geibert et al., 2008). Nozaki first proposed 227 Ac as a deep-sea tracer in 1984, but for years, 227 Ac went relatively unmeasured due to its low concentrations and difficulties with detection in ocean waters (Geibert and Vöge, 3 2002). However, during the last 30 years better methods have evolved that can measure small concentrations of trace elements and isotopes (TEIs), such as Radium Delayed Coincidence Counters (RaDeCC; Moore and Arnold, 1996; Shaw and Moore, 2002; Dulaiova et al., 2013) and more precise mass spectrometers (Levier et al., 2021). The last 4 major US GEOTRACES cruises have collected Ra and Ac from seawater by pumping large volumes of seawater through acrylic fibers impregnated with MnO2. The efficiency of these MnO2 fibers to adsorb Ra and Ac are 100% at flow rates below 1 L/min (Reid, 1979). However, the US GEOTRACES protocol, requiring rapid flow to minimize sample time, uses 1500 liters of seawater pumped in 4 hours, resulting in an adsorption efficiency of 65%. Having two MnO2 filters sit in series during the pumping time allows for a direct adsorption efficiency to be determined and allows 90% of the Ra and Ac to be captured. This has led to a large number of data sets to be produced for the radium quartet ( 223 Ra, 224 Ra, 228 Ra, and 226 Ra) and 227 Ac distribution throughout the oceans. High resolution sampling has occurred in the North Atlantic, North and South Pacific, and the Arctic oceans (Charette et al., 2015, Kipp et al., 2015; Sanial et al., 2018; Kipp et al., 2018; Hammond et al., in prep). While the radium quartet has been well studied and its source function assumed from mass balance models, 227 Ac has yet to be utilized to its full extent. For example, it has been shown that shelf-derived material has increased into the Arctic ocean in recent years due to rising temperatures by using 228 Ra as a tracer (Kipp et al., 2018; Rutgers van der Loeff et al., 2018). Profiles of 228 Ra were measured throughout the Arctic region in 2007 and 2015 and a significant increase in the concentration of dissolved 228 Ra was observed (Kipp et al., 2018). A mass balance 4 model was used to provide the source of 228 Ra into the Arctic ocean. This provided a tracer for other shelf derived material, such as organic carbon and nutrients into the Arctic region. 227 Ac shortcomings as a useful tracer lie in its source function not being well measured. While seawater measurements are fairly well constrained, the flux of 227 Ac from sediments is largely not well understood. This is because few measurements have been made in the sediments due to difficulty with detection (Ku and Luo, 2009; Geibert et al., 2002). However, during the past decade, better methods have evolved (Dulaiova et al., 2013) that allow 227 Ac to be measured in bottom sediments with relatively ease. This dissertation is the first study to measure the source function of 227 Ac along a transect in the North Pacific ocean. By comparing the distribution of 227 Ac in the water column to its source function, it can be determined what type of model should be applied to accurately constrain transport rates in the deep ocean. Chapter 2 presents results for estimates of benthic 227 Ac fluxes in the Northeast. Pacific Basin, based on using two independent techniques, measurement of solid phases and direct flux measurement using incubated cores. Samples were collected as part of the C-Disk-IV expedition during August. 2017, with Will Berelson and Jess Adkins as co-chief Scientists. These measurements provide a constraint for interpretation of water column measurements of 227Ac made as part of the GEOTRACES Leg GP15, described in Chapter 3. The GP15 cruise took place during Fall, 2018, with Karen Casciotti, Greg Cutter, and Phoebe Lam as co-chief Scientists. Collection of samples for 228Ra were overseen by our collaborators at WHOI, Paul Henderson and Emilie LeRoy, coordinated by the GEOTRACES in situ pump group overseen by Phoebe Lam. Interpretation of the results of sediment and water column studies includes contributions of co- authors identified in these two chapters. 5 1.2 Bioturbation and accumulation rates in sediments based on 210 Pb The 210 Pb method is a powerful geochronometer tool for recent (one century old) marine sediment deposits. Goldberg (1963) developed the first principles of the 210 Pb method by constraining accumulation rates in Arctic ice sheets. The 210 Pb method was further developed for marine sediments (i.e., lakes, deep-sea, and coastal environments) in the coming decades (Krishnaswami et al., 1971; Koide et al.,1973; Bruland et al., 1974). Today, the 210 Pb method is used frequently by oceanographers and limnologists, in conjunction with other geochronometers, when addressing recent depositional reconstructions (Swarzenski, 2014). 210 Pb (t 1/2 =22.2 y) is a naturally occurring radioisotope that is produced from the 238 U decay chain. The disequilibrium that occurs in the 238 U decay chain can be utilized if the geochemical behavior of each daughter-parent pair is known. For example, radium is quite soluble in seawater while Pb is quite insoluble and sorbs strongly to particulates. In marine sediments, 210 Pb may be in excess over its parent, 226 Ra (t 1/2 =1600 y) due to being scavenged from the overlying water column by particles falling to bottom sediments. This disequilibrium between the parent-daughter pair provides a unique opportunity to determine residence times for other solutes in the water column and recent sedimentation or mixing rates in sediments (Ku and Luo, 2009). 210 Pb in marine sediments is produced in two ways: first, the in-situ decay of 226 Ra to 210 Pb, which is called the ‘supported’ 210 Pb and second, as stated above, the production of 210 Pb in the atmosphere or water column where it is quickly scavenged by particles to bottom sediments (Bruland et al., 1974). The latter production is called ‘excess’ 210 Pb because it will be in excess 6 over its parent in marine sediments. The excess 210 Pb will decay with a 22-year half-life and can be used to establish sediment geochronologies. There are two models that have been successfully applied to the distribution of excess 210 Pb in marine sediments to determine sedimentation rates: the constant rate of supply (c.r.s.) and the constant initial concentration (c.i.c.) model. Both models assume a time-independent flux of 210 Pb across the sediment water interface (SWI) and the c.i.c. model additionally assumes that sedimentation rate is time-independent (Benninger and Krishnaswami, 1981; Robbins and Edington, 1975; Robbins, 1978; Appleby, 2001; Appleby and Oldfield, 1978; Kirchner, 2011). If both the flux and sedimentation of excess 210 Pb is constant over time at a given location, and no bioturbation is present, the down-core profile of excess 210 Pb will follow a simple exponential curve that will track its decay down-core. The equation for the c.i.c. models is expressed as: 𝐶 =𝐶 ! 𝑒 "#$ (1.1) where C is the activity of excess 210 Pb (dpm/g), C0 is the initial activity of excess 210 Pb at the SWI (dpm/g), z is depth at the midpoint of each sample interval, and a is the inverse scale length (cm -1 ) of the exponential curve. a is related to sedimentation rate by: 𝑎 = % & (1.2) where l is the decay constant of 210 Pb (0.0311 yr -1 ) and s is sedimentation rate (cm yr -1 ). 7 The c.r.s. model may be applied if sedimentation rate down-core has varied with time. Eq. 1.3 relates the age of the sediments at depth z: 𝑡 = ' % 𝑙𝑛( ((!) (($) ) (1.3) Where A(0) is the total inventory of excess 210 Pb (dpm cm -2 ), A(z) is the excess 210 Pb inventory from depth z to the bottom section of sediments (dpm cm -2 ), and l is the decay constant of 210 Pb (yr -1 ). The constant rate of excess 210 Pb supply to the first section (depth z to the bottom section of sediments) defined in eq. 1.3 is: 𝛼 =𝑟𝐶 ! (1.4) Where r is the mass accumulation rate (g cm -2 yr -1 ), C0 is the activity of excess 210 Pb in the first section of sediments (dpm g -1 ), a and is the rate of excess 210 Pb supplied to the first section of sediments (g cm -2 yr -1 ). And C0 is related to C(z) by the following equation: 𝐶 ! =𝐶(𝑧)𝑒 "%+ (1.5) where C(z) is the activity of excess 210 Pb at depth z and t is time (yr). a is related to A(0) and A(z) by the following relationship: 𝛼 =𝐴(0)𝜆 =𝐴(𝑧)𝑒 %+ 𝜆 (1.6) 8 Finally, by substituting C0 and a into eq. 1.4, the mass accumulation rate is: 𝑟 = (($)% ,($) (1.7) Both models can be modified to account for post-depositional compaction and varying porosities down-core by using integrated mass (i.e., g cm -2 for z). Profiles of 210 Pb can also be affected by bioturbation as well as accumulation. Assuming bioturbation can be described as a diffusive process, eq. 1.8 expresses the time dependent behavior: -, -+ =𝐷 . - ! , -$ ! −𝑆 -, -$ −𝜆𝐶 (1.8) where 𝐷 . = biodiffusivity (cm 2 s -1 ), C is excess 210 Pb, and other terms have been identified previously. The steady state solution to this equation, assuming the c.i.c. assumption applies, both 𝐷 . and 𝑆 are independent of depth, and two boundary conditions are applied: C = C0 at z = 0, and C à 0 as z à ∞ is: 𝐶 =𝐶 ! 𝑒 ".$ (1.9) where 𝑏 = "& 0 1& ! 0 23 " % 43 " 9 Eq. 1.9, like Eq 1.1, suggests an exponential profile for excess 210 Pb, but the value of the attenuation parameter 𝑏 depends on both 𝑆 and 𝐷 . . If laminated sediments are found, this is evidence that 𝐷 . is small. Or if a second chronometer is available, the importance of 𝐷 . and s can be determined. In Chapter 2, this model is applied to 210 Pb profiles to determine 𝐷 . at stations in the Northeast Pacific Basin. Accumulation rates in this deep ocean setting are so slow that excess 210 Pb profiles are dominated by 𝐷 . . Chapter 4 presents an application of 210 Pb to examine sedimentation rates in the Santa Monica Basin (SMB) throughout the past 250 years, with an emphasis on the last 40 years (Chapter 4). Laminated sediments in the central basin are evidence that excess 210 Pb profiles are not affected by Db and can be used to calculate S. Starting with a study by Bruland et al. (1974), investigators have been using 210 Pb profiles as a means of documenting sediment accumulation and sediment mixing in the SMB (Bruland et al., 1974; Huh et al., 1989; Christensen et al., 1993). A compilation of core analyses was published by Huh et al. (1989), and further work by Alexander and Lee (2009) provided a record of sedimentation in SMB from the 1970’s through the 1990’s. The recent study (conducted by Kemnitz et al., 2020) aimed at augmenting this record of coastal sedimentation, quantified by analyses of 210 Pb and 14 C profiles in the SMB. A focus of this study was to determine if development in the coastal zone over the past century has impacted sediment accumulation during the past century. 10 1.3 References Alexander, Clark R., and Homa J. Lee. “Sediment Accumulation on the Southern California Bight Continental Margin during the Twentieth Century.” Earth Science in the Urban Ocean: The Southern California Continental Borderland, 2009. https://doi.org/10.1130/2009.2454(2.4). Appleby, P. G. “Chronostratigraphic Techniques in Recent Sediments.” Tracking Environmental Change Using Lake Sediments, 2001, 171–203. https://doi.org/10.1007/0-306-47669-x_9. Appleby, P.G., and F. 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Broecker, Wallace S., and Tsung-Hung Peng. Tracers in the Sea. S.l., New York: s.n., 1982. Broecker, Wallace S., Yuan Hui Li, and John Cromwell. “Radium-226 and Radon-222: Concentration in Atlantic and Pacific Oceans.” Science 158, no. 3806 (1967): 1307–10. https://doi.org/10.1126/science.158.3806.1307. Bruland, Kenneth W., Kathe Bertine, Minoru Koide, and Edward D. Goldberg. “History of Metal Pollution in Southern California Coastal Zone.” Environmental Science & Technology 8, no. 5 (1974): 425–32. https://doi.org/10.1021/es60090a010. Charette, Matthew A., Paul J. Morris, Paul B. Henderson, and Willard S. Moore. “Radium Isotope Distributions during the US GEOTRACES North Atlantic Cruises.” Marine Chemistry 177 (2015): 184–95. https://doi.org/10.1016/j.marchem.2015.01.001. Chung, Y., and K. Kim. “Excess 222Rn And the Benthic Boundary Layer in the Western and Southern Indian Ocean.” Earth and Planetary Science Letters 49, no. 2 (1980): 351–59. https://doi.org/10.1016/0012-821x(80)90078-3. Chung, Yu-chia, and Harmon Craig. “Excess-Radon and Temperature Profiles from the Eastern Equatorial Pacific.” Earth and Planetary Science Letters 14, no. 1 (1972): 55–64. https://doi.org/10.1016/0012-821x(72)90079-9. 11 Dulaiova, Henrieta, Kenneth W. Sims, Matthew A. Charette, Julie Prytulak, and Jerzy S. Blusztajn. “A New Method for the Determination of Low-Level Actinium-227 in Geological Samples.” Journal of Radioanalytical and Nuclear Chemistry 296, no. 1 (2012): 279–83. https://doi.org/10.1007/s10967-012-2140-0. Geibert, W., M.M. Rutgers van der Loeff, C. Hanfland, and H.-J. Dauelsberg. “Actinium-227 as a Deep-Sea Tracer: Sources, Distribution and Applications.” Earth and Planetary Science Letters 198, no. 1-2 (2002): 147–65. https://doi.org/10.1016/s0012-821x(02)00512-5. Geibert, Walter, and Ingrid Vöge. “Progress in the Determination of 227 Ac in Sea Water.” Marine Chemistry 109, no. 3-4 (2008): 238–49. https://doi.org/10.1016/j.marchem.2007.07.012. Geibert, Walter, Matt Charette, Guebuem Kim, Willard S. Moore, Joseph Street, Megan Young, and Adina Paytan. “The Release of Dissolved Actinium to the Ocean: A Global Comparison of Different End-Members.” Marine Chemistry 109, no. 3-4 (2008): 409–20. https://doi.org/10.1016/j.marchem.2007.07.005. Goldberg, E D. “. Geochronology with Pb-210. Radioactive Dating.” International Atomic Energy Agency, 1963, 121. Hammond, D. E., M. Charette, W. Moore, P. Henderson, V. Sanial, L. Kipp, R. Anderson, and N. J. Kemnitz. “Actinium-227 as a Tracer for Solute Transport in the Deep Ocean: Results from the GEOTRACES Pacific Transect.” in prep, 2022. Hammond, Douglas E., Richard A. Marton, William M. Berelson, and Teh-Lung Ku. “Radium 228 Distribution and Mixing in San Nicolas and San Pedro Basins, Southern California Borderland.” Journal of Geophysical Research 95, no. C3 (1990): 3321. https://doi.org/10.1029/jc095ic03p03321. Huh, Chi-An, Lawrence F. Small, Sommart Niemnil, Bruce P. Finney, Barbara M. Hickey, Nancy B. Kachel, Donn S. Gorsline, and Peter M. Williams. “Sedimentation Dynamics in the Santa Monica-San Pedro Basin off Los Angeles: Radiochemical, Sediment Trap and Transmissometer Studies.” Continental Shelf Research 10, no. 2 (1990): 137–64. https://doi.org/10.1016/0278-4343(90)90027-j. Kemnitz, Nathaniel, William M. Berelson, Douglas E. Hammond, Laura Morine, Maria Figueroa, Timothy W. Lyons, Simon Scharf, et al. “Evidence of Changes in Sedimentation Rate and Sediment Fabric in a Low-Oxygen Setting: Santa Monica Basin, CA.” Biogeosciences 17, no. 8 (2020): 2381–96. https://doi.org/10.5194/bg-17-2381-2020. Kipp, Lauren E., Matthew A. Charette, Douglas E. Hammond, and Willard S. Moore. “Hydrothermal Vents: A Previously Unrecognized Source of Actinium-227 to the Deep Ocean.” Marine Chemistry 177 (2015): 583–90. https://doi.org/10.1016/j.marchem.2015.09.002. 12 Kipp, Lauren E., Matthew A. Charette, Willard S. Moore, Paul B. Henderson, and Ignatius G. Rigor. “Increased Fluxes of Shelf-Derived Materials to the Central Arctic Ocean.” Science Advances 4, no. 1 (2018). https://doi.org/10.1126/sciadv.aao1302. Kirchner, Gerald. “ 210 Pb As a Tool for Establishing Sediment Chronologies: Examples of Potentials and Limitations of Conventional Dating Models.” Journal of Environmental Radioactivity 102, no. 5 (2011): 490–94. https://doi.org/10.1016/j.jenvrad.2010.11.010. Koide, Minoru, Kenneth W. Bruland, and Edward D. Goldberg. “Th-228/Th-232 and Pb-210 Geochronologies in Marine and Lake Sediments.” Geochimica et Cosmochimica Acta 37, no. 5 (1973): 1171–87. https://doi.org/10.1016/0016-7037(73)90054-9. Krishnaswamy, S., D. Lal, J.M. Martin, and M. Meybeck. “Geochronology of Lake Sediments.” Earth and Planetary Science Letters 11, no. 1-5 (1971): 407–14. https://doi.org/10.1016/0012- 821x(71)90202-0. Ku, Teh-Lung, and Shangde Luo. “Chapter 9 Ocean Circulation/Mixing Studies with Decay- Series Isotopes.” Radioactivity in the Environment, 2008, 307–44. https://doi.org/10.1016/s1569- 4860(07)00009-5. Levier, M., M. Roy-Barman, C. Colin, and A. Dapoigny. “Determination of Low Level of Actinium-227 in Seawater and Freshwater by Isotope Dilution and Mass Spectrometry.” Marine Chemistry 233 (2021): 103986. https://doi.org/10.1016/j.marchem.2021.103986. Moore, Willard S. “Sampling 228 Ra in the Deep Ocean.” Deep Sea Research and Oceanographic Abstracts 23, no. 7 (1976): 647–51. https://doi.org/10.1016/0011-7471(76)90007-3. Moore, Willard S., and Ralph Arnold. “Measurement of 223 Ra and 224 Ra in Coastal Waters Using a Delayed Coincidence Counter.” Journal of Geophysical Research: Oceans 101, no. C1 (1996): 1321–29. https://doi.org/10.1029/95jc03139. Nozaki, Yoshiyuki, Masatoshi Yamada, and Hirofumi Nikaido. “The Marine Geochemistry of Actinium-227: Evidence for Its Migration through Sediment Pore Water.” Geophysical Research Letters 17, no. 11 (1990): 1933–36. https://doi.org/10.1029/gl017i011p01933. Nozaki, Yoshiyuki. “Actinium-227: A Steady State Tracer for the Deep-Sea Basin-Wide Circulation and Mixing Studies.” Deep Ocean Circulation - Physical and Chemical Aspects, 1993, 139–56. https://doi.org/10.1016/s0422-9894(08)71323-0. Nozaki, Yoshiyuki. “Excess 227 Ac in Deep Ocean Water.” Nature 310, no. 5977 (1984): 486–88. https://doi.org/10.1038/310486a0. Reid, David F., Robert M. Key, and David R. Schink. “Radium, Thorium, and Actinium Extraction from Seawater Using an Improved Manganese-Oxide-Coated Fiber.” Earth and Planetary Science Letters 43, no. 2 (1979): 223–26. https://doi.org/10.1016/0012- 821x(79)90205-x. 13 Robbins, J A. “Geochemical and Geophysical Applications of Radioactive Lead.” Biogeochemistry of Lead in the Environment, Elsevier Scientific, Amsterdam, 1978, 285–393. Robbins, John A., and D.N. Edgington. “Determination of Recent Sedimentation Rates in Lake Michigan Using Pb-210 and Cs-137.” Geochimica et Cosmochimica Acta 39, no. 3 (1975): 285– 304. https://doi.org/10.1016/0016-7037(75)90198-2. Rutgers van der Loeff, Michiel, Lauren Kipp, Matthew A. Charette, Willard S. Moore, Erin Black, Ingrid Stimac, Alexander Charkin, et al. “Radium Isotopes across the Arctic Ocean Show Time Scales of Water Mass Ventilation and Increasing Shelf Inputs.” Journal of Geophysical Research: Oceans 123, no. 7 (2018): 4853–73. https://doi.org/10.1029/2018jc013888. Sanial, V., L.E. Kipp, P.B. Henderson, P. van Beek, J.-L. Reyss, D.E. Hammond, N.J. Hawco, et al. “Radium-228 as a Tracer of Dissolved Trace Element Inputs from the Peruvian Continental Margin.” Marine Chemistry 201 (2018): 20–34. https://doi.org/10.1016/j.marchem.2017.05.008. Sarmiento, J. L., and C. G. Rooth. “A Comparison of Vertical and Isopycnal Mixing Models in the Deep Sea Based on Radon 222 Measurements.” Journal of Geophysical Research 85, no. C3 (1980): 1515. https://doi.org/10.1029/jc085ic03p01515. Sarmiento, J. L., C. G. Rooth, and W. S. Broecker. “Radium-228 as a Tracer of Basin Wide Processes in the Abyssal Ocean.” Journal of Geophysical Research 87, no. C12 (1982): 9694. https://doi.org/10.1029/jc087ic12p09694. Sarmiento, J.L., H.W. Feely, W.S. Moore, A.E. Bainbridge, and W.S. Broecker. “The Relationship between Vertical Eddy Diffusion and Buoyancy Gradient in the Deep Sea.” Earth and Planetary Science Letters 32, no. 2 (1976): 357–70. https://doi.org/10.1016/0012- 821x(76)90076-5. Shaw, Timothy J, and Willard S Moore. “Analysis of 227Ac in Seawater by Delayed Coincidence Counting.” Marine Chemistry 78, no. 4 (2002): 197–203. https://doi.org/10.1016/s0304-4203(02)00022-1. Swarzenski, Peter W. “ 210 Pb Dating.” Encyclopedia of Scientific Dating Methods, 2014, 1–11. https://doi.org/10.1007/978-94-007-6326-5_236-1. 14 Chapter 2: Actinium & Radium Fluxes from the Seabed in the Northwest Pacific Basin Nathaniel Kemnitz, Douglas E. Hammond, Paul Henderson, Emilie Le Roy, Matthew Charette, Willard Moore, Robert F. Anderson, Martin Q. Fleisher, Annie Leal, Erin Black, Christopher T. Hayes, Jess Adkins, William Berelson, Xiaopeng Bian (co-authors) Abstract Five sediment cores were collected and sampled along a cruise tract from Hawaii to Alaska in August 2017 (C-Disk-IV cruise) with the objective of characterizing the behavior of 227 Ac, 228 Ra, and 226 Ra and their fluxes into the overlying water column. Solid phase profiles of these isotopes were measured, and reaction-transport models were applied that account for molecular diffusion, bioturbation, sedimentation, distribution coefficient (kd), and fraction of each isotope released to pore water by parent decay (called F). Most solid phase 226 Ra profiles showed a large deficiency compared to parent 230 Th in the upper 15 cm of sediments, while the 228 Ra profiles showed a modest deficiency relative to its 232 Th parent in the top 3 cm of sediments. About half of the 227 Ac profiles showed a large deficiency relative to 231 Pa in the upper few cm of sediments, while the other half showed a very small deficiency. Sediment composition, loss of surficial material, non-steady state behavior, and non-local bioturbation transport of sediments might explain the discrepancy between the two types of 227 Ac profiles. Model fits to the 226 Ra profiles showed F and sedimentation rate values between 57 - 83% and 0.10 – 0.40 cm kyr -1 for C-Disk-IV sediments. The 228 Ra profiles were difficult to measure due to high counting uncertainties, but F values obtained from 228 Ra profiles were 15 similar to 226 Ra values. F values for 227 Ac had a much larger range than the Ra isotopes, ranging from 7 - 95% for the same sediments. The fits to the solid phase Ac and Ra profiles used kd values determined in lab experiments for C-Disk-IV sediments. Ra kd values were between 1000-3000 mL g -1 , which agree with previous author’s kd values for deep-sea sediments. The Ac kd values were between 3500-22000 mL g -1 for the same sediments. There is a strong positive correlation between Ra and Ac kd values, with Ac kd values being about 6.6 times greater than those of Ra. Two independent approaches were used to quantify the benthic fluxes of 227 Ac and 228 Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter-parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agreed within uncertainty, and the average 227 Ac and 228 Ra sediment fluxes for the Northeast Pacific are 90 ± 20 and 600 ± 200 dpm m -2 -yr - 1 , respectively. The 226 Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific. The average sediment flux for 226 Ra along the C-Disk-IV cruise is 1300 ± 200 dpm m -2 -yr -1 , which is over 2x higher than the water column inventory of 226 Ra in this region (600 dpm m -2 -yr -1 ), and indicates the importance of lateral 226 Ra export from the N. Pacific. 227 Ac and Ra fluxes are influenced primarily by the parent activities in sediments and bioturbation. The smallest 227 Ac, 228 Ra and 226 Ra fluxes in the Northeast Pacific are located north of 40˚N primarily due to dilution of their Pa and Th ancestors by higher sediment accumulation 16 rates, although higher bioturbation in this area partially compensates for the dilution. The largest 227 Ac and Ra isotope fluxes are near the center of the Northeast Pacific (~37˚N); 227Ac decreases by 50% south of this latitude , with smaller decreases for the radium isotopes. 2.1 Introduction 227 Ac (t1/2=21.77 y) has shown potential as a tracer for mixing on the basin scale range (Nozaki, 1984; Nozaki et al., 1990; Geibert and Vöge, 2008; Geibert et al., 2002). Nozaki first proposed 227 Ac as a deep-sea tracer in 1984, but for years, 227 Ac went relatively unmeasured due to its low concentrations and difficulties with detection in ocean sediments and waters (Geibert and Vöge, 2008). However, during the last 30 years, better methods have evolved that can measure small concentrations of this isotope, due to development of the Radium Delayed Coincidence Counters (RaDeCC; Moore and Arnold, 1996; Shaw and Moore, 2002; Dulaiova et al., 2013), and more sensitive mass spectrometers (Levier et al., 2021). GEOTRACES is an international collaboration whose main objective is to understand the processes that control the distribution and transport of trace elements and isotopes (TEIs) in the oceans (GEOTRACES Planning Group, 2007). One important component for understanding these processes is quantifying transport using naturally occurring radioisotopes, including 228 Ra and 227 Ac. The last 4 major US GEOTRACES cruises (GA01, GP16, GN01, GP15) have collected Ra and Ac from seawater by pumping large volumes through acrylic fibers impregnated with MnO2. This has led to acquisition of a number of data sets for the radium quartet ( 223 Ra, 224 Ra, 228 Ra, and 226 Ra) and 227 Ac distribution throughout the oceans. High 17 resolution sampling has occurred in the North Atlantic, North and South Pacific, and the Arctic oceans (Charette et al., 2015, Kipp et al., 2018; Sanial et al., 2018; Hammond et al., in prep; Kemnitz et al., in prep). Applying these isotopes as tracers can be facilitated if their source functions are defined. While 226 Ra and 228 Ra in deep ocean sediments have been well studied at several locations to constrain their source functions, limited data exists for 227 Ac (Cochran and Krishnaswami, 1980; Kadko, 1980; Moore et al., 1996; Rama and Moore, 1996; Kipp et al., 2018; Rutgers van der Loeff et al., 2018). This limits its utility (Ku and Luo, 2009). A few studies have assumed mass balance models (Geibert et al., 2008), but Nozaki et al. (1990) is the only study to date to have directly measured 227 Ac in sediments. Thus, this study aims to improve knowledge of the geochemical behavior of 227 Ac, 226 Ra, and 228 Ra in marine sediments and estimate their fluxes into deep ocean waters. One objective of this study is to compare methodologies to measure 227 Ac fluxes from marine sediments by using two independent methods: indirectly by modeling solid phase profile activity measurements and directly by core incubations. The former approach applies previously published reactive-transport models to predict geochemical behaviors of 227 Ac and other radionuclides ( 226 Ra & 228 Ra) in the sediments sampled during this study, while the second approach has not previously been utilized in the deep sea. A second objective is to evaluate spatial variability in sediment fluxes for 227 Ac, 228 Ra, and 226 Ra in a section through the Northeast Pacific Basin (NEPB) and show how the water column distribution of these radionuclides is related to these fluxes. 18 2.2 Theory Cochran and Krishnaswami, (1980) developed a reactive transport model for the behavior of 226 Ra and 228 Ra in marine sediments. This model can be adapted for 227 Ac as well (Nozaki et al., 1990). The model assumes parents of these isotopes are associated with solids, while one fraction of the more mobile daughters may diffuse as solutes or reversibly adsorb to solid phases, and a second fraction of these daughters is tightly bound within solid phases and does not exchange with the fluid phase. This model is idealized, but accounts for the major processes that govern reaction and transport in sediments, including molecular diffusion, radioactive decay, burial, and bioturbation (treated as a diffusive process). Sediments are divided into an upper, bioturbated layer that overly a deeper layer that is no longer bioturbated. The governing one-dimensional steady state equations are given by: 0=(𝐷 5 +𝐾𝐷 . ) - ! , -$ ! −𝑆(1+𝐾) -, -$ −𝜆(1+𝐾)𝐶+𝑃 𝑓𝑜𝑟 0≤𝑧 ≤𝐿 (2.1a) 0=𝐷 5 - ! , -$ ! −𝑆(1+𝐾) -, -$ −𝜆(1+𝐾)𝐶+𝑃 𝑓𝑜𝑟 𝑧 ≥𝐿 (2.1b) where: 𝐶 = concentration of dissolved radioisotope in porewater (atoms cm -3 ) 𝐷 5 = molecular diffusion corrected for tortuosity (cm 2 yr -1 ) 𝐷 . = bioturbation (cm 2 yr -1 ); assumed constant in the upper layer 𝑆 = sedimentation rate (cm yr -1 ) 𝜆 = decay constant of daughter (yr -1 ) 𝑃 = production rate of daughter atoms per unit of porewater from parent decay (atoms cm -3 yr -1 ) 19 𝐾 = partition coefficient (dimensionless) 𝑧 = depth in sediments (cm) 𝐿 = thickness of bioturbation zone (cm) A solution to eq. 2.1 assumes sedimentation rate, bioturbation, molecular diffusion, and K are all constant and independent with respect to depth (Cochran and Krishnaswami, 1980; Kadko, 1980). Solid phase 231 Pa and 230 Th profiles in the bioturbated layer (shown later) have been homogenized by bioturbation throughout the upper 6-8 cm. The production term (P) can be further expanded and expressed as: 𝑃 =𝐹𝐴 6' +𝑓𝐴 64 𝑓𝑜𝑟 0≤𝑧 ≤𝐿 (2.2a) 𝑃 =𝐹𝐴 6' 𝑒 "7($"8) +𝑓𝐴 64 𝑓𝑜𝑟 𝑧 ≥𝐿 (2.2b) where: F = fraction of parent isotope decay which recoil daughter products into porewater f = fraction of grandparent isotope decay which recoil daughter products into porewater 𝐴 6' = parent activity ( 231 Pa or 230 Th) per unit of porewater in atoms cm -3 yr -1 𝐴 64 = grandparent activity ( 234 U or 235 U) per unit of porewater in atoms cm -3 yr -1 𝜇 = ! ! " (cm -1 ) 𝜆 6 =decay constant of parent Note: The parent-daughter pair for F and f are assumed to be in equilibrium and f is typically minor. The parent and grandparent activities per unit of porewater are calculated from solid 20 phase activity multiplied by solid phase grams per cm -3 of porewater, based on porosity. Substituting eq. 2.2 into eq. 2.1, solving for the concentration, multiplying by ld, and expressing concentrations as activities (lC=A): 𝐴 ' (𝑧)=𝑄 ' 𝑒 9 # $ +𝑅 ' 𝑒 : # $ + ;( $# 0<( $! '0= 𝑓𝑜𝑟 0≤𝑧 ≤𝐿 (2.3a) 𝐴 4 (𝑧)=𝑄 4 𝑒 9 ! $ + ;( $# % 3 # 𝑒 "7($"8) + <( $! '0= 𝑓𝑜𝑟 𝑧 ≥𝐿 (2.3b) where Q, R, a, and d are all constants that depend on Ds, Db, S, and K (See SI S.1 for full definition of these terms). 𝐴 6' is the parent activity of daughter in dpm cm -3 of porewater, 𝐴 64 is the grandparent activity of daughter in dpm cm -3 of porewater, 𝐴 ' is the porewater activity of daughter in the bioturbated zone in dpm cm -3 , 𝐴 4 is the porewater activity of daughter below the bioturbated zone in dpm cm -3 , and z is depth in cm. Eq. 2.3 depends on 4 boundary conditions: 1.) 𝑎𝑡 𝑧 =0, 𝐴 ' = 𝐴 ! 2.) as z à∞, 𝐴 4 → <( $! '0= 3.) (𝐴 ' ) 8 =(𝐴 4 ) 8 4.) 𝐷 5 ( -( # -$ ) 8 =(𝐷 5 +𝐾𝐷 . )( -( ! -$ ) 8 To relate the dissolved concentration to the solid phase, the partition coefficient (K) can be related to the distribution coefficient, kd (in units of cm 3 g -1 ) as: 𝐾 = > % ('"?)@ & ? (2.4) 21 where rs = solid phase density (g cm -3 ). kd relates the mobile solid and dissolved phase concentration 𝐶 A (dpm g -1 ) as: 𝑘 B = , ' , (2.5) The following equations relate the dissolved phase (dpm cm -3 ) to the solid phase (dpm g -1 ) by multiplying eq. 2.3 by kd to find the mobile activity and adding the immobile crystalline activity to obtain the total activity: 𝐴 "#$%$ (𝑧)=𝑘 & (𝐴 " )+(1−𝐹)+ 𝐴 '"#$%$ −𝐴 '(#$%$ ,+(1−𝑓)𝐴 '(#$%$ 𝑓𝑜𝑟 0≤𝑧 ≤𝐿 (2.6a) 𝐴 (#$%$ (𝑧) =𝑘 & (𝐴 ( )+(1−𝐹)+ 𝐴 '"#$%$ −𝐴 '(#$%$ ,𝑒 )(+#,) +(1−𝑓)𝐴 '(#$%$ 𝑓𝑜𝑟 𝑧 ≥𝐿 (2.6b) where 𝐴 '"+C+ and 𝐴 4"+C+ is the total activity of daughter products in the solid phase (dpm g -1 ), 𝐴 6'"+C+ is the total parent activity of daughter ( 231 Pa and 230 Th) in dpm g -1 , 𝐴 64"+C+ is the total grandparent activity of daughter ( 234 U and 235 U) in dpm g -1 , (1−𝐹)H𝐴 6'"+C+ −𝐴 64"+C+ I is the immobile fraction of 226 Ra/ 227 Ac from excess 230 Th/ 231 Pa decay (dpm g -1 ), and (1−𝑓)𝐴 64"+C+ is the immobile fraction of 226 Ra/ 227 Ac from 234 U/ 235 U decay (this decay is from within the crystal lattice of sediments). Lastly, a one-layer model can be derived from eq. 2.1a if the parent activity and bioturbation are assumed constant throughout the entire profile. Due to its small scale-length, a one-layer model was used for 228 Ra in this study. Two boundary conditions were applied: at z=0, the concentration of daughter products = A 0 and as z à∞, the concentration of daughter 22 products à P/(1+K), where P is parent activity. Solving for the concentrations with respect to the previous boundary conditions and expressing the concentrations as activities (A=lC): 𝐴 3 (𝑧)=𝐴 ! 𝑒 "9$ + D '0= (1−𝑒 "9$ ) (2.7) 𝛼 = "&('0E)0 F G"&('0E)H ! 02(3 & 0=3 " )G% () ('0E)H 4(3 & 0=3 " ) (2.8) AD = concentration of dissolved daughters in porewater (dpm cm -3 ) A0 = initial concentration of dissolved daughters in porewater (dpm cm -3 ) 𝑃 = activity of parent isotope (dpm cm -3 ) For a one-layer model, the following equation relates the dissolved phase (dpm cm -3 ) to the solid phase (dpm g -1 ) by multiplying eq. 2.7 by kd and adding the total parent decay: 𝐴 IC+#J 3 A (𝑧)=𝐴 ! 𝑘 B 𝑒 "9$ +𝐹𝐴 6'"+C+ (1−𝑒 "9$ ) +(1−𝐹)𝐴 6'"+C+ (2.9) 𝐴 IC+#J 3 = the total activity of daughters in solid phase (dpm g -1 ). Fluxes of 227 Ac, 226 Ra, or 228 Ra can be calculated by applying Fick’s first law to the fitted equations: 𝐹𝑙𝑢𝑥 = −(𝐷 5 +𝐷 . 𝐾)𝜙 B, B$ (2.10) where B, B$ is the derivative of eq. 2.6a, 2.9. 23 2.3 Study Area Figure 2.1 shows the C-Disk-IV (Carbonate Dissolution Kinetics-IV) and US GEOTRACES PMT (GP15) transects in the NEPB. All deep-sea sediment samples were collected during the C- Disk-IV Cruise aboard the R/V Kilo Moana in August 2017. Profiles of dissolved 227 Ac, 231 Pa, 226 Ra, and 228 Ra in the water column were collected during GP15 cruise aboard R/V Roger Revelle between September and November 2018. Figure 2.1: Study Area of the Northeast Pacific Basin (NEPB). Red dots and black triangles refer to stations where 227 Ac was measured in the water column for GEOTRACES PMT (GP15) and C-Disk-IV cruises. Numbers next to symbols refer to station numbers. All sediment measurements (Ra, Th, Ac, Pb, and Po) were made on C-Disk-IV cruise samples. 24 Location, depth, surface porosity, sedimentation, and description of sediments for each C-Disk-IV station are summarized in Table 2.1. Stations 1, 2, and 3 are located within the subtropical gyre, where oligotrophic waters dominate and organic export to the seafloor is low. The sediments at these stations are characterized by fine-grained, clay material with low organic content. Stations 4 and 5 are in the subarctic gyre, where primary production is high. Their sediments are characterized by larger grain size material with higher biogenic fraction in comparison to stations 1-3. A distinctive fluff layer was observed above the sediments at station 4, indictive of high surface production above this station (Hou et al., 2019). Table 2.1: Station information for C-Disk-IV cruise. Sediment description is taken from Hou et al. (2019). Porosity was estimated from water loss of sediments for top 3 cm. Density of sediments was assumed 2.5 g cm -3 . Sedimentation rates were estimated from 230 Thex mass accumulation rates (MAR) in the mixed layer. Station Location Bot. Dep. Surf. Por. Sed. Rate Sediment Description (˚N, ˚E) (m) (%) (cm kyr -1 ) 1 22.8, 202.0 4730 86 0.40 uniform light reddish-brown color, tube/worm visible at the sediment surface 2 27.8, 204.8 5640 83 0.10 uniform light brown color, manganese nodules 3 35.3, 209.0 5560 87 0.20 uniform light brown color, manganese nodules, stiff/sticky texture of sediments below 2cm 4 41.8, 211.8 4970 89 0.17 very light brown color, 1-3cm fluff layer above sediment, burrows 5 49.8, 210.4 4725 80 1.64 greyish-brown color, darker between 5- 10cm, grittier below 5cm, worm/tube visible 25 2.4 Material and Methods 2.4.1 Sample Collection Sediment cores were collected using a multi-coring device (10cm ID) and sectioned every 1 cm between 0-10 cm and 2 cm between 10-30 cm. Sectioned mud was placed in a cold van after sectioning and then analyzed for porosity at the USC lab by weighing aliquots before and after drying at 50˚C. Samples were then dried, gently crushed, and placed in plastic bags until analysis. Weights for sediment samples were corrected for salt contribution, based on water content and salinity. In the water column, dissolved 227 Ac and the radium quartet were collected using a dual- flow path in-situ pump (McLane WRT-LV). Two to three casts were conducted at each station (shallow, intermediate, and deep) and five to eight pumps were deployed on each cast. Each pump had two filters in parallel: the higher volume flow path had a 1 µm quartz filter (Whatman QMA) and the lower volume flow path had a 0.8 μm polyethersulfone filter (Pall Supor800). This allowed sufficient flow to pass about 1.5 m 3 during a 4-hour pump. The filters on the GP15 cruise captured particulate matter that was measured for 234 Th, 230 Th, 232 Th, and 231 Pa. Downstream of the filter heads, flow then passed through two grooved acrylic cartridges impregnated with MnO2 that sat in series. After collection, cartridges were rinsed with DIW for several minutes to remove sea-salt and then dried to 50-120% moisture using compressed air. MnO2 cartridges were then analyzed using RaDeCC immediately after collection to measure 26 excess 223 Ra and 224 Ra. Cartridges were re-measured multiple times from a few months to 24 months later to determine 227 Ac and 228 Ra. 2.4.2 Standards and Tracers The 229 Th/ 225 Ac tracer (USC #89B: 9.32 ± 0.12 dpm g -1 ) used for this study was calibrated against an 227 Ac standard from Eckert and Zeigler (WHOI E&Z Actinium-227 CRM; See appendix A.4). The 227 Ac standard was prepared at Woods Hole and had a nominal activity of 5.90 dpm g -1 on 4/27/2017. The calibrated 229 Th/ 225 Ac tracer was prepared at USC by adding 0.1 mL from a non-calibrated 229 Th stock solution (USC #89A: ~24,500 dpm g -1 ) and diluted to 250 mL with 3 M HNO3. The calibrated 229 Th/ 225 Ac tracer was routinely measured by gamma spectroscopy to monitor changes in activity with time. Standards for gamma counting U, 226 Ra, and 228 Ra were obtained from the Environmental Protection Agency (SRM-1 diluted pitchblende and SRM-2 diluted monzonite). SRM-1 and SRM-2 had nominal activities of 562 dpm g -1 for 238 U and 333 dpm g -1 for 232 Th. All Th and Ra daughter products for the 238 U and 232 Th standards are assumed to be in secular equilibrium, with the exception of 210 Pb. The 227 Ac standard from Eckert and Zeigler was also measured by gamma spectroscopy and used as a standard for 227 Ac measurements in marine sediments. Standards were counted in 3.0 cm high geometry, and corrections were made to sample results to account for the different sample heights that were used. For low energy gamma regions (i.e., 46 keV: 210 Pb), a matrix correction of 10% was routinely applied to solid phase samples when liquid 27 solutions were used as standards based on spiking solid samples with high concentrations of known 210 Pb. 210 Pb was standardized using a NIST certified solution (SRM 4337). The nominal activity of the 210 Pb solution was 11,640 ± 300 dpm/g (7/15/2006). A few 210 Pb check samples were also run by alpha spectrometry using its 210 Po daughter and a 209 Po standard (USC #65D). This standard was calibrated against the NIST 210 Pb solution in the summer of 2018 and had a nominal activity of 24.1 ± 0.3 dpm/g (7/10/2018). The 209 Po standard was corrected for decay to the time of analysis using the recently determined half-life of 125 yr (Colle et al., 2014). 2.4.3 Distribution Coefficients for Ra and Ac Ac and Ra distribution coefficients (kd) were determined experimentally by taking Ra- free seawater, adding deep-sea sediments, and spiking it with an aged (>45 y) 232 Th (USC #106A: 492 dpm g -1 ) or 229 Th (USC #89E: 1500 dpm g -1 ) solution. Both spikes are assumed to be in secular equilibrium with all daughter products (Fig. 2.2). The radioisotopes of 228 Ra (t1/2 =5.75 y) and 225 Ac (t1/2 = 10.0 d) were used to determine Ra and Ac kd values. Due to the short half-life of 228 Ac relative to the counting time in these experiments, 228 Ac was not suitable for determining an Ac kd value. 28 Figure 2.2: Decay series of 238 U, 237 Np, 235 U, and 232 Th. 29 The procedure for determining Ra kd value was as follows: 30 mL of Ra-free seawater was weighed and placed into 50 mL centrifuge tube (some experiments required 80 mL of seawater and used 125 mL PET bottles). Then, 1500 dpm of 232 Th/ 228 Ra was added to the same centrifuge tube, along with enough Na2CO3 to neutralize acid and bring pH between 7.5-7.8. Next, ~ 14 grams of wet deep-sea sediments were added to tubes and shaken. The tubes were placed on a rotating disk (~30 rpm) to provide mixing for at least 1 hour, allowing radioisotopes to equilibrate between dissolved and solid phases. After equilibration, the tubes were centrifuged for 5 minutes at 2500 rpm and the seawater was decanted, filtered (0.45 µm), and a weighed aliquot passed through MnO2 fibers (~1g) 3 times. The fibers and wet sediments were then placed in separate polypropylene tubes and measured on HPGe detectors. The gamma rays from 228 Ac daughters (338 and 911 keV) that grew into equilibrium were used to determine the 228 Ra activity. The MnO2 fibers and sediments were counted 2 separate times to insure secular equilibrium between 228 Ra and 228 Ac. The final solid phase mass was determined by drying the wet sediments and weighing the final mass (corrected for salt content). The determination of Ac kd value was similar but required additional steps: First, a pure 225 Ac solution was needed. A 229 Th spike (matrix of 3 N HNO3) was weighed out and diluted with 15 mL of 3 N HNO3. 225 Ac was separated from 229 Th (and 225 Ra) by using a 2 mL, DGA chromatography cartridge resin from Eichrom Inc. (Martin, 1995). This solution was neutralized, mixed with sediments, and equilibrated as described for Ra, except that Savillex beakers were used instead of a centrifuge tube. The pH was kept between 7.3-7.6, to minimize 225 Ac sorption to the walls of the Savillex beakers. However, around 20% of 225 Ac did stick to walls of the 30 beaker after one hour of rotating the sediment slurry. This 225 Ac loss did not impact our calculations because both sediment and dissolved phases were measured to determine kd values. 2.4.4 Alpha Spectroscopy Procedures For analysis of sediments, 10 dpm of 229 Th/ 225 Ac tracer was added to a 250 mL Teflon beaker. 209 Po tracer was also added if 210 Po was to be analyzed. Sediments (typical activity of 3 dpm g -1 ) were crushed with mortar/pestle and weighed (0.5-1.0 g) to the Teflon beaker using aluminum foil to transfer sediments. The sediments and the tracers were then taken through a series of acid digestions that included HCl, HNO3, HF, and HClO4, to completely digest all solids (Fuller, 1982). Briefly, 50 mL of aqua regia was added and heated to 90˚C in covered beakers to dissolve organics. After 4 hours of heating, covers were removed, and samples taken to dryness. Then 20 mL HNO3 and 5 mL HClO4 were added to beakers and heated to dryness. This step ensures all organics have oxidized. Next, to dissolve the silicate materials, 10 mL HF along with 20 mL HNO3 and 5 mL HClO4 were added and taken to dryness at 90˚C. After drying, a few mL’s of HCl were added to dissolve left over solids. After this last step, the solution was dried and taken up in 1 M HCL to plate Po isotopes or proceed to the Ac separation step. A pre-concentration step needs to be performed before adding the solution to a chromatography column (PbSO4 co-precipitation technique, Dulaiova et al., 2013; Martin et al., 1995) to separate Ac from other metals. After solids were completely digested, the solution was dried completely and then taken up in 100 mL 0.1 M HNO3 (or 0.1 M HCl). Next, 1 mL of 31 concentrated H2SO4 and 2 g K2SO4 were added to the 100 mL solution and stirred. While stirring, 1 mL 0.24 M Pb(NO3)2 was added to the solution, drop-wise. The solution was then slightly heated for 30 minutes (~ 60 ˚C) and then cooled. After letting the solution cool, the 100 mL solution was poured into 2, 50 mL centrifuge tubes and centrifuged for 10 minutes at 3200 rpm, decanted, and supernatant discarded. The precipitate was then dissolved in 25 mL 4 M HCl by adding the 4 M HCl to each centrifuge tube, shaking the tube, then adding the solution to the original Teflon beaker. The solution was finally heated at 80˚C for 30 minutes to dissolve all residue. If residue still remained, more 4 M HCl was added, along with heating. The solution was then allowed to cool and loaded onto a prepacked, 2 mL DGA chromatography cartridge resin (DGA Resin, Normal; N,N,N’,N’-tetra-n- octyldiglycolamide). DGA is an extraction chromatographic resin that has a high adsorption capacity for rare earth elements and actinides at varying pHs. The 2 mL DGA cartridge needs to be connected to a vacuum due to very hydrophobic resins in the chromatography cartridge. Usually, the solution passed through the cartridge at 1-3 mL/min, but flow rates as high as 10 mL/min have had good separation results for Ac (Dulaiova et al., 2013). After the load solution passed through the DGA cartridge (25 mL 4 M HCl from above), the beaker was rinsed with 5-10 mL 4 M HCl and added to the DGA cartridge. Then 10 mL 3 M HNO3 was added to remove iron and any other left over alkaline metals, and the effluent discarded. Next, 25 mL of 2 M HCl was added to the DGA cartridge to remove Ac and collected for micro-precipitation. Th isotopes can also be removed from the DGA cartridge by subsequently adding 20 mL 0.1 M HCl. A flow diagram for Ac separation via DGA cartridge is shown in figure 2.3. 32 Figure 2.3: Flow diagram for separation of Ac from DGA resin. Before step #1, DGA cartridge is pre-conditioned with 25 mL 4 M HCl. Ten mL HNO3 is added in step #2. Finally, step #3 elutes Ac (20 mL 2M HCl). Alpha spectroscopy sources of Ac were prepared by CeF3 precipitation, following the procedure from Dulaiova et al. (2013) and Martin (1995). First, the Ac fraction, which was eluted from the DGA chromatography cartridge with 25 mL 2 M HCl, was collected and 100 µg of Ce was added to the solution. The solution was then stirred for 5 minutes, and 2 mL of concentrated HF was subsequently added while stirring. The solution was then allowed to sit for 30 minutes. After the Ac solution rested for 30 minutes, a 0.1 µm, 25 mm Eichrom Resolve TM Filter (Eichrom Inc.) was set up and prepared for filtration of the solution. The filter includes a 50 mL reservoir funnel. Before the CeF3 solution was passed through the filter, 3 mL of 80% Reagent alcohol (72% Ethanol with some methanol and isopropyl) was added to the filter to open the micro-filter pores, and about 5 mL of DIW was added to wash away the alcohol. The Ac solution (~25 mL) was poured into the funnel and passed through the filter once the vacuum 33 (~200 mm Hg) was applied. After filtration was complete (about 5 min), the filter was removed from the funnel apparatus and placed in the fume hood for 1 hour. It is important to have the filter completely dry so that HF fumes do not come off in the alpha detector. For alpha counting, the dry filter was mounted on a stainless-steel disk by gluing the filter and disk together with Elmer’s Glue (very light glue to have a smooth surface for the filter on the disk). Ac sources were measured by 3 Surface Barrier Silicon Detectors (ORTEC, 300mm active surface) at USC. Two measurements were made to calculate the 227 Ac activity: The first measurement was the determination of the net yield from counting 217 At (7.1 MeV), the granddaughter of 225 Ac (5.5-5.8 MeV). The net yield includes both the chemical yield and the detector efficiency. The two 225 Ac progeny, 221 Fr (6.1-6.3 MeV) and 217 At, have half-lives of 5 min and 32 ms (Fig. 2.2). Therefore, 217 At will be in secular equilibrium with 225 Ac within 20 minutes, which is relatively short compared to the alpha source preparation time described above (30 min). 225 Ac and 221 Fr are not used for the yield tracer due to possible interfering isotopes in that energy region (Geibert and Vöge, 2008). Furthermore, virtually all 217 At decays are observed in a single energy (7.1 MeV) and no other isotopes interfere in that energy. After 90 days, 225 Ac and its progeny have decayed, and the activity of 227 Ac is determined by counting its two alpha- emitting daughters, 227 Th (t 1/2 =18.7 d) and 223 Ra (t 1/2 =11.4 d). The activity of 227 Ac was determined by eq. 2.11. 𝐴 44K(L = * !!+,- . * !!/01 20 !!+,- 3 !!+,- . 20 !!/01 3 !!/01 M (2.11) 34 Where N 223Ra and N 227Th are the background corrected counts per minute (cpm) of 223 Ra and 227 Th, BR is the branching ratio, I is the ingrowth factor calculated from the Bateman equation, and e is the net yield determined from 217 At. Excellent separation and high yields (average of 50%) were observed for Ac in deep-sea sediments, suggesting this method works well for aluminosilicate clay material. 2.4.5 ICP-MS Procedure Uranium, thorium, and protactinium isotopes in sediments were measured by ICP-MS (Element 2, XR type) at Lamont Doherty Earth Observatory. Appropriate amounts (~100 µL) of a mixed 229 Th- 236 U and 233 Pa spike were added to 15-mL Savillex vials. Sediment samples were then crushed with mortar/pestle and weighed (100 mg) to the Savillex vials using aluminum foil to transfer sediments. The sediments and the spikes were then taken through a series of acid digestions that included HNO3, HClO4, HF, and HCl to completely digest all solids (Fleisher and Anderson, 1991). After solids were dissolved, samples were transferred to 15 ml centrifuge tubes and metal oxyhydroxides were co-precipitated with NH4OH. Samples were then centrifuged for 10 minutes at 2500 rpm. After centrifugation, samples were decanted, washed with DIW, and centrifuged again. Samples were then dissolved by adding 1 mL concentrated HNO3 to centrifuge tubes and transferred to clean 7 mL Savillex vials. Lastly, the purification of Th, U, and Pa from samples was then performed by column chromatography. Additional details can be found elsewhere (Anderson et al., 2012). 35 %Mn was measured at USC by following the dissolution protocol used for alpha spectroscopy described above, diluting samples in 2% HNO3 containing 1 ppb In tracer, and analyzing Mn with an Element 2 ICP-MS at mass 55 amu. 2.4.6 Gamma Spectroscopy Procedures Sediment samples were dried, gently crushed, and placed in polyethylene or polypropylene tubes (tubes were 5 cm in height and 0.5 cm in diameter). The weights and heights of the samples averaged between 0.5 - 2.0 g and 1.5 - 3.5 cm. Tubes were then placed in a high purity intrinsic germanium well-type detector (HPGe ORTEC, 120 cc active volume). Detector efficiencies were determined from counting the activities of known standards noted above (Table 2.2). Samples were counted for 2-8 days, and radioisotopes summarized in Table 2.2 were analyzed. 235 U (185.7 keV) interference at 186 keV was removed by using the 238 U estimate, the 235 U/ 238 U ratio, and appropriate branching ratios. These samples have high 226 Ra/ 235 U ratios, and use of the 186 keV peak appears to provide accurate estimates for 226 Ra activity. For 227 Ac, the 270 keV peak from its progeny was used. This peak requires correction for 228 Ac (BR=3.34%), typically 10% for C-Disk-IV sediments. For samples near the sediment- water interface, a fraction of the 222 Rn produced in situ will be lost by diffusion into the overlying water column. The scale length (1/e) of the Rn loss was assumed to be 3 cm and 50% of the Rn was assumed to be mobile, based on measurements of Rn profiles in equatorial sediments (Hammond et al., 1996). Excess 210 Pb ( 210 Pbex) was determined by subtracting the measured 226 Ra (corrected for 222 Rn loss in the upper 5 cm) from total 210 Pb activity and correcting for decay of 210 Pbex between collection and analysis. At depths below the bioturbated layer, 210 Pb and 226 Ra were in good agreement. 36 Table 2.2: Information for radioisotopes measured on HPGe well-type detector in this study. Branching Ratio (BR%) is the fractional percentage of decays emitting gammas at a particular energy. Detector efficiency was determined from branching ratio and standards with known activities of interfering isotopes, calculated for a sample height of 4.5cm, measured on HPGe detector MCB2 at USC. The comments column includes factors (in parentheses) used to correct observed counts for some isotopes having contributions from an interfering isotope at that energy. For these deep-sea red clays in a 0.5 cm ID plastic tube, the self-absorption amounted to 10% at 46 keV as determined from standard addition experiments. Isotope Energy (keV) Detector Eff. BR% Standard Comments 225 Ra 40.1 0.515 30.0 [1] Low energy: self-absorption 210 Pb 46.5 0.471 4.25 [2] Low energy: self-absorption 234 Th 63.3 0.436 4.8 [2] From 238 U, t1/2 = 24 d 235 U 185.7 0.393 57.2 [1] Calculate from 238 U via 234 Th 226 Ra 186.2 0.393 3.5 [2] 235 U Interference at 185.7 keV (0.50*63keV counts) 221 Fr 218.2 0.322 11.6 [1] From 225 Ac daughter , t1/2 = 5 min 224 Ra 241.0 0.289 3.97 [3] 214 Pb interference at 242 keV (0.40*295keV counts) 212 Pb 238.6 0.289 43.3 [3] Doublet with 224 Ra; Correct 214 Pb interference as above. 223 Ra 269.5 0.247 13.7 [4] 228 Ac interference at 270.2 keV (0.66*911keV counts) 219 Rn 271.2 0.247 10.8 [4] Doublet with 223 Ra; Correct 228 Ac interference as above. 214 Pb 295.2 0.218 18.5 [2] 228 Ac 338.3 0.164 11.3 [3] 223 Ra interference at 338.3 keV (BR = 2.79%, negligible) 214 Pb 351.9 0.176 35.8 [2] Minor Interference from 232 Th series (negligible) 214 Pb 609.3 0.052 44.8 [2] 228 Ac 911.2 0.048 26.6 [3] [1] 229 Th USC #89B; [2] SRM-1 diluted pitchblende; [3] SRM-2 diluted monzonite; [4] E&Z Actinium-227 CRM 2.4.7 Core Incubation Core incubations were carried out at each station using the methodology from Hammond et al (2004). Briefly, after retrieval of replicate cores from the multi-core device, the spring- loaded arms were released, and rubber stoppers were placed on the bottom of the cores. Next, a moveable piston was inserted into each core top, which could be advanced as water was withdrawn. Water height was adjusted to 12 cm of overlying water and advanced as samples were taken to measure nutrient fluxes. The incubation plug had a stirring device that 37 continuously stirred the overlying water for the entire incubation period. Results for nutrient fluxes in these cores have been published in Hou et al. (2019). After the incubation was completed, the remaining water from each core was siphoned out and all water from the two replicate cores at each site was combined for measurement of 227 Ac and 228 Ra (~1.0 L). This water was passed through loose MnO2 coated fibers (20 g) at least 3 times at < 1.00 L/min to insure complete absorption of radium, thorium and actinium (Reid et al., 1979). MnO2 fibers were then stored at room temp and shipped back to the USC lab. Upon arrival at USC, fibers were washed several times with DIW to remove sea-salt and then dried to 80-120% moisture to be analyzed for its 223 Ra and 224 Ra activity on RaDeCC (Moore and Arnold, 1996; Sun and Torgersen, 1998). Measurements began 7 to 20 days after incubation ended (longer delays for earlier stations). Subsequent measurements were made up to 2 years after collection, allowing 223 Ra, 224 Ra, 228 Ra, and 227 Ac activities at the end of the incubation period to be calculated using the Bateman equation fit to the data (Bateman, 1910). Eq. 2.12 presents expressions for 227 Acà 227 Thà 223 Raà 219 Rn and 228 Raà 228 Thà 224 Raà 220 Rn decay series: 𝐴 45678 = 227𝐴𝑐(1.004𝑒 9:.<5:=9>? −2.587𝑒 9@.@A<5? +1.582𝑒 9@.@B@B? )+227𝑇ℎ62.580(𝑒 9@.@A<5? − 𝑒 9@.@B@B? )7+223𝑅𝑎𝑒 9@.@B@B? (2.12a) 𝐴 44@78 = 228𝑅𝑎(1.50𝑒 9A.A=9C? −1.506𝑒 96.64>=9C? +0.00527𝑒 9@.5:6C? )+228𝑇ℎ61.005(𝑒 96.64>=9C? − 𝑒 9@.5:6C? )7+224𝑅𝑎𝑒 9@.5:6C? (2.12b) 38 where x is the time since the incubation ended and activities are values at that time. These equations were fit to the data using the program Kaleidagraph general curve fit, which calculated both activity for each isotope and its uncertainty. For the long-lived isotopes 227 Ac and 228 Ra the approach of Hammond et al. (2004) was used to calculate benthic flux J: 𝐽 =∆𝐴×∑ N + (2.13) where DA is the increase in activity (dpm m -3 ) of 227 Ac or 228 Ra during the incubation (in practice, initial concentrations were negligible), t is time of incubation period, and h is height of the overlaying water above the sediment-water-interface (SWI) in the core. Because aliquots of water were periodically removed to use for other analyses during the incubation, h/t must be summed for each incubation interval. For short-lived isotopes, the computation is more complex because these isotopes decay during the incubation period, and a parameter ai was computed (m - 1 ) to calculate flux. 2.5 Results and Discussion Estimates of benthic isotope fluxes have been made based on reaction-transport modeling of profiles of solid phase measurements, integrating profiles of daughter-parent deficiencies, and directly measuring isotope fluxes from incubated cores. Applying some of these approaches required measurements of distribution coefficients and evaluation of bioturbation and sediment accumulation rates. These approaches are detailed in sections below. 39 2.5.1 Distribution Coefficients (kd ) One important parameter needed to describe the behavior of radioisotopes in sediments is the partition coefficient (K), which is defined as the ratio of adsorbed solute to dissolved solute and is related to the distribution coefficient kd (eq. 2.4). While some estimates have been made for radium kd, (Cochran and Krishnaswami, 1980; Rama and Moore, 1996; Crotwell and Moore, 2003; Colbert and Hammond, 2008; Beck and Cochran, 2013), very little data is available for 227 Ac. The only study to provide K values for 227 Ac was Nozaki et al. (1990), and those values were estimated from Ra K values from an earlier study (Cochran and Krishnaswami, 1980). Tables 2.3 and 2.4 summarize Ra and Ac kd values determined experimentally in this study. The range of Ra kd values for C-Disk-IV sediments are between 1000-3000 mL g -1 . These values fall within the range of previous Ra kd estimates for deep-sea sediments. For Ac, kd values are much larger, but the patterns for different stations are similar. Station 1 shows constant Ra and Ac kd values in the upper 3 cm of sediments (1500 & 15,100 mL g -1 ), while other stations tend toward higher values at depth. Station 2 has the highest Ra and Ac kd values (3260 & 21,600 mL g -1 ), while results at other stations are about 2x smaller. This trend for decreasing kd values with increasing latitude may reflect a larger grain size that is visually apparent with latitude in the NEPB. Stations 1 and 2 are characterized by more clay-like material and its sediment composition has higher %Mn compared to the northern stations (Fig. 2.4). Ra kd values also seem to be influenced by %Mn in these sediments, but not as noticeably as Ac (Fig. 2.4). Previous authors have observed that Ra and other actinides are strongly influenced by adsorption onto Mn-oxides in marine sediments (Kadko, 1980; Kadko et al., 1987). %Mn differed at 40 different C-Disk-IV stations, but at each station it was nearly constant in the upper 7.5 cm, and Ac/Ra kd values at each station were also fairly uniform. Table 2.3: 228 Ra kd results using 232 Th spike solution. 228 Ra was measured in the dissolved and solid phase after rotating sediment slurry for at least one hour. CDISK depth pH mass:water 228 Ra # measured Station cm ratio mL g -1 n 1 0.5 7.6 0.003 1381±88 1 1 1.5 7.7 0.165 1456±77 2 1 2.5 7.9 0.004 1590±79 1 1 17 7.9 0.006 2117±138 1 2 0.5 7.9 0.005 2620±169 2 2 1.5 7.6 0.004 3804±195 2 2 2.5 7.9 0.004 3366±171 1 2 17 7.7 0.006 4438±216 1 3 0.5 7.8 0.005 1133±48 2 3 1.5 7.7 0.100 1464±180 2 3 2.5 7.7 0.120 1364±133 2 3 6.5 7.7 0.006 2357±121 1 4 1.5 7.8 0.004 1521±53 3 4 2.5 7.7 0.004 1540±63 1 5 1.5 7.6 0.006 1210±68 1 41 Table 2.4: 225 Ac kd results using 229 Th spike solution. 225 Ac was measured in the dissolved and solid phase after rotating sediment slurry for one hour. CDISK depth pH mass:water 225 Ac # measured Station cm ratio mL g -1 n 1 0.5 7.4 0.003 15532±1605 3 1 2.5 7.6 0.003 14722±1257 1 2 0.5 7.5 0.003 15931±1984 2 2 1.5 7.6 0.005 25786±2408 2 2 2.5 7.6 0.005 23000±2400 1 3 0.5 7.4 0.003 6846±382 2 3 1.5 7.5 0.003 7433±587 2 4 1.5 7.3 0.004 5354±369 2 5 1.5 7.5 0.003 3453±123 2 42 Figure 2.4: Ac and Ra kd vs. %Mn from aliquots taken from 0-7cm depth for all C-Disk-IV stations. Spikes of 225 Ac and 228 Ra were added to slurries of seawater and sediments, agitated for an hour, separated, and the activities of water and solids were measured by gamma spectroscopy. Fig. 2.5 shows Ac vs. Ra kd values for these experiments. It indicates that Ac kd values are 6.5 times higher than Ra kd values. While measurements are limited, this relationship could prove helpful in future modeling of 227 Ac in deep-sea sediments. Additional experiments (data 43 not shown) suggest that good pH control is required, as kd for Ac increases at higher pH. Previous studies have assumed that the Ac kd value is close to that of Ra (Nozaki et al., 1990). Figure 2.5: 225 Ac vs. 228 Ra k d values. The regression line is forced through zero and has a slope of 6.6. 2.5.2 Profiles of radioisotopes in solid phases 231 Pa in these sediments is largely derived from the overlying water column, where it is produced by decay of 235 U and scavenged by sinking particulates (Bacon and Anderson, 1982; Anderson et al., 1983). Based on 238 U (Table 2.5), less than 0.1 dpm g -1 of 231 Pa should be supported by its 235 U parent in sediments . Figure 2.6 displays 231 Pa concentrations in surface sediments vs. latitude in the Pacific Ocean near 150°W. There is a clear trend of increasing 231 Pa concentrations moving south from the Aleutians, likely reflecting less dilution of 231 Pa raining from the overlying water column by lower fluxes of detrital sediment (note accumulation rates in 44 Table 2.1). Both published (Lao et al., 1992) and this study’s values are shown. Data from two regions fall off this trend: one is located near the equatorial Pacific, where a high sedimentation rate should dilute the 231 Pa signal, and a second data point is near the Hawaii margin that is shallower and where the 231 Pa rain should be smaller. The profiles of 231 Pa and 230 Th (Fig. 2.7, 2.8) suggest bioturbation mixes Pa and Th fairly well throughout the upper 5-8 cm, due most likely to the long half-lives of each isotope relative to the mixing time scales. Figure 2.6: 231 Pa activity in surface sediments vs. latitude in the Pacific Ocean. Data taken from Lao et al. (1992) and this study (C-Disk-IV and GP15). Equatorial stations (2.5˚S - 2.5˚N) have lower 231 Pa activities than at surrounding sites because preservation of CaCO3 there dilutes the 231 Pa scavenged from the overlying water column. The site from the Hawaiian margin (20˚) has a lower 231 Pa value both due to higher sedimentation rate and shallower depth, which leads to less production of 231 Pa in the overlying water column. The general decrease in 231 Pa concentration northward of the equatorial cores reflects primarily a dilution effect of the northward increase in sediment mass accumulation rate. The low value at ~31 ˚N (GP15 Sta 14) is believed to reflect coring of an erosional surface. 45 Figure 2.7: Solid phase 231 Pa, 227 Ac, and 210 Pb ex profiles for all 5 C-Disk-IV stations. Eq. 2.6 was fit to the solid phase 227 Ac profile in the upper 3 cm of sediments (Top 5 profiles). Station 5 could not be fit due to equilibrium between 227 Ac and 231 Pa in the upper 3 cm. 210 Pb ex profiles were calculated by taking the difference of total 210 Pb and 226 Ra. Measured 226 Ra was corrected for in situ diffusive loss of 222 Rn from sediments based on deep-sea profiles in the equatorial Pacific loss (Hammond et al., 1996). An exponential function was fit to the 210 Pb ex profiles that gave values for initial 210 Pb ex at the SWI and attenuation (µ). µ is the inverse scale length of the exponential curve and it can be used to calculate the bioturbation rate (D b ) in units of cm 2 yr -1 . Bioturbation is assumed constant throughout the entire 210 Pb ex profile (Bottom 5 profiles). Figure 2.8: Solid phase 230 Th, 226 Ra, 232 Th, and 228 Ra profiles for all 5 C-Disk-IV stations. Eq. 2.6 was fit to the entire solid phase 226 Ra profile (Top 5 profiles). Station 5 could not be fit due to equilibrium between 230 Th and 226 Ra in the upper 8 cm. The 228 Ra profiles were fit with eq. 2.9, which is a solution to a one-layer model. All parameters such as 232 Th activity and Db are assumed constant throughout the entire 228 Ra profile (Bottom 5 Profiles). 46 210 Pbex profiles show varying bioturbation rates (Db) throughout the study area (Fig. 2.7). By fitting an exponential function to the 210 Pbex profiles in the top 4 cm (8 cm at sta. 4), incorporating biodiffusivity (Db) and radioactive decay, Db can be calculated at each site by fitting an exponential function to 210 Pbex (Tables 2.6 – 2.8; Db=l/b 2 , b=attenuation factor). Stations 1, 2 and 3 show low Db rates (<0.01 cm 2 yr -1 ) and stations 4 and 5 show high Db rates (>0.01 cm 2 yr -1 ), likely reflecting the latitude dependence of organic matter rain to the benthic community (Tromp et al., 1995). However, most 210 Pbex profiles have an erratic pattern: exponential decay in the upper 3 cm with a slight excess at some stations between 4-6. The excess in the 4-6 cm horizons is evidence of non-local transport, which is known to occur in deep-sea sediments (Teal et al., 2008; Smith et al., 1997; Boudreau, 1986). Station 4 has the highest 210 Pbex values at depth (~5.5cm). Station 2 has a relatively low inventory of 210 Pb in comparison to adjacent stations (Fig. 2.7), and its possible significance will be discussed later. 227 Ac is less strongly bound to solid phases than Th, Pa, and Pb, and can be transported within upper sediments by molecular diffusion and bioturbation as it is produced by decay of its 231 Pa parent (Nozaki et al., 1990). Consequently, profiles are expected to increase with depth toward secular equilibrium with the parent isotope, a pattern that is generally apparent. The scale length is comparable to the scale length for 210 Pbex at each station, which would be expected if bioturbation dominates transport, given the similar half-lives of these isotopes. There is little evidence of an 227 Ac deficiency at depths near 4-6 cm in association with the non-local mixing 210 Pbex. While the scale lengths for 227 Ac do not vary enormously among the stations, there are two types of 227 Ac profiles found in the North Pacific: One (Sta. 1 and 3) shows a large 227 Ac 47 deficiency relative to 231 Pa in the upper few cm of sediments, and the other (Sta. 2 and 4) shows a small deficiency in the upper few cm of sediments (Fig. 2.7). At all 5 stations, 227 Ac and 231 Pa activities in deeper samples show equilibrium (>5cm), which is expected and gives confidence in the 227 Ac analysis procedure used here (Table 2.5). Station 5 shows very little 227 Ac deficiency relative to its parent. The cause of differences among these profiles may be the result of differences in the fraction of 227 Ac that is mobile via molecular diffusion. At station 5, simple models do not account for many of the observations, and we suspect that non-steady state processes may have influenced distributions at this site. These issues are discussed further in later sections of the paper. The 226 Ra profiles in the North Pacific are as expected: Most profiles of 226 Ra increase in activity downcore to depths of 8 cm (Fig. 2.8), reflecting transport by diffusion and bioturbation, as well as ingrowth toward equilibrium with its 230 Th parent. The longer scale length for 226 Ra reflects its longer half-life (1600 y) and lower kd, as compared to 227 Ac and 210 Pb. Below this horizon, 226 Ra shows nearly constant activity until 17-20 cm, where it should approach equilibrium with its parent and decreases in activity as does its parent (Table 2.5). Stations 2 and 3 establish this behavior and is inferred at stations 1 and 4. The 226 Ra profile at station 5 was somewhat erratic, with equilibrium observed between 226 Ra and 230 Th throughout the core, similar to the 227 Ac/ 231 Pa profile. The absence of a deficiency at this station suggests erosion of overlying sediments may have occurred in the relatively recent past (decades to centuries). This theory is not supported by pore water profiles of silicic acid at this site (Hou et al., 2019), which do not indicate a disturbance; however, their response times to approach steady state may be only months to a few years. 48 Table 2.5: Activities of 238 U, 230 Th, 226 Ra, 210 Pb, 232 Th, 228 Ra, 231 Pa, and 227 Ac for C-Disk-IV sediments. 238 U, 230 Th, and 232 Th were measured by ICP-MS (Element 2). 226 Ra, 210 Pb, 228 Ra were calculated by gamma counting the 186, 46, and 240 keV peaks in a HPGe well-type detector. 227 Ac was determined by alpha spectroscopy. Depths ending in 0.5 are analyses of 1 cm intervals; those ending in 1.0 are for 2 cm intervals. Station depth 238 U 230 Th 226 Ra 210 Pb 232 Th 228 Ra 231 Pa 227 Ac (cm) (dpm g -1 ) (dpm g -1 ) (dpm g -1 ) (dpm g -1 ) (dpm g -1 ) (dpm g -1 ) (dpm g -1 ) (dpm g -1 ) 1 0.5 1.41±0.01 71.3±0.3 15.5±0.7 32.6±0.7 2.23+0.01 1.58±0.31 3.25±0.17 2.33±0.10 1.5 1.43±0.01 68.1±0.4 20.1±0.7 22.4±0.5 2.27±0.01 2.02±0.32 2.94±0.07 3.35±0.16 2.5 23.1±0.9 25.6±0.5 2.01±0.34 3.05±0.12 3.5 18.4±0.7 18.8±0.7 1.40±0.23 4.5 22.6±0.7 27.8±0.7 1.42±0.24 5.5 32.2±0.9 41.0±1.1 2.18±0.39 6.5 24.7±0.6 26.6±0.7 1.37±0.24 7.5 1.44±0.01 67.0±0.3 38.3±0.9 37.4±1.0 2.25±0.02 1.98±0.34 3.11±0.18 8.5 26.4±0.6 28.4±0.7 1.55±0.24 9.5 24.5±0.7 24.7±0.8 1.30±0.26 11.5 26.7±0.7 24.0±0.7 1.70±0.24 13.5 30.5±0.6 31.1±0.6 1.97±0.22 16.0 32.6±0.6 28.7±0.6 2.22±0.20 2 0.5 1.95±0.01 83.8±0.5 17.1±0.8 24.8±0.9 3.68±0.02 2.60±0.34 3.07±0.07 2.89±0.13 1.5 1.99±0.01 84.7±0.4 21.6±0.8 21.1±0.8 3.73±0.02 3.39±0.34 3.03±0.17 2.96±0.12 2.5 2.00±0.01 84.7±0.4 27.3±0.7 24.7±0.7 3.76±0.02 3.09±0.29 3.05±0.07 2.93±0.13 3.5 30.3±0.8 25.9±0.8 2.98±0.31 3.16±0.13 4.5 40.0±0.8 37.4±0.8 3.52±0.32 2.80±0.11 5.5 34.7±0.8 33.4±0.9 3.10±0.30 3.16±0.14 6.5 40.6±0.9 38.0±0.9 3.73±0.32 2.94±0.13 7.5 1.98±0.01 68.04±0.5 36.9±0.7 37.2±0.8 3.72±0.02 3.69±0.30 2.04±0.15 2.30±0.11 8.5 42.8±0.9 40.1±1.0 3.70±0.34 2.37±0.14 9.5 37.2±0.8 34.6±1.0 3.72±0.31 1.58±0.09 17.0 32.1±0.6 31.2±0.8 4.16±0.30 0.47±0.02 3 0.5 1.90±0.01 68.7±0.4 12.1±1.3 33.6±0.9 3.93±0.02 2.72±0.75 2.49±0.06 1.23±0.05 1.5 1.94±0.01 67.8±0.3 18.6±1.1 19.2±0.7 3.99±0.02 3.11±0.46 2.40±0.17 2.13±0.10 2.5 1.89±0.01 67.4±0.3 22.5±0.8 19.5±0.7 3.96±0.02 3.11±0.36 2.35±0.05 2.50±0.10 3.5 25.8±0.8 20.9±0.7 4.00±0.32 2.43±0.11 4.5 27.8±1.0 26.2±1.0 4.39±0.35 2.47±0.11 5.5 34.3±1.1 24.2±0.8 3.11±0.51 2.50±0.11 6.5 25.9±0.7 26.5±0.9 3.85±0.30 2.27±0.10 7.5 1.89±0.01 62.9±0.4 29.3±0.8 28.9±0.8 3.86±0.02 4.11±0.31 2.13±0.15 8.5 27.2±1.4 29.7±1.7 3.35±0.66 9.5 29.9±0.8 27.4±0.8 4.18±0.32 13.0 27.1±0.8 28.9±0.8 3.94±0.32 17.0 28.1±0.8 26.3±0.8 3.92±0.30 4 0.5 1.63±0.01 63.8±0.5 23.4±1.2 49.1±1.5 3.45±0.01 3.06±0.55 3.39±0.09 3.34±0.14 1.5 1.70±0.01 66.9±0.3 27.5±1.1 30.1±0.9 3.65±0.02 3.88±0.97 3.62±0.19 3.36±0.19 2.5 1.71±0.01 66.9±0.4 30.2±1.3 38.1±1.4 3.66±0.03 2.01±0.58 3.60±0.09 3.77±0.17 3.5 30.2±0.9 36.7±1.3 3.82±0.35 3.83±0.16 4.5 26.1±2.1 39.4±1.4 3.82±0.55 3.39±0.20 5.5 28.9±0.9 50.4±1.5 3.29±0.59 3.96±0.20 6.5 38.1±0.9 30.9±1.1 3.03±0.39 3.60±0.17 7.5 1.70±0.01 63.2±0.3 38.4±0.8 32.7±1.1 3.66±0.02 3.90±0.32 3.09±0.17 3.37±0.16 8.5 34.8±0.9 39.8±1.0 3.35±0.36 9.5 34.4±1.3 42.2±1.3 3.08±0.54 11.0 26.8±1.4 44.1±1.8 3.37±0.55 13.0 28.0±1.2 33.4±1.4 2.87±0.53 15.0 22.7±1.1 31.7±1.2 3.40±0.51 5 0.5 0.90±0.01 13.2±0.1 12.0±0.8 40.3±1.2 0.86±0.01 0.92±0.43 1.18±0.03 1.18±0.24 1.5 0.99±0.01 16.6±0.1 10.4±0.5 19.9±0.7 1.01±0.01 0.75±0.39 1.38±0.04 1.31±0.16 2.5 12.3±0.8 15.8±0.7 0.62±0.39 1.18±0.10 3.5 11.3±0.8 17.7±0.5 0.64±0.40 2.52±0.13 4.5 15.2±0.6 18.8±0.6 1.06±0.31 5.5 14.5±0.8 20.3±0.8 0.80±0.38 6.5 16.2±0.6 13.5±0.6 1.09±0.31 7.5 1.34±0.01 29.3±0.2 21.3±1.2 18.1±1.3 1.65±0.01 1.19±0.53 2.31±0.05 8.5 25.6±1.2 24.5±1.2 1.50±0.41 9.5 19.2±0.8 18.7±0.8 2.12±0.37 11.5 20.3±0.8 20.7±0.8 1.98±0.34 13.5 14.0±0.8 16.9±0.8 1.53±0.39 19.5 6.2±0.7 8.9±0.8 1.66±0.32 49 Despite relatively large uncertainties, 228 Ra profiles show a discernible increase with depth toward equilibrium with the parent 232 Th (Fig. 2.8). Measurement of 228 Ra was based on its 224 Ra progeny detected with gamma spectrometry, which is far less precise at these low concentrations than the ICP analysis of the 232 Th parent (Table 2.5). Most profiles have a small 228 Ra deficiency in the upper 2-3 cm of sediments. The much smaller scale length than 226 Ra is due to its much shorter half-life (t1/2=5.75 y), and while the uncertainties are large, 228 Ra appears to be in equilibrium with its parent at depth. The activities of 232 Th for these profiles are constant down to 7.5 cm, except station 5, which increases in activity by 7.5 cm. 2.5.3 Fitting reaction-transport models to solid phase profiles C-Disk-IV stations 1-4 227 Ac, 226 Ra, and 228 Ra profiles were fitted with the reaction- transport equations (eq. 2.6, 2.9) by using a least squares approach programmed into MATLAB. The model parameters for ‘best fit’ solid phase 227 Ac, 226 Ra, and 228 Ra profiles are summarized in Table 2.6, 2.7, 2.8. The mixing depth (L) averaged between 6-8 cm for stations 1-4 and was chosen based on profiles of the long-lived isotopes 230 Th and 226 Ra (Fig. 2.8). Based on averages for the mixed layer, 231 Pa and 230 Th activities were considered constant in this layer and declined exponentially thereafter, with a scale length µ equal to the ratio of sedimentation rate to radioactive decay as defined in equation 2.2b. A fit to the 226 Ra profile below the mixed layer constrained linear sedimentation rates in this zone. 232 Th activity was assumed constant throughout the entire 228 Ra profiles and thus a one-layer model was used for these profiles (eq. 2.9). 50 Table 2.6: Model parameters for best fit solid phase 227 Ac profiles (Eq. 6). D b , k d , f, D m , and 231 Pa were all measured independently or considered known variables. S was determined from 226 Ra model parameters. Mixing depth (L) was determined by change in 227 Ac or 231 Pa activity at depth. Porosity (f) was calculated by averaging the porosity in the upper 3cm of sediments. Only F was allowed to vary in the model until the best fit was achieved. Sta. 231 Pa Db kd L f Ds S F 227 Ac Flux # dpm g -1 cm 2 yr -1 mL g -1 dm cm cm 2 yr -1 cm kyr -1 dm dpm m -2 yr -1 1 3.09±0.12 0.007±0.001 15120±570 6.5 0.82 62 0.40 0.45±0.20 130±40 2 3.05±0.10 0.008±0.004 21570±6000 6.5 0.78 56 0.10 0.16±0.10 40±30 3 2.41±0.09 0.004±0.001 7140±420 6.5 0.81 60 0.20 0.95±0.10 290±70 4 3.53±0.12 0.450±0.160 5250±370 7.5 0.87 69 0.17 0.07±0.05 90±60 5 1.28±0.03 0.020±0.003 3450±120 N/A 0.79 57 N/A N/A N/A Table 2.7: Model parameters for best fit solid phase 226 Ra profiles (Eq. 6). D b , k d , f, D s , and 230 Th were all measured independently or considered known variables. Mixing depth (L) was determined by change in 226 Ra or 230 Th activity at depth. Porosity (f) was calculated by averaging the porosity in the mix layer (0 - L) Only F and S were allowed to vary in the model until the best fit was achieved. Sta. 230 Th Db kd L f Ds S F 226 Ra Flux # dpm g -1 cm 2 yr -1 mL g -1 cm dm cm 2 yr -1 cm kyr -1 dm dpm m -2 yr -1 1 64.9±0.3 0.007±0.001 1480±110 6.5 0.80 85 0.40 0.79±0.02 1690±490 2 80.3±0.4 0.008±0.004 3260±600 6.5 0.75 75 0.10 0.83±0.02 1540±780 3 64.0±0.3 0.004±0.001 1320±170 6.5 0.78 81 0.20 0.74±0.02 1580±360 4 60.3±0.4 0.450±0.160 1530±60 7.5 0.82 89 0.17 0.57±0.03 720±260 5 14.1±0.1 0.020±0.003 1210±70 N/A 0.76 77 N/A N/A N/A Table 2.8: Model parameters for solid phase 228 Ra profiles (Eq. 9). D b , k d , f, D s , and 232 Th were all measured independently or considered known variables. Porosity (f) was calculated by averaging the porosity in the upper 3cm of sediments. Only F was allowed to vary in the model until the best fit was achieved. Sta. 232 Th Db kd f Ds S F 228 Ra Flux # dpm g -1 cm 2 yr -1 mL g -1 dm cm 2 yr -1 cm kyr -1 dm dpm m -2 yr -1 1 2.25±0.02 0.008±0.001 1480±110 0.82 89 0.40 0.46±0.32 520±360 2 3.73±0.02 0.008±0.004 3260±600 0.78 81 0.10 0.66±0.25 900±570 3 3.92±0.02 0.004±0.001 1320±170 0.81 87 0.20 0.57±0.22 1440±660 4 3.58±0.02 0.450±0.160 1530±60 0.87 100 0.17 0.22±0.13 700±390 5 0.93±0.02 0.020±0.003 1210±70 0.79 N/A N/A N/A N/A Sedimentation rate (S) and the fraction released by parent decay (F) were allowed to vary in the reaction-transport model for 226 Ra until the ‘best fit’ was achieved. The model for 227 Ac and 228 Ra profiles used S from 226 Ra profiles and only varied F, the amount of 227 Ac, 226 Ra, or 51 228 Ra that is released into pore water. Krishnaswami and Cochran (1980) predicted this value should be around 50% since half of all alpha decays from an adsorbed parent should either release daughters into the surrounding pore waters or inject it into the solid on which it is sorbed. The fraction of Ac and Ra ejected into pore waters can either remain in solution or be adsorbed onto sediment surfaces, but it should remain mobile. The 226 Ra profiles had F values ranging between 50-80%, while 228 Ra had F values between 25-70% (Tables 2.7 – 2.8). The 228 Ra F values are surprisingly high, assuming most of its 232 Th parent is probably contained in crystalline lattices. The 227 Ac F values had a much broader range. Stations 2 & 4 had F values below 20% while the station 3 F value was above 90%. Only station 1 had an F value around 50%. Both 227 Ac and 226 Ra should be primarily produced from their adsorbed parents, and the much lower F values for 227 Ac at stations 2 and 4 are unexpected. 2.5.4 230 Th and Accumulation Rates S in the upper layer can also be estimated from a mass balance for 230 Th in the mixed layer using logic presented by Bacon (1984) and reviewed by Francois et al. (2004) and Costa et al. (2020). At steady state, the rain of 230 Th to sediments divided by its production from 234 U in the overlying water column can be used to define a sediment focusing factor 𝑔: 𝑔 = O( D P (2.14) Where: 𝑀 = mass accumulation rate (g cm -2 y -1 ) 𝐴 Q = 230 Th in sediment raining to the sea floor and being preserved (dpm g -1 ) 𝐼 = production from 234 U in overlying water (dpm cm -2 y -1 ) 𝑔 = (rain of 230 Th to sediments)/(integrated water column 230 Th production) 52 The value of 𝑔 is near 1.0 in many parts of the ocean, reflecting the relatively short residence time of 230 Th in the water column. However, preferential boundary scavenging of 230 Th in high particle flux areas, sediment winnowing, or lateral inflow of water low in 230 Th may reduce 𝑔 below 1.0. Alternatively, lateral inflow of 230 Th-rich water or sediment focusing at a coring site may increase 𝑔 above 1.0. The rain of 230 Th, averaged over its lifetime, can also be estimated by integrating its downcore activity, summing its decay rate in the bioturbated mixed layer with the integration of its exponential profile based on fits below this layer in Fig. 2.8. Results for this approach using cores in this study (Table 2.9) indicate that 𝑔 is often less than 1. Values indicate that either notable sediment winnowing or lateral thorium transport in the water column has been occurring at stations 2, 4, and 5. However, some localized thorium removal is occurring at station 1. It is interesting to note that these stations with low values of 𝑔 are also those with much smaller values for 227 Ac F than expected. An additional factor of importance for these sediments is the relatively thick bioturbated layer that exists, with 230 Th homogenized to depths of 6-7 cm. Rather than using 14 C to correct for the effective age of this layer (Francois et al., 2004), the 230 Th balance can be modified to include its presence and determine the residence time of 230 Th in this layer. The calculation assumes the excess 230 Th leaving the base of the mixed layer equals its input at the top (using eq. 2.14), minus decay in the mixed layer: 𝑀𝐴 R =𝑔𝐼−𝜆𝜌𝐿(1−𝜙 S8 )𝐴 R (2.15a) 𝑀 = TP ( E −𝜆𝜌𝐿(1−𝜙 S8 ) (2.15b) 53 where l = decay constant of 230 Th (yr -1 ) A m = activity of excess 230 Th in mixed layer (dpm g -1 ) 𝜙 S8 = Upper layer average porosity The term on the far right of eq. 2.15a corrects for radioactive decay of 230 Th during passage of sediment through the bioturbated layer, and the equation can be re-arranged to find the mass accumulation rate in this layer using eq. 2.15b. Below this layer, the LL (Lower Layer) MAR was derived by multiplying S determined in the LL by r(1-fLL), where = Lower layer porosity. Table 2.9 summarizes the results for mass accumulation rates (MAR) in the upper and lower layers using the above equations along with 230 Th inventories, sedimentation rates (S), and mixed layer age. Most stations have similar MAR in the upper and lower layers, suggesting that the thorium dynamics have not changed significantly over the time defined by sampling (about 50 ky). Table 2.9: Mix layer depth (L), 230 Th activity, mass accumulation rates (MAR), sedimentation rate (S), inventory of 230 Th (Inv.), and ages are shown for 5 C-Disk-IV profiles. Upper and lower layers, denoted as UL and LL are referenced to layers below or above the mixing depth. Lastly, total inventory of 230 Th in the sediments (Tot. Inv.) and water column (WC) is shown. STA. L UL UL UL UL UL UL LL LL LL Tot. WC WC # 230 Th MAR S Inv. Flux Age MAR S Inv. Inv. Flux Inv. cm dpm g -1 g cm -2 kyr -1 cm kyr -1 dpm cm -2 dpm cm -2 kyr -1 kyr g cm -2 kyr -1 cm kyr -1 dpm cm -2 dpm cm -2 dpm cm -2 kyr -1 dpm cm -2 1 6.5 65 0.166 0.33 212 12.4 19.9 0.248 0.40 1559 1771 10.8 1356 2 6.5 80 0.152 0.24 322 14.8 25.7 0.070 0.10 588 910 12.2 1617 3 6.5 64 0.201 0.37 231 14.6 17.8 0.135 0.20 905 1136 13.9 1594 4 7.5 60 0.192 0.43 161 13.1 12.8 0.095 0.17 496 657 11.5 1425 5 6.5 15 0.917 1.53 75 12.4 3.7 N/A N/A 543 618 13.8 1355 2.5.5 Observed Deficiency Method The observed 226 Ra, 228 Ra, and 227 Ac deficiency was calculated by the following equation: 54 𝐹𝑙𝑢𝑥 = ∑𝜌(1−𝜙 U )∆𝑧 U (𝑃−𝐷) U 𝜆 (2.16) where r is density (g cm -3 ), ∆z is each depth interval (cm), l is decay constant (yr -1 ), and P and D are the activities of parent-daughter pair in dpm g -1 . A density of 2.5 g cm -3 was assumed for the solids. Integrated depth intervals were 1 or 2 cm, and each profile was integrated to a depth where equilibrium was reached between parent-daughter pair. 227 Ac and 228 Ra total integration averaged 3 cm, while 226 Ra averaged 20 cm. Table 2.10 shows the results for the flux to the water column calculated from the observed deficiency for each station. The 226 Ra, 228 Ra, and 227 Ac fluxes calculated by this method should, in theory, be the most accurate approach, since all variables in the observed deficiency calculation are directly measured (i.e., 226 Ra/ 230 Th, 232 Th/ 228 Ra, 227 Ac/ 231 Pa, and f). 230 Th, 232 Th, and 231 Pa activities were assumed to be constant in the mixed layer, and below this layer, an exponential function was used that depends on l, S, and parent activity in the mixed zone (PLexp(-l/S)). The mixing depths for most stations were about 6.5 cm, depending on where parent activity started to decrease (Table 2.6, 2.7, 2.8). Uncertainties were calculated from error propagation. Results for 228 Ra have relatively higher uncertainties than the other two isotopes due to the large analytical uncertainties. Table 2.10: Observed deficiencies in sediments for 226 Ra, 228 Ra, and 227 Ac along C-Disk-IV transect. Deficiencies were calculated by taking the difference between parent and daughter isotopes and multiplying by the interval sampled (∆z), density of the solids (𝜌(1−𝜙)), and the decay constant (l) (eq. 2.13). A solid density of 2.5 g cm -3 was assumed for the solids. Intervals not measured for either 227 Ac, 226 Ra, 228 Ra, 230 Th, 231 Pa, or 232 Th were interpolated. STATION 226 Ra 228 Ra 227 Ac # dpm m -2 yr -1 dpm m -2 yr -1 dpm m -2 yr -1 1 1310±100 480±310 100±40 2 1330±180 1140±380 70±50 3 1410±90 1420±490 180±40 4 670±110 400±520 40±40 5 190±100 460±440 50±50 55 2.5.6 Isotope Fluxes Based on Analytical and Numerical Fits As described in the theory section, the analytical equation fits were used with eq.10 to calculate benthic fluxes for each isotope (Table 6-8). The analytical solution for eq. 2.1 to find the isotope profiles described by eq. 2.3a and 2.3b required the assumption that porosity was constant in each layer. However, changes in porosity with depth do occur and affect the diffusivity of solutes (Ds) and the ratio of sorbed to dissolved solute (K). To evaluate this effect on 227 Ac behavior, the reaction-transport equation (eq. 2.1) was applied numerically in a MATLAB script. The most rapid porosity change occurs in the uppermost portion of sediments (~3cm), where the 227 Ac deficiency is observed. The model was applied to the upper 10 cm of sediment, which were divided into 100 boxes. Boundary conditions were imposed by fixing the uppermost box as the bottom of the water column, and 227 Ac activity in the bottom box was assumed equal to its parent 231 Pa activity in the bottom box. The model ran for 100 years (5 half- lives of 227 Ac) and produced values for 227 Ac for every interval. The 227 Ac flux was calculated by taking the upper most slope (0.0 - 0.1 cm depth) and multiplying by (Ds+KDb) and f for this interval (eq.2.13). Table 2.11 summarizes the results for the numerical model. After the model ran for 100 years, the 0.1 cm intervals were averaged for every 1 cm and then compared against measured 227 Ac values. F values were then varied until the sum of squares for differences in calculated and observed values was minimized. F values and 227 Ac fluxes calculated for the numerical model were almost identical to the reaction-transport model, suggesting that changes in porosity in the upper most centimeters of sediments do not have a significant effect on the reaction-transport model fits where porosity is assumed constant. While the increase in porosity should reduce the 56 production of 227 Ac, there is a corresponding increase in Ds that largely cancels this. Uncertainties in fitted parameters were derived from a Monte Carlo approach that used the analytical uncertainties in each measurement of concentration vs. depth and in the input variables (Db, kd) to compute a distribution of values for F and flux that was assessed to find their uncertainties. In comparison to the results for the analytical model results for 227 Ac (Table 2.6), F typically changed by 0.01 to 0.03 and flux decreased by an average of 20%, less than the uncertainty in the estimates. Table 2.11: Numerical model results for 227 Ac flux and F. Db and kd, are also shown for reference. Only porosity was allowed to vary with depth, which affected Ds and K downcore, since Ds=Dmf 2 and K=kdr(1-f)/f. Dm was set to 91 cm 2 yr -1 (Li and Gregory, 1974; Nozaki et al., 1990). Similar to the analytical approach, F was the only parameter allowed to vary until the best fit was achieved. Sta. 231 Pa Db kd F 227 Ac Flux # dpm g -1 cm 2 yr -1 mL g -1 dm dpm m -2 yr -1 1 3.09±0.12 0.007±0.001 15120±570 0.49±0.37 90±40 2 3.05±0.10 0.008±0.004 21570±6000 0.11±0.04 30±10 3 2.41±0.09 0.004±0.001 7140±420 0.94±0.08 250±60 4 3.53±0.12 0.450±0.160 5250±370 0.05±0.05 60±60 5 1.28±0.03 0.020±0.003 3450±120 N/A N/A 2.5.7 Core Incubation Method The 227 Ac and 228 Ra concentrations in overlying water collected during core incubations were determined by following the activity of 223 Ra and 224 Ra in these samples over a 3-year period. As mentioned earlier, 223 Ra is the granddaughter of 227 Ac, with a half-life of 11 days and is much more soluble than its ancestors 227 Th and 227 Ac, producing a higher flux during the incubation. Consequently, 223 Ra on these MnO2 fibers was initially in excess over its ancestors 57 but grew into equilibrium with its grandparent activity by 120 days. 224 Ra and 228 Ra show a similar pattern, but over a much longer time scale. Table 2.12 summarizes the results for stations 1-4. Results for station 5 indicated unreasonably high fluxes. In part, this can be attributed to a short incubation time that would result in large uncertainty, but it might reflect a problem with filtration or perhaps a non-steady state distribution of its 227 Ac parent profile in the sediments, as discussed earlier. Figure 2.9 shows Bateman equation fits to the 223 Ra and 224 Ra activity for a typical station. Table 2.12: Fluxes derived from Bateman equation fits to the core incubation samples measured over 1200 days. These fits provided the 227 Ac, 223 Ra, and 228 Ra activity in the core-top water at the conclusion of the incubation. Time/height is the summation of time incubated divided by height of the remaining water in the core for all segments of the incubation. Volume indicates the amount of seawater that was passed through the Mn-fibers. ai is the effective inverse height needed to calculate flux for a short-lived radioisotope. 227 Ac, 223 Ra, and 228 Ra fluxes were calculated by eq. 2.15. Sta. time/height Vol. 227 Ac 223 Ra 228 Ra 227 Ac Flux 228 Ra Flux ai 223 Ra Flux days m -1 L dpm dpm dpm dpm m -2 yr -1 dpm m -2 yr -1 m -1 dpm m -2 yr -1 1 142 1.35 0.07±0.01 0.83±0.12 0.22±0.06 120±30 410±200 5.60 2430±850 2 171 1.29 0.09±0.03 0.90±0.22 0.24±0.07 140±40 390±130 6.49 2400±620 3 128 1.16 0.12±0.02 0.82±0.17 0.37±0.05 280±60 880±220 5.49 2870±560 4 95 1.00 0.03±0.01 0.41±0.06 0.15±0.05 100±50 650±220 4.35 2080±920 5 51 1.00 0.12±0.02 0.48±0.06 0.21±0.05 850±110 850±490 2.70 3980±1200 58 Figure 2.9: Bateman equation fits to determine 223 Ra, 224 Ra, 228 Ra, and 227 Ac activities in incubation samples at the conclusion of the incubation. Samples were stored and counted for up to 3 years while 223 Ra and 224 Ra grew into equilibrium with their longer-lived ancestors. Only one station is shown for reference (STA. 3). 223 Ra grows into equilibrium with its grandparent activity, 227 Ac, by 125 days, while ingrowth of 224Ra toward equilibrium with its 228Ra parent is still continuing. Samples from most stations had very little excess 224 Ra activity due to long wait times between collection and counting ( 224 Ra: t1/2=3.4 days). 2.5.8 Summary of Ac and Ra Inputs throughout the NEPB 59 227 Ac Figure 2.10 summarizes all 227 Ac fluxes determined by the methods discussed in this paper; in theory, the observed deficiency and core incubation approaches should be the most accurate approach for determining 227 Ac fluxes. Given the uncertainties, these approaches agree at most C-Disk-IV stations, as do the fluxes based on the numerical and reaction-transport (analytical) models. Although the incubation-based fluxes are often slightly less precise, they typically agree within about 35% of the mean for the first two approaches. Furthermore, the analytical model results are comparable to the numerical approach. This suggests that a simple model that accounts for major processes governing a radionuclide in marine sediments can describe solid phase 227 Ac profiles fairly well. To test the accuracy of these 227 Ac benthic inputs, they can be compared to water column inventories of excess 227 Ac, computed along the GP15 transect (Table 2.13). The 227 Ac sediment fluxes are an average of the results from the core incubation and observed deficiency methods. Water column fluxes were averaged from GP15 station observations. The computation and interpretation of the water column data is the subject of a companion paper (Kemnitz et al., in prep). It is clear that the results are comparable within the uncertainties. Sediment fluxes for 227 Ac show a localized maximum at station 3 and a pronounced drop toward the north (sta. 5). Changes in the water column inventory are less apparent, but clearly are lower at the northern end of the transect. Lateral mixing in the water column is likely to mute spatially varying inputs, and these patterns will be discussed in the companion paper. 60 Figure 2.10: Top: 227 Ac flux was determined by four methods: subtracting 227 Ac and 231 Pa from each sediment horizon in the top 3 cm of sediments (observed deficiency), curve fitting solid phase 227 Ac vs. depth using eq.2.6 (reaction-transport model), creating a model that accounts for varying porosity downcore (numerical model), and direct measurements via core incubation (core incubation). Observed deficiency and core incubation should give the most accurate results. Middle: 228 Ra flux was determined by observed deficiency, core incubation, and curve fitting solid phase 228 Ra vs. depth (eq.2.9). Note that core incubation results are most precise for this isotope. Bottom 226 Ra flux was determined by two methods: observed deficiency and curve fitting solid phase 226 Ra vs. depth (eq.2.6). 61 Table 2.13: 227 Ac, 226 Ra, and 228 Ra fluxes for water column (WC) and sediments along C-Disk- IV transect. The water column flux for 227 Ac was estimated from a two-layer model along GP15 transect. 226 Ra and 228 Ra water column flux was estimated from numerically integrated 226 Ra and 228 Ra in the water column. The 227 Ac sediment flux is an average of the observed deficiency and core incubation methods, while the 228 Ra sediment flux is core incubation method only. The 226 Ra sediment flux is an average of observed deficiency and reaction-transport model flux results. STATION WC SEDIMENT WC SEDIMENT WC SEDIMENT # 227 Ac FLUX 227 Ac FLUX 226 Ra FLUX 226 Ra FLUX 228 Ra FLUX 228 Ra FLUX dpm m -2 yr -1 dpm m -2 yr -1 dpm m -2 yr -1 dpm m -2 yr -1 dpm m -2 yr -1 dpm m -2 yr -1 1 80 ± 20 115 ± 25 640±10 1500±250 180±100 410 ± 200 2 95 ± 15 125 ± 30 660±10 1440±400 440±130 400 ± 100 3 90 ± 10 240 ± 40 630±10 1500±190 1460±130 900 ± 200 4 80 ± 25 90 ± 40 600±10 700±140 660±100 550 ± 200 5 40 ± 20 30 ± 40 600±10 200±100 380±30 N/A Db, F and 231 Pa should be the major controls on 227 Ac fluxes in the NEPB (Nozaki et al., 1990). There is a positive correlation between 227 Ac fluxes and F, while there is little correlation between 227 Ac fluxes and kd (Fig. 2.11). The large variation in F values in the NEPB masks the correlation of 227 Ac fluxes with the other variables. Despite the low value for F at Station 4, its high flux reflects its large bioturbation rate, which is 50 times greater than any other station in the NEPB (Guinasso and Schink, 1975; Kadko and Heath, 1984; Nozaki et al., 1990). Perhaps non-local burial of young surface material that should be deficient in 227 Ac has played some role in exhuming deeper material that is less deficient in 227 Ac. At station 2, as noted previously, the inventory of 210 Pb is relatively low near the surface. This might reflect a recent erosion event of the top ~1 cm at that site, perhaps a decade ago, so that pore water profiles (Hou et al., 2019) have re-adjusted, but 210 Pb and 227 Ac have not returned to steady state. 62 Fig. 2.11: 227 Ac Flux vs. F, k d , and 231 Pa for C-Disk-IV sediments. The results for F and k d are taken from the reaction-transport model and 231 Pa is 231 Pa activity in the mixed layer. 63 226 Ra Similar to 227 Ac fluxes in this region, 226 Ra fluxes are lower above 40˚N and then increase below this latitude (Fig. 2.12). The fluxes for 226 Ra range from 200 - 1700 dpm/m 2 -yr throughout 5 C-Disk-IV stations (Table 2.13). The highest 226 Ra fluxes were observed between 25˚N-40˚N (Sta.1-3). The 226 Ra fluxes in this region are consistent, averaging around 1350 dpm m -2 yr -1 (observed deficiency). North of 40˚N, 226 Ra fluxes decrease toward the Alaskan margins (650 – 200 dpm m -2 yr -1 ). Topography steps up a few hundred meters of elevation around 40˚N, which is the boundary of the Mendocino Fracture Zone (MFZ) (Hautala, 2018). North of this boundary, 226 Ra and 227 Ac fluxes are much lower, likely due to the shallowing of bottom depths and increase in sedimentation, which dilutes both the 231 Pa and 230 Th concentrations in sediments. Fig. 2.12: 226 Ra, 228 Ra, and 227 Ac Flux vs. Station # for C-Disk-IV sediments. The 228 Ra and 227 Ac fluxes were multiplied x2 and x10 as to scale the fluxes properly, since 226 Ra flux was so much larger. 64 These estimates are in good agreement with the 226 Ra sediment flux data from Cochran and Krisnaswami (1980), which reported 226 Ra fluxes between 800-2000 dpm m -2 yr -1 for the North Equatorial Pacific. They noted that the local 226 Ra in the overlying water column required a benthic input of only 600 dpm m -2 yr -1 , indicating that North Equatorial Pacific was a net source of 226 Ra to the world’s oceans. For this study, the 226 Ra sediment fluxes required to explain the average standing crop of 226 Ra in the water column above the C-Disk-IV sites (Table 2.13) are a factor of ~ 2 lower than the measured sediment fluxes. Thus, the NEPB is comparable to the North Equatorial Pacific region in that 226 Ra has a large sediment flux that results in a net source of 226 Ra to other parts of the world’s oceans. 228 Ra Lastly, 228 Ra sediment fluxes were measured by core incubation and curve fitting solid phase 228 Ra profiles using eq. 2.9. A discrepancy is apparent between the two methods, with core incubation being half of the model results. However, the model calculation has large uncertainties and is therefore not inconsistent with the more precise core incubations. Table 2.14 also shows the sediment and the water column flux for 228 Ra along C-Disk-IV transect. Core incubation results were used for the sediment flux, and the water column flux was calculated by averaging 228 Ra activity in the bottom 500 m. The highest 228 Ra flux is located around 37˚N and decreases south and north of this latitude (Fig. 2.10). Comparing the 228 Ra flux to both 226 Ra and 227 Ac in the NEPB, the highest fluxes occur south of the MFZ (~37˚N) and then decrease moving toward the Alaskan margin. In the center of the NEPB, above oligotrophic waters, fluxes remain high and constant towards the Hawaiian margin. Comparing 228 Ra water column fluxes to the 65 sediment flux, most C-Disk-IV stations agree with each other, except station 3 (37˚N). At station 3, the water column estimate is 60% higher than the sediment-based approach for the 228 Ra flux. The activity of 228 Ra near the bottom 200 m at GP15 stations 12 and 14 shows an activity of 25- 30 dpm m -3 , almost 2-3x larger than stations north or south of this area. This large activity near the bottom significantly increases the 228 Ra water column flux around 37˚N. Due to 228 Ra short mean half-life relative to 227 Ac (8 vs. 31 y), the source of this large bottom activity of 228 Ra is integrating over a shorter distance compared to 227 Ac. The 227 Ac in this region is being transported father away and thus is being sourced from an area of lower 227 Ac flux. Both radionuclides are likely experiencing significant horizontal transport. 2.6 Conclusions 227 Ac, 226 Ra, and 228 Ra profiles in sediments were measured and modeled at 5 stations along the C-Disk-IV transect in the Northeast Pacific. The procedure of Dulaiova et al. (2013) was applied to these deep-sea sediments to measure 227 Ac and showed accurate and precise results. Distribution coefficients (kd) were measured for Ac and Ra in C-Disk-IV surface sediments. These Ra kd values were comparable to previously published work, with values between 1000-3000 mL g -1 , which agree with Ra kd values for similar sediments (Beck and Cochran, 2013). The Ac kd values were between 3500-22000 mL g -1 for the same sediments. There was a strong positive correlation between Ra and Ac kd values, with Ac being almost 6.6 66 times higher than Ra kd values, and both co-varying with the MnO2 in solid phases. This is the first study to directly measure Ac kd values in marine sediments. Two independent approaches were carried out to quantify the source function of 227 Ac in the Northeast Pacific: Core incubation and solid phase measurements of 227 Ac by alpha spectroscopy. The latter approach used analytical and numerical reaction-transport models to describe the behavior of 227 Ac in sediments. Both models showed comparable results for 227 Ac fluxes when compared to core incubation results. 227 Ac profiles showed a large range in the fraction of mobile 227 Ac released from 231 Pa in the upper few cm of sediments, versus the expected value of about 50%. The cause of this wide range is undetermined. At one site (station 2) a relatively low value of excess 210 Pb suggests an erosion event could have occurred in recent decades. At another site that underlies higher productivity waters (station 4), we speculate that exhumation of deeper sediment by bioturbation might play a role, but there is little other apparent evidence this is occurring. It is notable that about half of the expected 230 Th is missing from stations 2, 4 and 5, suggesting winnowing is taking place at these sites. Interestingly, these are also sites where the near surface 227 Ac deficiency is much smaller than expected, possibly indicating exhumation of older sediments. Most 226 Ra profiles for C-Disk-IV sediments have a large deficiency relative to 230 Th. Similar to the 227 Ac profile at station 5, 226 Ra and 230 Th also show equilibrium throughout the entire profile at this station. C-Disk-IV 226 Ra F values at other sites range between 57-83%, which shows agreement from previous results. Benthic input from the NEPB is a net source of 67 226 Ra to the global ocean, averaging around 1400 dpm m -2 -yr -1 vs. the 600 dpm m -2 -yr -1 needed to supply the 226 Ra in the overlying water column. 228 Ra profiles along C-Disk-IV transect also had a modest deficiency relative to its parent, 232 Th. However, 228 Ra profiles were difficult to measure due to high counting uncertainties. F values obtained from 228 Ra profiles were similar to 226 Ra values. Core incubation results provided the most precise estimates of benthic input for this isotope. The smallest 227 Ac fluxes in the Northeast Pacific are located north of 40˚N and decrease towards the Alaskan margins. The largest 227 Ac fluxes are near the center of the Northeast Pacific (~37˚N) but decrease by 50% south of this latitude. 227 Ac fluxes are influenced by the 231 Pa concentration in sediments, sediment composition, and mass accumulation rate. The largest 226 Ra and 228 Ra fluxes are also located near the center of the Northeast Pacific, but 226 Ra fluxes remain high from Hawaii to 37˚N. 228 Ra fluxes decrease south of 37˚N and both 226 Ra and 228 Ra fluxes decrease north of this latitude until the continental margins. Finally, we note that a surprisingly large number of the 5 sites we have sampled do not perfectly match the ideal radioisotope distributions we anticipated. Inventories of 230 Th are about half the expected values at stations 2, 4 and 5. Station 5 isotope profiles appear to be non-steady state, with evidence of erosion in recent times. 227 Ac deficiencies in the top 1 cm are often smaller than expected, suggesting that older material has been exhumed. This suggests that the NEPB sediments may experience far more disturbance than expected. 68 2.7 References Anderson, Robert F., and Alan P. 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Geibert, W., M.M. Rutgers van der Loeff, C. Hanfland, and H.-J. Dauelsberg. “Actinium-227 as a Deep-Sea Tracer: Sources, Distribution and Applications.” Earth and Planetary Science Letters 198, no. 1-2 (2002): 147–65. https://doi.org/10.1016/s0012-821x(02)00512-5. Geibert, Walter, and Ingrid Vöge. “Progress in the Determination of 227 Ac in Sea Water.” Marine Chemistry 109, no. 3-4 (2008): 238–49. https://doi.org/10.1016/j.marchem.2007.07.012. 70 Geibert, Walter, Matt Charette, Guebuem Kim, Willard S. Moore, Joseph Street, Megan Young, and Adina Paytan. “The Release of Dissolved Actinium to the Ocean: A Global Comparison of Different End-Members.” Marine Chemistry 109, no. 3-4 (2008): 409–20. https://doi.org/10.1016/j.marchem.2007.07.005. “GEOTRACES – an International Study of the Global Marine Biogeochemical Cycles of Trace Elements and Their Isotopes.” Geochemistry 67, no. 2 (2007): 85–131. https://doi.org/10.1016/j.chemer.2007.02.001. Guinasso, N. L. and D. R. Schink. “Quantitative Estimates of Biological Mixing Rates in Abyssal Sediments.” Journal of Geophysical Research 80, no. 21 (1975): 3032–43. https://doi.org/10.1029/jc080i021p03032. Hammond, D.E., J. McManus, W.M. Berelson, T.E. Kilgore, and R.H. Pope. “Early Diagenesis of Organic Material in Equatorial Pacific Sediments: Stoichiometry and Kinetics.” Deep Sea Research Part II: Topical Studies in Oceanography 43, no. 4-6 (1996): 1365–1412. https://doi.org/10.1016/0967-0645(96)00027-6. Hammond, Douglas E., Kathleen M. Cummins, James McManus, William M. Berelson, Gerry Smith, and Federico Spagnoli. “Methods for Measuring Benthic Nutrient Flux on the California Margin: Comparing Shipboard Core Incubations to in Situ Lander Results.” Limnology and Oceanography: Methods 2, no. 6 (2004): 146–59. https://doi.org/10.4319/lom.2004.2.146. 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Adkins, and Abby Lunstrum. “Spatial Patterns of Benthic Silica Flux in the North Pacific Reflect Upper Ocean Production.” Deep Sea Research Part I: Oceanographic Research Papers 148 (2019): 25–33. https://doi.org/10.1016/j.dsr.2019.04.013. Kadko, David, and G. Ross Heath. “Models of Depth-Dependent Bioturbation at MANOP Site H in the Eastern Equatorial Pacific.” Journal of Geophysical Research 89, no. C4 (1984): 6567. https://doi.org/10.1029/jc089ic04p06567. 71 Kadko, David, J Kirk Cochran, and Mitchell Lyle. “The Effect of Bioturbation and Adsorption Gradients on Solid and Dissolved Radium Profiles in Sediments from the Eastern Equatorial Pacific.” Geochimica et Cosmochimica Acta 51, no. 6 (1987): 1613–23. https://doi.org/10.1016/0016-7037(87)90342-5. Kadko, David. “A Detailed Study of Some Uranium Series Nuclides at an Abyssal Hill Area near the East Pacific Rise at 8°45′N.” Earth and Planetary Science Letters 51, no. 1 (1980): 115– 31. https://doi.org/10.1016/0012-821x(80)90260-5. Kipp, Lauren E., Matthew A. Charette, Willard S. Moore, Paul B. Henderson, and Ignatius G. Rigor. “Increased Fluxes of Shelf-Derived Materials to the Central Arctic Ocean.” Science Advances 4, no. 1 (2018). https://doi.org/10.1126/sciadv.aao1302. Ku, Teh-Lung, and Shangde Luo. “Chapter 9 Ocean Circulation/Mixing Studies with Decay- Series Isotopes.” Radioactivity in the Environment, 2008, 307–44. https://doi.org/10.1016/s1569- 4860(07)00009-5. Lao, Yong, Robert F. Anderson, Wallace S. Broecker, Susan E. Trumbore, Hansjakob J. Hofmann, and Willy Wolfli. “Transport and Burial Rates of 10 Be and 231 Pa in the Pacific Ocean during the Holocene Period.” Earth and Planetary Science Letters 113, no. 1-2 (1992): 173–89. https://doi.org/10.1016/0012-821x(92)90218-k. Levier, M., M. Roy-Barman, C. Colin, and A. Dapoigny. “Determination of Low Level of Actinium-227 in Seawater and Freshwater by Isotope Dilution and Mass Spectrometry.” Marine Chemistry 233 (2021): 103986. https://doi.org/10.1016/j.marchem.2021.103986. Martin, P., G.J. Hancock, S. Paulka, and R.A. Akber. “Determination of 227 Ac by α-Particle Spectrometry.” Applied Radiation and Isotopes 46, no. 10 (1995): 1065–70. https://doi.org/10.1016/0969-8043(95)00222-y. Moore, Willard S. “Fifteen Years Experience in Measuring 224 Ra and 223 Ra by Delayed- Coincidence Counting.” Marine Chemistry 109, no. 3-4 (2008): 188–97. https://doi.org/10.1016/j.marchem.2007.06.015. Moore, Willard S., and Ralph Arnold. “Measurement of 223 Ra and 224 Ra in Coastal Waters Using a Delayed Coincidence Counter.” Journal of Geophysical Research: Oceans 101, no. C1 (1996): 1321–29. https://doi.org/10.1029/95jc03139. Moore, Willard S., David J. DeMaster, Joseph M. Smoak, Brent A. McKee, and Peter W. Swarzenski. “Radionuclide Tracers of Sediment-Water Interactions on the Amazon Shelf.” Continental Shelf Research 16, no. 5-6 (1996): 645–65. https://doi.org/10.1016/0278- 4343(95)00049-6. Nozaki, Yoshiyuki, Masatoshi Yamada, and Hirofumi Nikaido. “The Marine Geochemistry of Actinium-227: Evidence for Its Migration through Sediment Pore Water.” Geophysical Research Letters 17, no. 11 (1990): 1933–36. https://doi.org/10.1029/gl017i011p01933. 72 Nozaki, Yoshiyuki. “Actinium-227: A Steady State Tracer for the Deep-Sea Basin-Wide Circulation and Mixing Studies.” Deep Ocean Circulation - Physical and Chemical Aspects, 1993, 139–56. https://doi.org/10.1016/s0422-9894(08)71323-0. Nozaki, Yoshiyuki. “Excess 227 Ac in Deep Ocean Water.” Nature 310, no. 5977 (1984): 486–88. https://doi.org/10.1038/310486a0. Rama, and Willard S. Moore. “Using the Radium Quartet for Evaluating Groundwater Input and Water Exchange in Salt Marshes.” Geochimica et Cosmochimica Acta 60, no. 23 (1996): 4645– 52. https://doi.org/10.1016/s0016-7037(96)00289-x. Reid, David F., Robert M. Key, and David R. Schink. “Radium, Thorium, and Actinium Extraction from Seawater Using an Improved Manganese-Oxide-Coated Fiber.” Earth and Planetary Science Letters 43, no. 2 (1979): 223–26. https://doi.org/10.1016/0012- 821x(79)90205-x. Rutgers van der Loeff, Michiel, Lauren Kipp, Matthew A. Charette, Willard S. Moore, Erin Black, Ingrid Stimac, Alexander Charkin, et al. “Radium Isotopes across the Arctic Ocean Show Time Scales of Water Mass Ventilation and Increasing Shelf Inputs.” Journal of Geophysical Research: Oceans 123, no. 7 (2018): 4853–73. https://doi.org/10.1029/2018jc013888. Sanial, V., L.E. Kipp, P.B. Henderson, P. van Beek, J.-L. Reyss, D.E. Hammond, N.J. Hawco, et al. “Radium-228 as a Tracer of Dissolved Trace Element Inputs from the Peruvian Continental Margin.” Marine Chemistry 201 (2018): 20–34. https://doi.org/10.1016/j.marchem.2017.05.008. Shaw, Timothy J, and Willard S Moore. “Analysis of 227 Ac in Seawater by Delayed Coincidence Counting.” Marine Chemistry 78, no. 4 (2002): 197–203. https://doi.org/10.1016/s0304- 4203(02)00022-1. Smith, Craig R., Will Berelson, David J. Demaster, Fred C. Dobbs, Doug Hammond, Daniel J. Hoover, Robert H. Pope, and Mark Stephens. “Latitudinal Variations in Benthic Processes in the Abyssal Equatorial Pacific: Control by Biogenic Particle Flux.” Deep Sea Research Part II: Topical Studies in Oceanography 44, no. 9-10 (1997): 2295–2317. https://doi.org/10.1016/s0967-0645(97)00022-2. Sun, Yin, and T. Torgersen. “The Effects of Water Content and Mn-Fiber Surface Conditions on Measurement by Emanation.” Marine Chemistry 62, no. 3-4 (1998):299–306. https://doi.org/10.1016/s0304-4203(98)00019-x. Teal, LR, MT Bulling, ER Parker, and M Solan. “Global Patterns of Bioturbation Intensity and Mixed Depth of Marine Soft Sediments.” Aquatic Biology 2, no. 3 (2008): 207–18. https://doi.org/10.3354/ab00052. Tromp, T.K., P. Van Cappellen, and R.M. Key. “A Global Model for the Early Diagenesis of Organic Carbon and Organic Phosphorus in Marine Sediments.” Geochimica et Cosmochimica Acta 59, no. 7 (1995): 1259–84. https://doi.org/10.1016/0016-7037(95)00042-x. 73 Chapter 3: 227 Ac Distribution along the GEOTRACES Meridional Transect as an Indicator of Solute Transport Nathaniel Kemnitz, Douglas Hammond, Susan Hautala, Paul Henderson, Emilie Le Roy, Matt Charette, Willard Moore, Bob Anderson, Marty Fleisher, Chris Hayes, Erin Black (co-authors) Abstract 227 Ac (t1/2=21.8 y) is produced by decay of 231 Pa (t1/2=32.8 ky). 231 Pa is scavenged in the water column by falling particulates that carry it to sediments, where it decays to its more soluble daughter. A fraction of this 227 Ac diffuses out of deep-sea sediments and is transported vertically and horizontally as it decays in the water column, providing a distribution that can be used to infer transport rates for other solutes in the deep ocean. The water column distribution of excess 227 Ac ( 227 Acex) was measured along the U.S. GEOTRACES Meridional Transect (GP15) from Alaska to Tahiti in fall 2018. To constrain its benthic input, cores from 5 stations near the northern half of the GP15 transect were collected in the summer of 2017 (C-Disk-IV transect stations 23-50°N). The GP15 transect parallels the C-Disk-IV cruise track in the Northeast Pacific, offset by a few hundred km. Two independent approaches were used to quantify the source function of 227 Ac in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter-parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agree within uncertainty, and the average 227 Ac 74 sediment fluxes for the Northeast Pacific are 90 ± 20 m -2 yr -1 . 227 Ac fluxes for the southern half of the GP15 transect were calculated by estimating F (fraction released by 231 Pa decay) and using 231 Pa measurements in the upper few cm of sediments. Profiles of 227 Ac and 231 Pa were measured and modeled in the water column along the GP15 transect. Along the GP15 transect, 227 Ac and 231 Pa typically remain in equilibrium between 0-3000m depths, and below this horizon, 227 Ac is in excess over its parent. Excess 227 Ac ( 227 Acex) generally increases in activity with increasing depth and the highest concentrations of 227 Acex are found within the bottom 1000 m. The highest concentrations of 227 Acex in the Eastern Pacific are located near the center of the Northeast Pacific Basin (NEPB) and south of 10˚S. These areas are dominated by high inputs due to low sedimentation and high 231 Pa activity in sediments. Along the southern leg of the GP15 transect (Sta.19 - 37), some elevated activities of 227 Acex are found at mid-depths (~2600m). Based on their proximity to d 3 He anomalies, these horizons appear to be influenced by hydrothermal activity from the East Pacific Rise (EPR), although maxima in the two tracers are not perfectly coincident. Mismatched details may result from spatial or temporal variability in the tracer sources, with 227 Ac reflecting more recent (last century) or more proximal inputs. Three types of 227 Acex profiles were found in the water column along the GP15 transect: (1) an expected, exponential decrease of 227 Acex away from the seafloor; (2) a well-mixed 500m thick bottom layer with very little 227 Acex above; and (3) an unexpected, erratic distribution of 227 Acex that has local maxima in 227 Acex overlying bottom waters of lower concentration. The first type of 227 Acex profile is found toward the northern end of the GP15 transect (Sta.5 – 10). 75 These profiles reflect a combination of diapycnal and isopycnal transport from the seabed. These profiles can be generated if water flows along a constant depth, where the 227 Ac bottom source is constant. If vertical diffusion of 227 Acex is constant, it should produce an 227 Acex distribution that is exponentially decreasing away from the seafloor, due to the combination of diffusion and radioactive decay. This is the most ideal profile and can be used to find apparent vertical eddy diffusivity rates (Kz), if the bottom is flat, or inclined with a constant slope. The apparent vertical diffusivity will be affected by both diapycnal and isopycnal transport, in situations where the isopycnals and/or the bottom topography are inclined. The second type of 227 Acex profile found along the GP15 transect shows constant 227 Acex activities near the bottom few hundred meters. These profiles are found in the middle of the Northeast Pacific (30˚ - 40˚N) and indicate that the bottom 500 meters are rapidly mixed, reflecting the density structure. Above this benthic layer, little 227 Acex is found, indicating low vertical transport. The third type of 227 Acex profile, with an erratic distribution of 227 Acex in the bottom few hundred meters is commonly found in the southern half of the GP15 transect. The localized maxima correlate with the depth distribution of regional topographic features. The irregularity appears to reflect high density stratification, coupled with vertically varying input from irregular topography in the Southeast Pacific, with horizontal transport of 227 Acex along isopycnals. A weighting function for several of these stations has been developed to predict profile shapes, based on known topography over the likely Ac source region, and assuming likely values of advection, coupled with vertical and horizontal diffusivities. This function matches the erratic profiles of 227 Ac fairly well. 76 The role of horizontal transport of 227 Acex is significant in some parts of the transect, as shown by comparing the integrated decay of 227 Acex in the water column to the benthic source: between 40˚ - 30˚N and 10˚ - 0˚N, water column decay is smaller than benthic input, and south of 10˚S, it is larger than benthic input. Areas where horizontal transport does not significantly affect 227 Acex are: north of 40°N, areas around the equator, and between 30˚ - 10˚N. In these areas, water column decay is comparable to benthic input. This pattern is consistent with predictions from an inverse model in the Northeast Pacific (Hautala, 2018) that indicates deep- water advection is strongest between 40˚ - 30˚N and circulation flows in a west-east direction (along 150˚W). North of 40˚N, the model suggests that circulation is moving in a north-south direction. 3.1 Introduction The GEOTRACES program is an international effort to map the distribution of trace elements and their isotopes. The main objective is to advance the current knowledge of biogeochemical cycles of these elements in the oceans (GEOTRACES Planning Group, 2007). These TEIs can either serve as essential nutrients for life, tracers of physical ocean processes, or contaminants. For example, essential nutrients such as iron and zinc in the oceans can have significant impacts on carbon sequestration from the atmosphere and its ultimate export to the seafloor. Knowing what controls the distribution of these essential nutrients can provide insight into the marine carbon cycle and its future in a changing marine environment. 77 Tracers in the oceans, especially radiotracers, are uniquely suited to define transport in that they have a built in “clock” that can give rates of oceanic processes, depending on their half- life and chemical properties. 227 Ac (t1/2=22.8 yr), the longest-lived isotope of Ac, is one such radiotracer. 227 Ac is moderately soluble in seawater and does not react biologically or chemically within marine environments on the time scales of its mean life (Nozaki, 1984; Geibert et al., 2002). Furthermore, the 227 Ac half-life makes it uniquely suited to give rates of transport on the basin-scale range (~100 yr) (Nozaki, 1984; Nozaki et al., 1990). Its distribution will only depend on its source function, half-life, and transport. However, 227 Ac has had little utility in the last 40 years since its discovery by Nozaki, (1984). Difficulties with detection and sampling made this radiotracer of little use to oceanographers (Geibert et al., 2002). Nonetheless, better measurement techniques and sampling methods during the last 10 years have resulted in dozens of 227 Ac profiles being generated in the oceans. The GP16 cruise in 2013 was the first oceanic transect to provide high resolution sampling of 227 Ac in the oceans. Since then, high-resolution sampling has occurred in the North Atlantic, Arctic, and South Pacific Oceans (Charette et al., 2015, Kipp et al., 2017; Sanial et al., 2018; Kipp et al., 2017; Hammond et al., in prep). This is the first study to provide high-resolution 227 Ac water column profiles that are coupled with measurements of its source function. Comparing the water column distribution with its source function can provide insight into what controls the 227 Ac distribution in the water column and rates of mixing (Ku and Luo, 2009). This knowledge can then be used to infer transport pathways of other solutes in the oceans, including trace metals and carbon. 78 This study will begin with an overview of the geochemistry and behavior of 227 Ac in the water column and sediments. Building on this understanding of 227 Ac in marine environments, the theory for using 227 Ac as a tracer for mixing will be described. Next, this study will compare the distribution of 227 Ac in the water column to its source function. Finally, bottom topography, density gradients, circulation and mixing will be correlated with 227 Ac distribution to provide insights into what controls the transport of solutes in deep waters along the GP15 transect. 3.2 Geochemistry of 227 Ac Figure 3.1 illustrates the sources and transport of 227 Ac in the water column. 227 Ac is the daughter of 231 Pa, which is produced by 235 U decay in the water column; 235 U has nearly constant concentrations throughout the world’s oceans. Because 231 Pa is strongly particle-reactive, models of reversible adsorption/desorption indicate its profiles should increase with increasing depth in the water column (Anderson et al., 1982). Removal from the water column should also result in in abyssal sediment concentrations that are in excess of its 235 U parent activity (Nozaki, 1984; Geibert et al., 2002). 231 Pa decays in the sediments and releases some 227 Ac into pore waters, with some 227 Ac diffusing out of the sediments, where it is mixed vertically and horizontally in the water column (Nozaki, 1984). 227 Ac will be in excess over its parent near the SWI (sediment water interface) and approaches equilibrium with its parent in the upper ocean. Little excess 227 Ac is observed in shallow marine environments because high inputs of detrital sediments strongly dilute the modest excess 231 Pa generated in the shallow overlying water column. This 79 leads to the conclusion that the dominant source of excess 227 Ac is deep-sea sediments, providing a constraint on boundary conditions for modeling. Figure 3.1: Schematic of the geochemical behavior of the 235 U series in the ocean. 3.3 Theory relating 227 Ac profiles to mixing processes The equation defining change of concentration of a dissolved radioactive solute in a 1- dimensional section in the water column is as follows (Berner, 1980): B, B+ =𝐾 $ B ! , B$ ! −𝑤 B, B$ −𝜆𝐶 (3.1) 80 where: C (dpm m -3 ) is the excess activity of the dissolved radioisotope, z (m) is the distance above the SWI (sediment water interface), Kz (m 2 yr -1 ) is the apparent vertical eddy diffusivity, w (m yr -1 ) is vertical advection, and l (yr -1 ) is the decay constant. If the primary transport is diffusive, Kz is assumed to be constant over the entire water column profile, steady state is invoked, and vertical advection w is assumed to be very small eq. 3.1 can be simplified to eq. 3.2: 0=𝐾 $ B ! , B$ ! −𝜆𝐶 (3.2) The apparent vertical eddy diffusivity (Kz) can be determined by solving eq. 3.2 to find eq. 3.3 for the excess 227 Ac profile in the water column, assuming the boundary condition that C à 0 as z à ¥: 𝐶 =𝐶 ! 𝑒 ".$ (3.3) where: x is distance from the bottom in meters, C is the activity of excess 227 Ac (dpm m -3 ), C0 is the activity of excess 227 Ac (dpm m -3 ) at the SWI, and b (m -1 ) is the inverse scale length of the exponential curve. Kz (m 2 yr -1 ) and b are related by the following relationship: 𝑏 =V % = F (3.4) However, the ocean has both diapycnal and isopycnal diffusion, causing two factors to add complexity to the apparent Kz: (1) the seafloor may slope rather than being flat, and (2) isopycnals may tilt relative to the bottom. Sarmiento and Rooth (1980) pointed out how these factors can affect the apparent vertical diffusivity and noted that if diffusive transport is resolved 81 into diapycnal and isopycnal components, and if the angle alpha between isopycnals and the bottom is small: 𝐾 $ =𝐾 V +𝛼 4 𝐾 6 (3.5) where Kn is diapycnal diffusivity, Kp is isopycnal diffusivity, and in this case a should be the angle between isopycnals and the slope of the seabed that supplies tracer. In the case where these parameters are constant, the exponential profile for a radioactive tracer coming from below should still apply (eq. 3.3), but the apparent Kz reflects the two components of diffusivity as well as the geometry defined by a. An important aspect pointed out by Sarmiento and Rooth (1980) is that because Kp is so much larger than Kn, it is difficult to use radiotracers to establish the diapycnal component of diffusion that is needed to determine buoyancy fluxes. However, for solutes introduced at the seafloor, if a radioisotope has a similar source function, the apparent Kz can be applied to estimate fluxes of the solute from below. 3.4 Study Area Figure 3.2 shows the C-Disk-IV (Carbonate Dissolution Kinetics-IV, sampled August, 2017) and GEOTRACES PMT (GP15, sampled September-November, 2018) transects in the Northeast and Southeast Pacific Ocean. The deep northeast Pacific is largely filled by Pacific Deep Water (PDW), which is predominately formed via upwelling, diffusion, and mixing of 82 other mode waters that enter from the South Pacific (Talley, 2013; Talley, 2011; Kawabe and Fujio, 2012). The PDW is characterized by its low oxygen, high nutrients, and large 14 C age. Below the PDW is an abyssal layer (> sq ~ 28.0 kg/m 3 ) composed of denser waters that originate from the Southern Ocean, which is largely fed into the Northeast Pacific through a northern route at a rate of 3 Sverdrup (Sv). The boundary between these two deepest layers is highly eroded (Talley, 2011). The top two ocean layers in the Northeast Pacific are the upper mixed layer, which occupies a density range between sq ~ 26.7 kg/m 3 to 27.6 kg/m 3 , and the intermediate layer, which occupies a density range between sq ~ 24.5 kg/m 3 to 26.5 kg/m 3 . Both of these have sources at the surface in the North Pacific. 83 Figure 3.2: Study Area of C-Disk-IV and GP15 cruise. Red dots refer to stations where 227 Ac was measured in the water column (GP15). Black triangles refer to stations where 227 Ac was measured in sediments (C-Disk-IV). Numbers next to symbols refer to station numbers. The deep southeast Pacific is largely composed of AABW, characterized by low temperature (< 1˚C), high density (g n > 28.12 kg m -3 ), and high salinity (34.6-34.7 PSU) (Talley, 84 2013). Above this deep layer is a mixture of LCDW and AABW with g n between 28.06-28.1 kg m -3 . Above 3500 m, a mixture of PDW and LCDW is present, with PDW flowing southward from the Northeast Pacific (Kawabe and Fujio, 2012). The south Pacific is more stratified compared to the Northeast Pacific, due to its age and the interaction with topography that mixes the PDW as it flows northward. Abyssal and Deep Circulation in Northeast Pacific The abyssal waters that occupy the Northeast Pacific are derived from the western boundary current system in the South Pacific (Hautala, 2018). The main pathway of these abyssal waters into the Northeast Pacific is from the western side of the Emperor Seamount Chain, mostly along a northern route that travels through the Aleutian Trench at a rate of 3 Sv (Kawabe and Fujio, 2012). Another branch is funneled through the main gap of the Emperor Seamounts around 40˚N and joins with a westward flowing current that then moves eastward along the Mendocino Fracture Zone at a rate of 1 Sv (Fig. 3.3). The topography in the Northeast Pacific is relatively smooth, with a gentle west-dipping slope between 140˚W and 180˚E, north of 40˚N. Rough topographies are present around the Hawaiian Ridge and Mendocino Fracture Zone near 40°N, where the seafloor topography steps up by 1 km to the north (Hautala, 2018). Hautala (2018) used an inverse model, based on high quality hydrographic data and conserving vorticity, to calculate advective flow at various depths, with the pattern in the deepest layer (~4200 m) shown in Fig. 3.4. Abyssal velocities are typically 0-4 mm/s. At a mean speed of 2 mm s -1 , Ac could be transported about 2000 km (about 85 18° latitude) from its source in one mean life. Hautala and Hammond (2020) modified this inverse model to include re-mineralization of silicic acid and carbon from particles raining from above, constrained by estimates of particle fluxes and the pattern of remineralization. Isopycnal diffusion was also incorporated, and a reasonable match of the model tracer fields with observed data was obtained with an isopycnal diffusivity of 240 m 2 s -1 . The expected diffusion distance for Ac from its source in one mean life (as sqrt(Kp/l)) is 500 km (about 5° latitude). These length scales provide rough estimates for the lateral scales which should be considered in evaluating source functions and transport. Figure 3.3: Abyssal Circulation Pathways in the Pacific Ocean. Numbers on the figure refer to Sverdrup’s (Sv). Open circles are suggested areas of upwelling. The black solid line indicates the GP15 transect (Figure modified from Kawabe and Fujio, 2012). 86 Figure 3.4: Inverse model using hydrography data in the NEPB. Figure is taken from Hautala (2018) and is overlaid with GP15 cruise tract. Colors indicate vorticity and red arrows are advection transport vectors. This model represents 4200 meters depth, the bottom-most layer in Hautala (2018) model. 3.5 Materials and Methods 3.5.1 Sample Collection Profiles of dissolved 227 Ac and 231 Pa in the water column were collected during the GP15 cruise aboard R/V Roger Revelle between September and November 2018 (Fig. 3.2). Deep-sea sediment samples were collected during the C-Disk-IV Cruise aboard the R/V Kilo Moana in August 2017 and during GP15 using a gravity corer suspended from the deep CTD deployment. Details about sediment work can be found elsewhere (Chapter 2 in thesis). 3.5.2 Dissolved 227 Ac and 231 Pa 87 Dissolved 227 Ac was collected using a dual-flow path in-situ pump (McLane WRT-LV). Two to three casts were conducted at each station (shallow, deep, and, at super stations, intermediate). Five to eight pumps were deployed on each cast. Each pump had two filters in parallel: the higher volume flow path had a 1 µm quartz filter (Whatman QMA) and the lower volume flow path had a 0.8 μm polyethersulfone filter (Pall Supor800). This allowed sufficient flow to pump about 1.5 m 3 during a 4-hour deployment. The QMA/Supor filters on the GP15 cruise captured particulate matter that was measured for 234 Th, 230 Th, 232 Th, and 231 Pa. Downstream of the filter heads, flow then passed through two grooved acrylic cartridges impregnated with MnO2 that sat in series (called commercial cartridge or CC for short). These were labeled A and B. Having two identical cartridges mounted in series allows determination of absorption efficiencies of Ra and Ac from seawater (Charette et al., 2015). For the GP15 cruise, half of the cartridges used were prepared at USC (usually the B cartridge) and half were prepared at WHOI (usually in the A position). On some casts, USC cartridges were used in both positions, and the average Ac absorption efficiency for these was found to be 65%. This is quite close to results for previous cruises in the EPTZ (64% on GP16, using acetate cartridges) and Arctic (62% on GN01). Cartridges prepared at WHOI for GP15 often had lower and more variable efficiencies. Consequently, for stations with WHOI cartridges in the A position and USC cartridges in the B position, the total actinium activity for the pumped sample was calculated as: 𝐴 +C+#J =𝐴 B6R + W %$E !.YZ (3.6) 88 This calculation for Ac was validated by comparing total dissolved 227 Ac and 231 Pa at shallow depths where the two isotopes should be in equilibrium (for all GP15 stations). The regression line between 227 Ac and 231 Pa shows very good agreement, suggesting that an efficiency of 65% is accurate for these MnO2 cartridges (Fig. 3.5). Furthermore, the efficiency for 227 Ac was also verified by comparing the cross-over stations for GP15 (Sta.35) and GP16 (Sta.36) cruises. Both stations agreed within uncertainty with each other (Fig. 3.6). To determine activity on cartridges, they were rinsed with DIW after collection for several minutes to remove sea-salt and then dried to 50-120% moisture using compressed air. All GP15’s MnO2 cartridges A cartridges were measured immediately after collection using RaDeCC, and thus were able to measure initial 223 Ra and 224 Ra (results to be discussed elsewhere). Both A and B MnO2 cartridges were then returned to USC and WHOI a few months later, to analyze 227 Ac and 228 Ra via analysis of their progeny using RaDeCC. Figure 3.5: Total dissolved 227 Ac vs. 231 Pa for GP15 stations. Depths shown were between 0 and 1900 m, where 227 Ac and 231 Pa are found to be in secular equilibrium. Cartridge efficiency for 227 Ac in the plot was 65%. 231 Pa samples were collected by Niskin bottles and measured by ICP- MS at LDEO. The regression line is forced through zero. 89 Figure 3.6: Total dissolved 227 Ac vs. depth (m) for cross-over stations 35 and 36 of GP15 (PMT) and GP16 (EPZT). While details of the profiles differ, the profile patterns for the two stations agree within uncertainty, with the exception of near-bottom features. 3.5.3 Preparation of MnO2 Commercial Cartridges (CC) The MnO2 commercial cartridges (CC) created for the GP15 cruise followed the procedure from Henderson et al. (2013) with modifications. First, cartridge acrylic fibers (3M catalogue #G80B81N, supplied cut to 4”) were purged with DIW and then soaked in a DIW bath for 48 hours. It is important to purge acrylic cartridges with DIW as to fill all voids, which will help KMnO4 to diffuse through the cartridge during the permanganate soaking. Next, a bath of 0.5 M KMnO4 was prepared that could hold 12 acrylic cartridges at one time. Cartridges were removed from the DIW bath and immediately placed in a 30 L bath of 0.5 M KMnO4, which was continually stirred at room temperature for 48 hours. Once the cartridges were removed, they were then allowed to sit for 48 hours to dry, flushed with copious quantities of DIW until 90 effluent became clear, then placed back into the KMnO4 bath for another 48 hours of soaking. The second KMnO4 bath was done to ensure that enough MnO2 reacted with the acrylic fibers. The cartridges were brown after the first KMnO4 soak, which is evident after washing off with DIW. The cartridge color should be black as possible, thus showing a higher yield of KMnO4 reduced to MnO2 on the acrylic cartridge. More MnO2 on the cartridge should allow more surface area for Ra and Ac to bind to, and thus provide have higher efficiency as described above. After the second, ~48 hour soaking in the KMnO4 bath, cartridges were removed, allowed to dry for another 48 hours, then flushed with DIW until the effluent became clear (this flushing usually takes 10 minutes). After the second wash, cartridges were usually very black in color. It is also important to check that MnO2 reacted through the whole cartridge, if not, another KMnO4 soaking will suffice. Most batches only need two KMnO4 soaks, but some batches were given a third soaking in the KMnO4 bath. Just allowing the cartridges to sit in the bath for a single soak of more than 2 days seemed not to work as well. We hypothesized that KMnO4 crystals may form on the outside of the cartridges after 2 days in the bath, and do not allow for any more reduction of KMnO4 to MnO2 onto the acrylic fibers. However, if the cartridges are washed and thoroughly flushed with DIW, then we believe this removes KMnO4 crystals that might have formed on or, in the inside the cartridges. This procedure is time consuming and tedious, but adsorption efficiencies for this cruise were high. Each 30 L KMnO4 bath solution was used for 5-8 batches (12 CC in each batch). 3.5.4 RaDeCC 91 For RaDeCC analysis (Moore and Arnold, 1996), moist MnO2 cartridges were placed in a closed system device that re-circulates helium through the cartridges and a Lucas scintillation cell that detects alpha decays of 219 Rn and 220 Rn using a photomultiplier tube (PMT). MnO2 cartridges are kept moist to achieve the maximum emanation efficiency of the two radon daughters (Sun and Torgersen, 1998a; Moore and Cai, 2013). The delayed coincidence system detects 219 Rn (t1/2 = 3.96 s) and 220 Rn (t1/2 = 55.6 s) by detecting decay of their polonium daughters ( 215 Po = 1.78 ms; 216 Po = 145 ms;), in one of the two-time gated windows that open after detection of an initial alpha decay. The 219 Rn and 220 Rn activities reflect the activities of 223 Ra and 224 Ra that are absorbed onto the Mn-fibers. If it is assumed that 223 Ra is in secular equilibrium with 227 Ac, then the activity of 219 Rn will equal the activity of 227 Ac on the MnO2 cartridges. Similarly, 220 Rn and 224 Ra will grow into secular equilibrium with 228 Th, and ultimately 228 Ra. Six RaDeCC detectors were operated to measure 223 Ra and 224 Ra from the MnO2 cartridges (Detectors 1, 3, 2, 4, 5, and 7). Detector efficiencies ranged from 26 – 44% for detecting 223 Ra and 224 Ra. Detector efficiencies were determined by measuring standards spiked with known amounts of 227 Ac and 232 Th (STDK21 and STDK16). STDK21 and STDK16 had geometries like the commercial cartridges used for water column samples. Standards were routinely analyzed on all 6 detectors on a weekly basis, to monitor efficiency drifts with time. Each count typically lasted 4 hours, and each cartridge was measured 2 to 6 times, on multiple detectors. Counts were rejected if a decline in activity of more than 20% was noted over 4 hours, or if activity differed by more than 3 standard deviations from replicate analyses. Blanks varied 92 slightly for different detectors but were typically 0.001 and 0.03 cpm for 219 and 220. Some cross talk from high 220 activity can appear in the 219 channel (Scholten et al., 2013), but this correction was generally negligible. Additional details about data reduction and analytical protocols are in Hammond et al. (in prep). 3.6 Results and Discussion 3.6.1 227 Ac Distribution in the Water Column compared to Benthic Input The distribution of 227 Ac along the GP15 transect in the water column increases in activity with increasing depth towards the bottom (Fig. 3.7, 3.8). 227 Ac activity is close to equilibrium with its 231 Pa parent above 3000 m, and below this horizon it is often in excess. This is as expected, since the source function of excess 227 Ac is deep-sea sediments, and 227 Ac decreases in activity as it transported away from its source (Nozaki, 1984, Nozaki et al., 1990; Geibert et al., 2002). A few areas in the Southern Pacific showed elevated 227 Ac concentration at mid-depth (~2500m). These areas are likely influenced by hydrothermal activity of the East Pacific Rise (EPR), and others have described hydrothermal vents as being a significant source of 227 Ac into the oceans (Kipp et al., 2015, Hammond et al., in prep). These inputs will be described in more detail in a later section. 93 94 Figure 3.7: All profiles of 227 Acex vs. depth along GP15 transect. 227 Acex was calculated by taking difference of total dissolved 231 Pa and 227 Ac. 231 Pa data was measured at almost every depth interval as 227 Ac. Intervals where 231 Pa was not sampled were interpolated between nearby depths. The bold dashed lines below the bottom of the profiles are station bottom depths. Note, station 36 has 227 Acex axis between -0.5 and 6 dpm m -3 , and all other profiles are -0.5 and 5 dpm m -3 . Figure 3.8: 227 Acex activity vs. depth along the GP15 transect. The figure shows 3000-6000 m depths only since 227 Ac above this horizon is practically zero. The white dots are sample depth for 227 Ac via McLane in-situ pumps. 95 Concentrations of 227 Ac in bottom waters varies across the GP15 transect (Fig. 3.8). The highest concentrations are seen near the southern end of the transect and the center of the Northeast Pacific, with lower concentrations north of 40°N and near the equator. Higher concentrations are found in the southern hemisphere. The inventory of excess 227 Ac at each station was computed, summing the inventory in near-bottom mixed layers (if present) with the integral of an exponential function fitted to excess 227 Ac where profiles decrease exponentially with distance from the bottom. This inventory is plotted in Fig. 3.9, along with benthic inputs of 227 Ac based on profiles of 227 Ac/ 231 Pa in sediments and core incubations discussed by Kemnitz et al. (in prep). In addition, inputs from sediments can be estimated from 231 Pa in surficial sediments using an equation presented by Nozaki et al. (1990) to calculate the flux of 227 Ac across the SWI: 𝐽 44K(L = 𝐹𝜌 5 (1−𝜙)𝐴 4['D# W𝐷 . 𝜆 (L (3.7) where: F is the fraction of 227 Ac that escapes the solid grain when it decays from 231 Pa (dpm g -1 ), f is the porosity, 𝜌 (g cm -3 ) is the density of deep-marine sediments, A231Pa is the activity of 231 Pa in the sediments, Db (cm 2 yr -1 ) is the bioturbation coefficient, and 𝜆 is the decay constant for 227 Ac. The derivation assumes all parameters are independent of depth in sediments, and the water column concentration is negligible. Furthermore, this equation assumes that the partition coefficient K is large for Ac in sediments, so that bioturbation is the main mechanism that transports 227 Ac upward. The pattern for the water column inventory generally matches the measured and estimated sediment fluxes (Fig. 3.9). The match largely reflects the benthic input, as under subtropical gyres, sediment accumulation is low, resulting in higher concentrations of 96 231 Pa in sediments. Near the equator and north of 40°N, the 231 Pa raining from the water column is diluted by higher sedimentation rates. Figure 3.9: 227 Ac flux vs. latitude along the GP15 transect. 227 Ac Flux (measured) were sediment fluxes measured by core incubation and summation of 227 Ac deficiency downcore. 227 Ac Flux (predicted) represents sediment fluxes measured by using eq. 3.7, which uses 231 Pa activity in core tops. Lastly, 227 Ac Flux (water column) were water column fluxes measured by fitting eq. 3.3 to the exponential portion of the 227 Acex profile, integrating the results through this portion, and adding this to an average integrated value for any benthic mixed layer that was present. The similarity in water column inventories and benthic input suggests 227 Acex is not transported over large horizontal distances, at least not perpendicular to the latitudinal gradients for input. As noted earlier, Hautala (2018) has estimated abyssal currents in the Northeast Pacific to be a few mm s -1 . In one 227 Ac mean life (30 y), a net velocity of 2 mm s -1 should result in in a transport of 2000 km, or 18° of latitude. This suggests that much of the advective flow along 150°W may be roughly west-east, as predicted by the hydrographic calculations of Hautala (2018) (Fig. 3.4). Also based on the isopycnal diffusivity of 240 m 2 s -1 (Hautala and Hammond, 2020) the diffusion scale length of (Kh/l) 0.5 indicates mean transport of 500 km, or 5° of latitude. This may explain why the water column fluxes directly north and south of the equator appear 97 slightly higher than the local sediment flux. Stations south of 10˚S indicate the water column has higher 227 Acex than the sediment input would predict. This may indicate net advective transport toward the west from shallower regions, while near 15°N, the lower water column inventory suggests that abyssal transport from deeper areas to the west may reduce the Ac inventory. 227 Ac was introduced into the model of Hautala and Hammond (2020), and predictions for the bottom water concentrations (Fig. 3.10) show general similarity to the observed patterns at 4600 m (Fig. 3.8), with higher concentrations north of 37°N and lower concentrations 25-37°N. Unfortunately, the model domain does not extend south of 20°N. Figure 3.10: Inverse model using 227 Acex data in the NEPB at 4600 dbar. Figure is provided from Susan Hautala at Washington University. This model is similar to the inverse model from above with circulation based on hydrography, with horizontal diffusivity of 240 m 2 s -1 and vertical diffusivity dependent on distance from the bottom (Hautala, 2018). Three types of 227 Acex profiles were found along the GP15 transect: (1) an expected, exponential decrease of 227 Acex away from the seafloor; (2) a well-mixed 500m thick bottom layer with very little 227 Acex above; (3) and lastly, an unexpected, erratic distribution of 227 Acex that has local maxima in 227 Acex that overlie bottom waters of lower concentration. 98 The first type of 227 Acex profile is found toward the northern end of the GP15 transect (Sta.5 – 10). These profiles reflect a combination of diapycnal and isopycnal transport from below. As noted in the theory section, a fit to this type of profile can be used to calculate apparent vertical diffusivity. Figure 3.11 shows the northern GP15 stations curve fits to 227 Acex vs. DAB profiles. Kz values are also shown on the profiles. Results for this calculation (Table 3.1) can then be used to estimate an upper limit for isopycnal diffusivity, assuming diapycnal diffusivity is small (Table 3.1), as predicted from physical measurements of turbulence in the ocean interior (Kunze et al., 2008). An apparent trend is seen with the Kz values along the northern transect of GP15: Kz values roughly increase from the Aleutian trench until the Hawaiian margins. Around station 12, the bottom 500m is well-mixed and the bottom layer Kz is approximately infinity. However, Kz values above this layer are in the same magnitude as the stations north of 40˚N. Stations 12 and 14 Kz values are between 2-3 cm 2 yr -1 above the mixed layer. The low vertical resolution for 227 Ac may limit the precision for Kz estimates, particularly at station 16, however. The latitudinal trend of decreasing Kz to the north may reflect the latitudinal pattern of input, as noted in the caption for Fig. 3.11. Table 3.1: b, Kz, bottom and isopycnal slopes, a, and Kp values for GP15 stations 8, 10, 14, and 16. The b value is the attenuation of eq. 3.3, Kp is the isopycnal diffusivity, and alpha is found by adding the bottom and isopycnal slopes. Kp is calculated by eq. 3.5, assuming Kn is small. STA. # b Kz* Bottom Slope Isopycnal Slope a Kp m -1 m 2 s -1 m km -1 m km -1 m km -1 m 2 s -1 8 0.013±0.002 200±30 -0.227 1.049 0.0011 10±1 10 0.004±0.001 2400±400 0.511 1.049 0.0011 30±5 14 0.002±0.000 11100±1600 0.429 0.853 0.0009 200±30 16 0.001±0.000 31700±9600 0.464 0.853 0.0009 560±170 99 Figure 3.11: 227 Acex vs. DAB for GP15 stations 8, 10, 14, and 16 (latitude range from 47°N to 27°N). All four stations have similar standing crop inventories that agrees with the benthic input of 227 Acex in the water column. The upper boundary of the mixed layer was chosen based on a rapid increase in the density gradient. Note that apparent Kz values increase from high to low latitudes (Sta. 8 à Sta. 16), with stations 14 lower layer Kz value being too large to model. The change in apparent Kz may reflect the lower input into less dense waters that contact the seabed further north (Fig. 3.13). This would attenuate a vertical profile for 227 Acex more rapidly with increasing distance from the sea floor. 100 The second type of 227 Acex profile found along the GP15 transect shows constant 227 Acex activities within the bottom few hundred meters. These profiles are found in the middle of the Northeast Pacific (30˚ - 40˚N, Sta. 12 & 14) and indicate that the bottom 500 meters are rapidly mixed, reflecting the density structure. Above this benthic layer, little 227 Acex is found, indicating low vertical transport. Two stations in the Northeast Pacific showed constant activity for 227 Acex in the bottom 500m, and above this horizon showed little to no 227 Acex (Sta. 12 and 14). Thus, 227 Acex was also modeled by treating the water column profile as a two-layered structure at these stations. Boundary conditions for this model were determined from the break in slope from the neutral density vs. depth profiles (Fig. 3.12). The bottom layer is assumed to be well-mixed, and its concentrations are constant throughout the deep layer interval. Each two-layer profile was also fit with eq. 3.3 in the upper layer. The values of b were then used with eq. 3.4 to find Kz values for this upper layer. The third type of profile has an erratic distribution of 227 Acex in the bottom few hundred meters. These are common in the southern half of the GP15 transect. This type correlates with the depth distribution of regional topographic features. The irregularity appears to reflect high density stratification, coupled with regional input from irregular topography in the Southeast Pacific that inputs 227 Acex at multiple depths, which then travels horizontally along isopycnals (Fig. 3.13). The correlation with topography will be discussed in a later section. 101 Figure 3.12: Neutral density vs. depth juxtaposed with 227 Acex vs. depth for GP15 stations 12 and 14. The mixed layer boundary was determined from a break in slope on Neutral density vs. depth plots. 102 Figure 3.13: Neutral Density vs. depth along the GP15 transect. Note the differences in vertical and horizontal isopycnals for the North and South Pacific. In the S. Pacific, isopycnals are nearly horizontal to the bottom. In the N. Pacific, isopycnals are horizontal at shallower depths, but dip steeply into the bottom. The 227 Ac profiles roughly reflect the density structures, with bottom mixed layers present where isopycnals are nearly vertical. 3.6.2 Advective Transport revealed by vertical mass balances Horizontal transport of 227 Acex appears significant in some parts of the transect, as shown by comparing the integrated decay of 227 Acex in the water column to the benthic source. Between 40˚ - 30˚N and 10˚ - 5˚N, water column decay is smaller than benthic input and south of 10˚S, it is larger than benthic input. Areas where horizontal transport does not significantly affect water column inventories are: north of 40°N, and between 30˚ - 10˚N. In these areas, water column decay is comparable to benthic input. This pattern is consistent with predictions from an inverse model in the Northeast Pacific (Fig. 3.4) that indicates deep-water advection is toward the east between 40˚ - 30˚N. This should transport water overlying deeper sediments, and with lower 227 Ac, into the stations sampled. North of 40˚N, the model suggests that circulation is moving in a north-south direction. The water column inventory is close to the flux measured in this region 103 (Fig. 3.8), matching expectations. Near the equator (5°S to 5°N), as mentioned previously, lateral diffusive transport is likely to augment the low equatorial input from below, causing the water column inventory to exceed the sediment input slightly (Fig. 3.8). 3.6.3 Topography influence on 227 Ac distribution Sarmiento et al. (1982) used existing 228 Ra profiles in the North Atlantic water column and analyzed them using a 1-D vertical mixing model. The results from these profiles produced apparent vertical diffusivity rates (Kz) on the order of 10 cm 2 s -1 for the bottom 1000m and 100 cm 2 s -1 near the ocean floor. These rates were much too high for diapycnal diffusivities and were inconsistent with mass balance models for buoyancy. Thus, they determined 228 Ra is traveling predominantly along isopycnals rather than across them. The source of 228 Ra they observed is largely from topographic highs within the bottom 2000m of the North Atlantic ocean. 227 Ac in deep waters should behave similarly to 228 Ra. However, abyssal topography could add excess 227 Ac into the ocean (>4000m). Many 227 Ac profiles in the South Pacific show an irregular distribution of 227 Ac near the bottom 1000m (Sta. 21 -39). Density stratification is high in these profiles, and it is assumed 227 Ac is largely traveling along isopycnals, producing an unexpected 227 Acex profile (Fig. 3.7). To investigate topography’s influence on the distribution of 227 Ac in the deep ocean, a model was developed to semi-quantitatively assess 227 Ac sediment input from topographic highs. 104 Several stations with erratic profiles were modeled. The model was developed as followed: bottom depths were considered for 15° in all directions from the station (~1600 km in all directions around station). This produced a square box with the station in the center of the box (Fig. 3.14). Next, relative 227 Ac inputs were calculated for various depth horizons, based on the bottom distances (using the 2’ topography of Smith and Sandwell, 1997) and horizontal and vertical diffusion and advection. The horizontal and vertical diffusivity were assumed: Kh=250 m 2 s - 1, Kz=0.1 cm 2 s -1 (Hautala and Hammond, 2020). Predominant velocities were assumed to be less than a few mm, with the direction of flow assumed to be east or west, depending on the station and published estimates of flow direction (Reid 1979; Hautala and Riser, 1992; Reid 1997; Reising et al., 2015). Eq. 3.8 summarizes 227 Ac transport in the model. 𝑊 =𝑒 " \ ] G. HG ! . IJ - K !J - ^ ! ∆` ! 0 K∆M ! J - 0 K∆F ! J F (3.8) where: 𝑊 is the weighting function for input for each depth, based on topography, l is the decay constant for 227 Ac, 𝑣 is velocity in the x direction (EW), Kh is horizontal diffusivity, Kz is vertical diffusivity, and ∆x and ∆y are East-West and North-South distance of the station from bottom exposure in a certain depth range, while ∆z is distance from the bottom. This function computes the effective input from each grid point based on the vertical and lateral distance along with the effective transport over that distance. Using 50 m depth range bins, relative 227 Ac inputs for each depth around the station were then summed, scaled, and plotted as effective distance from bottom vs. water depth horizon (Fig. 3.14). The model is designed so that it only considers 105 depths within the square area relative to each station as shown in Fig. 3.14, breaking water depth horizons into 50 m layers. The function was arbitrarily scaled by dividing the total by the weight in the depth range having the maximum weight and multiplying this by 5 as a scaling factor. This approach gave a profile that matched well at Station 21. To account for differences in regional inputs due to 231 Pa distributions in sediments, weights for other stations were multiplied by the ratio of sediment 231 Pa concentration to that for station 21 (data in Fig. 3.9). Scale distances for transport in any direction depend on diffusivities and velocity. As noted previously, likely scale distances for lateral diffusion may be 5°. Advective scale distances may be a bit longer and depend on the assumed velocity. To decide on an appropriate seabed area to consider for the weighting function, test calculations were carried out at station 21, Expected values for Kh, Kz, and v were used to compute weighting factors shown in Fig. 3.15, with the distance scale considered increasing from 5° to 15° in each direction. Little change in the scaling was observed as the distance increased beyond 10°. To allow for selection of somewhat greater transport parameters, a seabed area extending 15° from the station in each x and y direction was selected (resulting in a square of 30°x30°, centered on the station). 106 Figure 3.14: Example of GP15 station 21 topography (A) and its influence on the station sampled, shown as a color-coded weighting function that represents relative input of each location to different horizons in the water column (B). The station is located in the center of the figure (red dot on panel A). Locations nearest the station and in the direction of flow are weighted more heavily. Note the bright orange color that extends laterally from east to west. This reflects the influence of advection in the x-direction. In the y- and z-direction, only diffusion supplies transport. Figure 3.15: : Station 21 227 Acex vs. DAB juxtaposed with topography weighting values vs. depth. The model size was changed in order to see how the weighting function structure changes with depth. Model size = 15˚ is suggested as the best size since the weighting function is close to most 227 Acex data points. A B 107 Different combinations of the 3 transport parameters were considered, and results for station 21, 33, and 37 are shown in Fig. 3.16. Beginning with station 21, a few noticeable trends with the topography weighting function are evident with changing of the three parameters: as velocity is increased, the topography effect between 0 – 700 meters DAB becomes more pronounced especially around 700 meters DAB. The area around 700 meters is likely a topographic high located to the east that is getting weighted more heavily as velocity is increased. For the other two parameters: as Kh is increased, the depths of maxima and minima do not change greatly, but the weighting near 1200 m DAB increases to match the data point there. Changes in Kz reduce the near-bottom gradient slightly, but do not provide a major influence. The middle panel for this station seems to be a somewhat better fit than other combinations. At station 37, after multiplying scaling by a factor of 1.5 to account for the higher 231 Pa in sediments at that site, similar effects of transport parameters occur. Combinations of the higher velocity and Kh seem to offer slightly better simulations. At station 33, Reid (1997) has suggested flow may be from west to east (negative velocities in this computation) and this is supported by simulations that better match the observed concentrations. Consequently, it appears that simulations are primarily sensitive to flow directions, and that the horizontal eddy diffusivity recommended by Hautala and Hammond (2020) offers a reasonable choice for this transport parameter. 108 109 110 Figure 3.16: . Different combinations of the parameters velocity (v), horizontal diffusion (Kh), and vertical diffusion (Kz) are shown. The scale max is the weighting function max value for the entire profile. Note how the scale max changes as v, Kh, and Kz change. Station 33 and 37 are scaled by 1.4 x more than station 21 since its 231 Pa activity is 1.4 more than station 21 231 Pa values (4 vs. 6 dpm g -1 ). Note that the direction of flow reverses is negative (west-east) at station 33. 111 3.6.4 Hydrothermal influence on 227 Ac distribution Kipp et al. (2015) measured elevated activities of 227 Acex near the Mid-Atlantic Ridge (MAR) in the North Atlantic and concluded that hydrothermal vents are an unrecognized source of 227 Ac into the oceans. Their study used d 3 He profiles within the hydrothermal plume, which is a well-known tracer from hydrothermal activity, and compared it to the distribution of 227 Acex (Jenkins, 2013). They found a tight correlation between elevated d 3 He and 227 Acex at two stations along the US GEOTRACES GA03 transect near the MAR. The two stations near the MAR had depths of only 3000m, unlikely to have a large enough 231 Pa signal from sediments to generate any 227 Acex in the immediate water column. Thus, the study concluded that the only likely source of 227 Acex in this vicinity is the nearby hydrothermal vents of the MAR. The GP15 transect shows hydrothermal activity in the water column between 20˚N – 15˚S. d 3 He anomalies were measured between stations 19 – 37 and show elevated values at 2600m (Jenkins group). Only two stations in the South Pacific had high resolution sampling for intermediate depth ranges for 227 Acex (sta. 37 & 35). The distribution of 227 Acex and d 3 He anomalies are shown in Fig. 3.17 for GP15 stations 35 and 37. Station 37 shows the clearest maximum peak for 227 Acex at 3000m. Station 37 227 Acex activity is 0.5 dpm m -3 near 2600m and increases to a maximum of 1.0 dpm m -3 by 3000m. Below this horizon, 227 Acex decreases in activity until the bottom 500m, where bottom sediment sources add high concentrations of 227 Acex. Due to high density stratification near the bottom 1000m at station 37, the bottom source of 227 Acex is unlikely to migrate vertically more than a few hundred meters off the seafloor. 112 Lastly, local topographic highs are also absent near the hydrothermal plume depth, although the previous topographic weighting analysis suggests that some of these features may represent advective transport from seabed inputs on the East Pacific Rise, rather than hydrothermal plumes. The Station 35 mid-depth 227 Acex peak is not as clear as station 37. The first elevated mid-depth 227 Acex activity is at 2200m. After this depth, 227 Acex decreases to negligible levels by 2600m and then steadily increases until the seafloor. High activities near 3500m and 4000m are likely being sourced by topographic highs (Fig. 3.17). Similar to station 37, high density stratification throughout stations 35 suggest little vertical mixing. Thus, the 227 Acex seen between 2200-3200m at stations 35 and 37 may be sourced from the hydrothermal system of the EPR. Figure 3.17: 227 Acex vs. depth juxtaposed with d 3 He vs. depth for GP15 stations 35 and 37. d 3 He data is taken from Jenkins Group. However, it is perplexing why the 227 Acex peak within the hydrothermal plume does not align with the d 3 He maxima. Kipp et al. (2015) witnessed a similar offset with d 3 He and 227 Acex 113 (~200m offset in the North Atlantic) and suggested that sampling the two isotopes on separate casts resulted in the plume migrating due to turbulence (short-term fluctuations). They used transmissometer data taken on each cast and notice that the plume’s peak particle concentration moved by 200m in the water column. However, this study’s d 3 He and 227 Acex mid-depth maximum were offsets by more than 400m and it is unlikely that such short-term fluctuations can move the 227 Acex peak by this much. Suggested reasoning for the offset could be related to 227 Ac half-life of 22 years, and its distribution within the plume could be reflecting a newer hydrothermal vent system at deeper depth, or a more proximal source that has added less d 3 He. The d 3 He distribution has a long residence in the oceans and its structure may take hundreds of years to reflect a different vent at a shallower depth. 3.7 Conclusions Profiles of 227 Ac and 231 Pa were measured and modeled in the water column along the GP15 transect in the Eastern Pacific. Along the GP15 transect, 227 Ac and 231 Pa remain in equilibrium between 0-3000m depths and below this horizon, 227 Ac is in excess over its parent. Excess 227 Ac ( 227 Acex) increases in activity with increasing depth toward the bottom and the highest concentrations of 227 Acex is contained within the bottom 1000 meters. The highest concentrations of 227 Acex in the Eastern Pacific is found the center of the Northeast Pacific Basin (NEPB) and south of 10˚S. These areas are dominated by low sedimentation and high 231 Pa activity in sediments. A few areas along the GP15 transect show elevated activities of 227 Acex at mid-depths (~2600m). These areas may be influenced by hydrothermal activity from the East 114 Pacific Rise (EPR). The main input of 227 Acex into the Eastern Pacific is deep-sea sediments with some possible addition from hydrothermal vents. Two independent approaches were used to quantify the source function of 227 Ac in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore 227 Ac- 231 Pa deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agree within uncertainty, and the average 227 Ac sediment fluxes for the Northeast Pacific is 90 ± 20. Estimates of 227 Ac fluxes in the Southeast Pacific was determined by measuring the 231 Pa activity in the upper few cm of sediments and applying the former reactive-transport model that depended on F and Db. The lowest 227 Ac fluxes along the GP15 transect are found near the equator, Hawaiian, and Alaskan margins. The highest 227 Ac fluxes are near the center of the Northeast Pacific (~37˚N) south of 10˚S. 227 Ac fluxes are influenced by 231 Pa signal in sediments, sediment composition, and distance from continental margins. Horizontal transport is significant in some parts of the transect, as shown by comparing the integrated decay of 227 Acex in the water column to the benthic source: between 40˚ - 30˚N and 10˚ - 0˚N, water column decay is smaller than benthic input, and south of 10˚S, it is larger than benthic input. Areas where horizontal transport is not significant is north of 40°N, areas around the equator, and between 30˚ - 10˚N. In these areas, water column decay is comparable to benthic input, and the water column was treated using a 1-D model. 115 Three types of 227 Acex profiles were found along the GP15 transect: an expected, exponential decay of 227 Acex away from the seafloor; a well-mixed 500m thick bottom layer with very little 227 Acex above; and an unexpected, erratic distribution of 227 Acex that has 227 Acex increasing in activity away from the seafloor. The first type of profile was used to calculate a vertical eddy diffusivity. Model results found Kz values between 0.01 – 10 cm 2 s -1 . These Kz values are much larger than diapycnal diffusivities in the deep North Pacific, inferred by previous authors and are considered an upper limit as they reflect both isopycnal and diapycnal diffusivity. The other two profiles are dominated by density stratification and circulation of water masses moving along a constant depth. Inputs from a seabed with irregular topography were modeled by considering the effective transport paths and their rates, and this simulation offered insight into the role of lateral transport in creating excess 227 Ac profiles with erratic shapes. The unexpected erratic profiles are strongly influenced by local topographic highs and density stratification, which transports 227 Acex primarily along isopycnals. 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Swift. “Introduction to Descriptive Physical Oceanography.” Descriptive Physical Oceanography, 2011, 1–6. https://doi.org/10.1016/b978-0-7506-4552-2.10001-0. Yong Lao, Robert F. Anderson, Wallace S. Broecker, Susan E. Trumbore, Hansjakob J. Hofmann, and Willy Wolfli. “Transport and Burial Rates of10be and231pa in the Pacific Ocean during the Holocene Period.” Earth and Planetary Science Letters 113, no. 1-2 (1992): 173–89. https://doi.org/10.1016/0012-821x(92)90218-k. 120 Chapter 4: Evidence of Changes in Sedimentation Rate and Sediment Fabric in a Low Oxygen Setting: Santa Monica Basin, CA Nathaniel Kemnitz, William M. Berelson, Douglas E. Hammond, Laura Morine, Maria Figueroa, Timothy W. Lyons, Simon Scharf, Nick Rollins, Elizabeth Petsios, Sydnie Lemieux, and Tina Treude (co-authors) Abstract The Southern California Bight is adjacent to one of the world’s largest urban areas, Los Angeles. As a consequence, anthropogenic impacts could disrupt local marine ecosystems due to municipal and industrial waste discharge, pollution, flood control measures and global warming. Santa Monica Basin (SMB), due to its unique setting in low oxygen and high sedimentation environment, can provide an excellent sedimentary paleorecord of these anthropogenic changes. This study examined ten sediment cores, collected from different parts of the SMB between spring and summer 2016, and compared them to existing cores in order to document changes in sedimentary dynamics during the last 250 years, with an emphasis on the last 40 years. 210 Pb-based mass accumulation rates (MAR) for the deepest and lowest oxygen- containing parts of the SMB basin (900-910m) have been remarkably consistent during the past century, averaging 17.1 ± 0.6 mg cm -2 yr -1 . At slightly shallower sites (870-900m), accumulation rates showed more variation, but yield the same accumulation rate, 17.9 ± 1.9 121 mg/cm 2 -yr. Excess 210 Pb ( 210 Pbex) sedimentation rates were consistent with rates established using bomb-test 137 Cs profiles. We also examined 14 C profiles from two cores collected in the deepest part of the SMB, where fine laminations are present up to about 450 years B.P. These data indicate that MAR was slower prior to ~ 1900 CE (rates obtained = 9 and 12 mg/cm 2 -yr). d 13 Corg profiles show a relatively constant value where laminations are present, suggesting that the change in sediment accumulation rate is not accompanied by a change in organic carbon sources to the basin. The increase in sedimentation rate towards the recent occurs at about the time previous studies predicted an increase in siltation and the demise of a shelly shelf benthic fauna on the SMB shelf. X-radiographs show finely laminated sediments in the deepest part of the basin only, with cm-scale layering of sediments or no layering whatsoever in shallower parts of the SMB basin. The absence of finely laminated sediments in cores MUC 10 (893 m) and MUC 3 (777 m) suggest that the rate at which anoxia is spreading has not increased appreciably since cores were last analyzed in the 1980s. Based on core top data collected during the past half century, sedimentary dynamics within SMB have changed minimally during last 40 years. Specifically, mass accumulation rates, laminated sediment fabric, extent of bioturbation, and % Corg have not changed. The only parameter that appeared to have changed in the last 450 years was the MAR with an apparent >50% increase occurring between CE ~1850 and the early1900s. The post-1900 CE constancy of sedimentation through a period of massive urbanization in Los Angeles is surprising. 122 4.1 Introduction The use of laminated sediments as a record of environmental change has many historical precedents (Koivisto and Saarnisto, 1978; Gorsline, 1992; Algeo et al., 1994). The deepest portion of Santa Monica Basin (SMB, Fig. 1) has been accumulating finely laminated sediments for the past approximately 400 years (Christensen et al., 1993). The presence of fine lamination is evidence that macrofaunal activity on or in the sediment has been minimal to absent. Savrda et al. (1984) documented the transition from laminated to bioturbated sediments as corresponding to a change in oxygen concentration in the bottom water, which is the chief control of benthic macrofauna presence (Levin, 2003). Yet two things are necessary to produce laminated sediments. First is the absence of disturbance or mixing, and the other is a pulsed delivery of sediment that produces a distinction in composition or sediment fabric (Kemp, 1996). Sediment trap studies at a long-term study site (SPOT, Fig. 4.1) in adjacent San Pedro Basin demonstrated a seasonal pattern of sedimentation with highest rates in late winter and spring (Collins et al., 2011). Similarly, Haskell et al. (2015) documented seasonality in upwelling velocity and biogenic particle export from the upper ocean at SPOT. In contrast to the annual forcing of sedimentation in local waters, sediments in the SMB show primarily non-annual laminations with a frequency of 3-7 year. This lamination cycle may be consistent with the frequency of heavy rainfall in Southern California during El Nino years (Quinn et al., 1978; Christensen et al., 1993). 123 Figure 4.1: All cores and coring locations presented in this paper. Acronyms ‘MUC’, ‘BC’, and ‘AHF’ indicate Multi-Core, Box Core, and Allan Hancock Foundation (see Table 2 for more details on coring locations). The present study considers changes in SMB sedimentation over the past 150 years, a time period when changes in ocean biogeochemistry have been observed both globally and regionally. For example, the large-scale changes in the size and intensity of global oxygen minimum zones (Stramma et al., 2010; Brietburg et al., 2018) have also been documented for Southern California waters (Bograd et al., 2008). Oxygen concentrations in near-surface waters of the Southern Californian shelf show a 20- to 50-year decline beginning in the early 1960s, which was attributed to increased stratification and/or increased productivity caused by enhanced nutrient supply (Booth et al., 2014). In concert with changes in upper ocean oxygen content, 124 research by Huh et al. (1989) and Christensen et al. (1993) have documented expansion of the area of laminated sediments in SMB over the past 400 years. Their work with X-radiography showed that homogenized sediment was covered by laminated sediment, marking a transition from bioturbation to lamination preservation. Age dating of this transition, as deduced by applying a 210 Pb-derived sedimentation rate, revealed concentric zones covering the entire basin floor, which accumulated laminated sediments from 400 (basin center) to 50 (shallower depths) years before present (ybp). This expansion in lamination is taken as evidence of expanding oxygen deficiency and is particularly interesting given the global and local changes mentioned above. Over the past 400 years of laminae accumulation in SMB, the southern California region has grown into one the world’s largest urban areas. Particularly notable was a change in ecosystem structure of benthic shelf fauna during the mid- to late 1800’s, which was attributed to an onset of higher coastal sediment delivery caused by grazing cattle (Tomašových and Kidwell, 2017). This new land use was proposed to increase the frequency and amount of sediment entering the coastal zone. Another notable anthropogenic impact is the introduction of sewage waste into the coastal system starting in the early 1900’s (Alexander and Venherm, 2003). Advanced treatment of this sewage did not start until the 1970’s. Furthermore, channelization of the LA River and construction of sediment- trapping flood basins up-river have occurred over the past century (See supplement section for LA land usage timeline). Thus, there is ample evidence of environmental change in and around the SMB over the past 150 years. 125 Starting with a study by Bruland et al. (1974), investigators have been using 210 Pb profiles of sediments as a means of documenting sediment accumulation and sediment mixing in the SMB. A compilation of core analyses was published by Huh et al. (1989), and further work by Alexander and Lee (2009) provided a record of sedimentation in the SMB from the 1970’s through the 1990’s. Our work here (conducted in 2016) aimed at augmenting this record of coastal sedimentation, quantified by analyses of 210 Pb and 14 C profiles. We sampled intact surface sediments (top ~30 cm) and also conducted analyses of (1) sediment fabric by x- radiography, (2) sediment macrofaunal composition, and (3) Corg content to study changes in sedimentation in the SMB over the past 150 years. Our study provides new information about sedimentation and the potential expansion or contraction of laminated sediments over the past 150 years with a focus on the past 40 years. 4.2 Study Area The San Pedro-Santa Monica Basins are ‘bathtub’-shaped basins, oriented north-west to south-east adjacent to the Los Angeles coastline. They are both approximately 900 m deep, separated by a sill. Water entering San Pedro Basin (SPB) from the south-east crosses the sill at ~740 m and then passes into the SMB (Hickey, 1991). Bottom water circulation below the sill depth is sluggish, < 1.0 cm/s and generally moves in a counter-clockwise direction. To the north- east of SMB is a slope and the broad Santa Monica shelf, which is incised by Redondo Canyon, in the south-east portion of the basin, and Santa Monica Canyon, which empties in the middle of SMB; Malibu and Pt. Dume Canyons drain into the northeast portion of the basin. Sedimentation 126 is characterized as hemi-pelagic, interrupted by sandy turbidites that primarily originate from the northeast canyons and spread onto the basin floor (Gorsline, 1992). The upper ocean waters (above 300 m) are a mixture of at least two distinctive water masses (Fig. 4.2) whereas the waters below sill depth have a T-S signature suggestive of mixing with a water mass that originates somewhere in the NW Pacific (Lynn and Simpson, 1987). All waters below 400 m are low in oxygen (<20 µM) although the deepest water sometimes has slightly higher concentrations compared to those immediately above (Fig. 4.2). This phenomenon is rare and identifies a basin ‘flushing’ event (Berelson, 1991). Generally, water enters SMB, and the sluggish circulation and slow rates of replenishment (deep water residence times on the order of 1-3 years; Hammond et al., 1990) tend to deplete oxygen further. Hence oxygen concentrations range between 1 - 9 µM (Berelson, 1985). Complete bottom water depletion of oxygen and/or the presence of sulfide in bottom waters has never been reported. The sediments of SMB have 15-20 wt. % CaCO3, 2-6 wt. % Corg and 2-8 wt. % SiO2 (Cheng et al., 2008). 127 Figure 4.2: Oxygen and T-S plot for SMB obtained in spring 2016 (MUC 9). 4.3 Methods 4.3.1 Water column and sediment sampling Temperature, salinity, and dissolved O2 concentrations were profiled in the water column of the SMB (0-907 m water depth) from aboard the RV Yellowfin (Southern California Marine Institute) in April 2016, using a CTD (Sea-Bird 25) with attached SBE43 oxygen sensor (calibrated by Winkler titration). For CTD calibration, automated bath systems, sensor stability, primary standards in temperature (water triple point and gallium melting point) and conductivity (International Association for the Physical Sciences of the Oceans: IAPSO) were maintained. Ten sediment cores were collected in April and July 2016 from eight stations (MUC 3, MUC 5- 11) between 319 and 907 m water depth, using a miniature multicorer (MUC, K.U.M. Kiel) equipped with four polycarbonate core liners (length: 60 cm, inner diameter: 9.5 cm). 0 200 400 600 800 0 50 100 150 200 250 Santa Monica Basin 2016 Depth (m) Oxygen (uM) 128 After cores were retrieved, one core was sectioned on shipboard in one-centimeter intervals through the upper 10 cm and two-centimeter intervals below 10 cm. Aliquots were sealed in porosity vials, and the remaining mud was placed in plastic bags. A second core from the same multicore deployment was preserved intact for x-radiography. 4.3.2 Porosity and Integrated Mass Wet mud from the sectioned core was placed in pre-weighed porosity vials (15 mL snap- cap glass vials), re-weighed and dried at 50°C for 48-96 hours. Vials were subsequently re- weighed to determine water loss. The dry weight was corrected for salt content, assuming a salinity of 35. Porosity was determined assuming a grain density of 2.5 g/cm 3 . Integrated mass to the mid-point of each sample interval was calculated from the porosity profile and this density, summing to numerically integrate eq. 4.1: 𝐼 = ∫(1−∅)𝜌𝑑𝑥 (4.1) where dx is the interval thickness, r is solid phase density, I is integrated mass, and ∅ is porosity. 4.3.3 Macrofauna Sediment from one of the cores collected from each site was used for faunal surveys. The first 5 cm of each core was sectioned into 1 cm intervals for the purposes of capturing faunal 129 variability near the sediment water interface. The remaining length of the core was sectioned into 2 cm intervals. The sediments from each interval were then washed with DI water through a 2mm sieve, and the residue collected. Macrofauna and meiofauna in each section were identified with the aid of optical light microscopy, and were preserved in an ethanol-glycol mixture (80% ethanol). 4.3.4 Organic Carbon Content Dried porosity samples were ground by mortar and pestle and this homogenized sediment was used for Corg, 210 Pb and 137 Cs analyses. A portion of the ground sediment was weighed (10- 150 mg) and was placed into a 10 mL exetainer tube and acidified with 10% phosphoric acid. The evolved gas was analyzed for CO2 using a Picarro CRDS, following procedures developed at USC (Subhas et al., 2015, 2017). This provided a measure of acid-reactive C, assumed to equal C bound as CaCO3. Another split of powder was weighed into tin capsules and combusted at 800° C on a Costech CN analyzer to measure total carbon, with the CO2 and d 13 C concentration also determined via the Picarro. USGS standards were used to calibrate wt. % Total C in samples. The difference between total C and CaCO3 carbon was taken as the % Corg. Replicates indicate analytical uncertainties in this measurement of ±0.2 wt. % Corg on samples that have 2-6 wt. % Corg. 4.3.5 Photographs and X-radiographs 130 Replicate cores from each multicorer sampling were photographed at University of California Los Angeles (UCLA). Cores returned to the University of Southern California (USC) and were stored for 2-4 months to air-dry, which allowed the sediment to lose water and consolidate. A router was used to remove a section of plastic core liner on opposite sides of the core tube. The core was split into two halves with smooth cut faces from top to bottom using a wire. One split core was transferred to a plastic tray with approximately a 2cm lip along the long edges. The wire was run along the top of the lip, yielding a uniform 2 cm thick slab of sample. Each slab was placed on a large sheet of Kodak film and x-rayed for 90-180 sec at 8 milliamps and 96 volts. Negatives were developed in a dark room. 4.3.6 Excess 210 Pb and 137 Cs Approximately 0.5- 1.0 g of dried, homogenized sediment was placed in 5 mL polypropylene test tubes for analysis by gamma spectroscopy. Excess 210 Pb and 137 Cs activities in sediments were measured using high purity intrinsic germanium well-type detectors (HPGe ORTEC, 120 cm 3 active volume). Detector efficiencies were determined by counting the activities of known standards in the same geometry as the samples. Standards used included IAEA-385 marine sediments, EPA Diluted Pitchblende SRM-DP2, and NIST 210 Pb liquid solution. Samples were counted for 2-4 days, and the spectra (keV) were analyzed for the following radioisotopes: 210 Pb (46), 214 Pb (295), 214 Pb (352), 214 Bi (609), and 137 Cs (661). The 226 Ra activity (termed the supported 210 Pb) was measured by counting the activity of the short- lived 222 Rn daughters ( 214 Pb and 214 Bi). A small 10% correction was applied to each sample to account for radon leakage, based on measurements of radon loss from similar sediments 131 (Hammond, unpub. data). Excess 210 Pb was determined by subtracting the supported 210 Pb ( 226 Ra, Fig. 6.8.1) from total 210 Pb activity and correcting for decay between collection and analysis (See Appendix 6.3 section for 210 Pb calibration). 4.3.7 Radiocarbon Radiocarbon values were measured using the accelerator mass spectrometry (AMS) at the University of California Irvine (UCI) Keck Carbon Cycle Accelerator Mass Spectrometry (KCCAMS) laboratory. Samples were subjected to HCl vapor for four hours to acidify calcium carbonate, dried on a vacuum line, combusted, graphitized and then counted on the AMS. Sample preparation backgrounds were subtracted, based on measurements of acidified glycine, ANU and Lycine. Radiocarbon results have been corrected for isotopic fractionation according to the conventions of Stuiver and Polach (1977), with 𝛿 13 Corg measured using a Costech ECS 4010 Analyzer - Delta V Plus IRMS at the University of California Riverside (UCR). The isotopic ratio is given in delta notation relative to Vienna Pee Dee Belemnite (VPDB) for 𝛿 13 C values. Glycene, peach, acetate and house soil were used as reference material, standard error (1𝛔) was <0.10‰. 4.4 Results Sediment porosity declined with depth, with generally higher values in cores collected at deeper stations (Fig. 4.3). At all sites, there was typically a porosity difference of ~0.2 between the sediment-water interface (SWI) and 30 cm depth horizon. However, several cores showed 132 notable interruptions in the monotonic decline in porosity with depth. Core MUC 9 and MUC 10 had intervals with lower porosities compared to the overall depth trend. Low porosity anomalies were observed below 25 cm in MUC 9 and at 13-15 cm, ~22, and ~28 cm in MUC 10. Figure 4.3: Porosity profiles for SMB 2016 MUC cores. Only three cores (of those collected at depths > 320 m) had macrofauna obtained from sediment sectioning and sieving (Table 4.1). Notable was the abundance of sponge spicule clusters found throughout much of core MUC 8. An intact annelid worm was found on the surface of MUC 11 (745 m), which had oxygen concentrations < 8 µM near the seafloor. 133 Weight percent Corg content of the upper cm of the cores collected in 2016 showed a distinct trend of increasing %Corg with water depth (Fig. 4.4). Basin sediments (MUC’s 9 and 10) had 5-6 wt. % Corg whereas slope sediments ranged from 2-5 wt. %. Cores collected in the 1970’s and 1980’s show the same trend for core top %Corg vs. water depth as the MUC cores (Gorsline, 1992; Fig. 4.4). Table 4.1: Macrofauna for selected SMB 2016 MUC cores. Bottom Depth Core ID Core Interval Description m cm 508 MUC 12 5-6 Annelid, Polychaete, Arenicola sp. 9-11 Porifera 695 MUC 8 0-1 Porifera, Demosponge-partially articulated 11-31 Porifera, Demosponge-abundant spicules 745 MUC 11 Annelid, Polychaete, Arenicola sp. Figure 4.4: %Corg content for the 0-1 cm intervals from MUC cores (red triangles) and data from Gorsline (unpublished box core results). Box core data also represent the upper (0-2 cm) sediment Corg fraction. 134 Photographs of MUC cores showed light reddish-brown colored sediment near the surface of each core and a progression in MUC 9 and 10 toward darker colored sediment with depth (Fig. 4.5). Only MUC 9 (907 m) had laminations visible by eye. The sediment in the upper 10 cm from other cores (MUCs 10, 3, and 11) appeared homogeneous. MUC 11 showed a living polychaete worm present at the sediment-water interface. Figure 4.5: Photographs of selected 2016 MUC cores. X-radiographs of MUC 9 and MUC 10 revealed distinct laminations (Fig. 4.6). MUC 9 showed clear sediment laminations down to approx. 15 cm. However, MUC 10, collected from a site only 14 m shallower did not show fine lamination, but broader banding was apparent down 135 to 12 cm. Both cores showed zones of higher density material (light-colored in x-radiograph negative). A distinct higher density zone is seen in MUC 9 below 25 cm. Three zones of dense material were detected in the MUC 10 x-radiograph; the first was between 12-17 cm, the second at ~22 cm, and the third below 28 cm. These zones of higher density material correspond with the zones of anomalously low porosity (Fig. 4.3). Figure 4.6: X-radiographs of cores MUC 9 and 10. Arrows designate location of turbidites which show up in x-ray as lighter colored (denser). Also note the fine laminations visible in MUC9. 4.4.1 Excess 210 Pb and 137 Cs 136 Values of excess 210 Pb in surface sediments varied from 25 dpm g -1 at the shallow water sites to 100 dpm g -1 in deeper waters near the mid-basin (Fig. 4.7). Many of the cores from the shallower sites (<800 m) showed a constant activity of excess 210 Pb in the top 1-3cm, below which activity decreased exponentially. MUC 8 deviated from this trend and showed an increase in excess 210 Pb at 9 cm. MUC 9 and MUC 10, which are the two cores in the central basin collected from water depths greater than 850 meters, showed high values of excess 210 Pb near the surface and an exponential decrease below the sediment-water interface. Excess 210 Pb in these two cores was restricted to the top 8 cm, whereas excess 210 Pb penetrated deeper into the sediment of cores from the basin slope (MUC’s 5, 6, 7, 8, and 3). 137 Cs profiles of MUC 9 and MUC 10 showed peaks between 4.5 and 2.5 cm depth, respectively (Fig. 4.8), whereas 137 Cs profiles of cores taken along the slope showed very low values with large uncertainties. 137 Figure 4.7: Eight multi-cores sampled for 210 Pb in the Santa Monica Basin. The points are plotted in the middle of the depth interval (given in Table 2). Note that the depth and activity scales are slightly different for different cores. 138 Figure 4.8: Eight multi-cores sampled for 137 Cs in the Santa Monica Basin. MUC 9 and MUC 10 were the only cores with a clear 137 Cs peak. All other cores had no defined peak. 4.4.2 Radiocarbon and d 13 Corg The organic carbon from selected intervals from MUC 9 and MUC 10 was measured for radiocarbon content and d 13 Corg to depths of 25 centimeters (Fig. 4.9, Fig. 4.10). ∆ 14 C (BP)* and d 13 Corg values were plotted vs. integrated mass to provide a normalization for the downcore porosity changes that occur downcore. ∆ 14 C (BP)* indicates a conventional radiocarbon age that was determined using the method of Stuiver and Polach (1977). A reservoir age adjustment was not applied to the ∆ 14 C (BP)* values. Between the depths equivalent to 2 to 6 mass units (g cm -2 ), there is a linear relation between age and integrated mass (Fig. 4.9), consistent with an assumption that reservoir age and mass accumulation rate at these sites remained constant 139 through this interval. In both cores, these intervals were fit with a regression to determine mass accumulation rate for the studied time period (depth ranges 7-16 cm in MUC9 and 7-14 cm in MUC10). Calculations of radiocarbon sedimentation rates for MUC 9 and 10 yield values of 9.0 and 12.0 mg/cm 2 -yr, respectively, spanning an interval of about 400 years between 2 and 6 mass units. This calculation excluded samples in the upper 2 integrated mass units due to apparent bomb 14 C contamination, as both cores show a much younger value of ∆ 14 C in the upper 1 cm of sediments relative to the profile below this depth. Below the zone that was fitted, ∆ 14 C (BP)* values for MUC 10 were quite erratic, due to several turbidites that were noted in this core. Turbidite influence is also evident in the d 13 Corg profiles (Fig. 4.10), introducing carbon that is isotopically lighter than the material immediately above and below. All 14 C values below 6 integrated mass units were deemed to have turbidite influence. and were also excluded from the fit. Figure 4.9: ∆ 14 C (BP)* vs. Integrated Mass (g/cm 2 ) for SMB cores MUC 9 and MUC 10. ∆ 14 C (BP)* denotes conventional radiocarbon age without assigning a reservoir age. If reservoir age 140 and mass accumulation rate are unchanging, the plots should be linear. A linear fit was applied to the solid circles (lying between 2 and 6 mass units, horizons 5-18 cm depth range). Data from above 2 mass units and below 6 mass units were excluded from the fits as they appear to be influenced by “bomb” carbon or turbidites (see Fig. 10). The regression slope defines mass accumulation in mg cm -2 y -1 . The depth equivalent to 2 g/cm 2 represents an age of approximately 120 years B.P. Figure 4.10: ∆ 14 C (BP)* vs. Integrated Mass (g cm -2 ) as in Fig. 9, including designation of turbidite and bomb carbon region (hachured) and corresponding d 13 C data. Dashed horizontal lines refer to year 1900 CE. 141 4.5 Discussion 4.5.1 Excess 210 Pb as a measure of sedimentation rate 210 Pb has proven to be a useful tracer for sediment accumulation rates in the Santa Monica Basin (Bruland et al., 1974; Huh et al., 1989; Christensen et al., 1993) and similar environments (Souza et al., 2012) during the last 100 years. Past studies derived mass accumulation rates (MAR) rates using 210 Pb by assuming a constant sedimentary flux of 210 Pb over the time scale concerned (~100 years), negligible bioturbation, and strong absorption of 210 Pb to particles (constant initial concentration method; Benninger and Krishnaswami, 1981; Robbins and Edington, 1975; Robbins, 1978; Appleby, 2001; Oldfield and Appleby, 1984). These assumptions should be valid in the deepest parts of the SMB where sediments are minimally disturbed by bioturbation as shown by laminations. Table 4.2 shows a compendium of mass accumulation rates for the central portion of SMB, obtained from cores collected during a 42-year interval from 1974 to 2016. MAR values were taken directly from Bruland et al. (1974) Huh et al. (1989), and Christensen et al. (1993), and all studies accounted for sediment compaction. All cores collected from depths >900 m showed MARs that were remarkably consistent, averaging 17.1±0.6 mg/cm 2 -yr (± 1 SDOM). There was also no noticeable trend in MAR (Fig. S3) or variation in the amount of excess 210 Pb at the sediment-water interface over time. Additionally, excess 210 Pb profiles were similar in structure downcore. All cores (Fig. 4.11), except for those obtained in the present study, were retrieved by box corers, which can disturb the top few centimeters of sediments (Huh et al., 142 1989). Yet all the cores collected from the deep basin showed remarkable consistency, with no evidence of sedimentation rate change between the 1970’s and 2016, as well as no evidence of core disturbance. Table 4.2: Station ID, year collected, mass flux, depth, inventory and excess 210Pb at sediment water interface (SWI) for all cores greater than 800 meters depth in the Santa Monica Basin. The first 13 cores are from deeper than 900 meters, and the last 8 cores are from 800-900 meter water depths. 210 Pb Inventory was also computed but values are not discussed in this article (* indicates a graphical integration was used, others are from fitting parameters). References: [1] = this work, [2] = Christensen et al., 1991, [3] = Huh et al., 1989; [4] = Bruland, 1974. Year Mass Flux Depth Excess 210 Pb @ SWI Inventory Collected Station ID mg/cm 2 -yr m dpm/g dpm/cm 2 Reference >900 m depth region 2016 MUC-9 16.8 ± 0.2 907 140 71 [1] 1990 DOE 65 13.6 ± 0.3 910 200 90 [2] 1988 DOE 25 20.8 ± 3.8 904 70 59 [2] 1988 DOE 26 17.7 ± 0.3 904 190 110* [2] 1987 CaBS V BC6 18.8 ± 0.8 910 163 114* [3] 1987 CaBS V BC7 18.5 ± 0.8 910 190 111* [3] 1987 CaBS V BC3 15.8 ± 0.9 906 160 76* [3] 1986 CaBS III BC31 14.9 ± 0.5 910 174 88* [3] 1986 CaBS III BC 1 15.8 ± 0.1 910 182 82* [3] 1985 CaBS I BC102 16.6 ± 1.6 910 159 92 [3] 1985 CaBS I BC89 14.1 ± 0.5 908 145 100 [3] 1976 AHF 25842 17.8 ± 1.4 902 97 53* [2] 1974 Bruland, 1974 20.7 ± 1.0 903 94 69 [4] Average (±SDOM) 17.1 ± 0.6 800-900 m depth region 2016 MUC-10 14.1 ± 0.8 893 120 54 [1] 1990 DOE 49 19.1 ± 1.2 890 200 111* [2] 1988 DOE 31 13.3 ± 0.4 890 130 78* [2] 1988 DOE 27 20.1 ± 1.6 860 120 90 [2] 1988 CaBS X BC3 17.0 ± 0.7 890 160 89* [3] 1988 CaBS X BC2 29.3 ± 2.1 870 107 96* [3] 1987 CaBS V BC 8 16.8 ± 0.8 880 N/A N/A [3] 1976 AHF 25511 15.1 ± 1.4 879 120 68* [2] Average (±SDOM) 17.9 ± 1.9 143 Figure 4.11: Semi-log plot of Excess 210 Pb activity vs. integrated mass for thirteen cores sampled in the Santa Monica Basin between the years 1974-2016. All 13 cores are from depths greater than 900 meters. The linear fit to these plots yield slopes that define the mass accumulation rate (see Table 2). Arrows on the y-axis indicate the integrated mass equivalence to the year 1900 CE. We also compared 210 Pb profiles in cores retrieved from water depths 870-900 m (Fig. 4.12) to those collected from deeper sites as to determine if a trend exists with water depth. These cores showed the same MAR as the deeper sites, although with more variation evidenced in the larger standard deviation of the mean (17.9±1.9 mg/cm 2 -yr). However, as with the deepest cores, we observed no systematic change as a function of year collected (Fig. 6.8.2). Much of the variability in MAR was driven by CaBS X BC2, which was collected at 870 m. Core CaBS V BC8 had a clear 210 Pb minimum in the upper 10 cm and featured a ‘typical’ 210 Pb profile only below this depth. The minimum and the offset of the extrapolated fit for the deeper points from the surface values suggest rapid input of material with low excess 210 Pb, most likely from a 144 localized turbidite in this core. The eight cores collected from 870-900 meters showed surface excess 210 Pb that were similar to cores collected from sites >900 m. Although cores from the shallower depth range averaged the same MAR as the deeper cores, the quality of the linear fit of excess 210 Pb versus integrated mass, as demonstrated by the average R 2 value, was poorer for cores 870-900 m (average R 2 = 0.90) compared to cores collected at depths of >900 m (average R 2 =0.99), suggesting either episodic input of sediment with varying excess 210 Pb or possibly minor episodic disturbances. Figure 4.12: Same as Fig. 4.11 but for 8 cores obtained from depths between 870-900 meters. 4.5.2 Changes in the areal extent of laminated sediments 145 Christensen et al. (1993) and Huh et al. (1989) documented the concentric areal expansion of laminated sediments throughout the floor of SMB starting about 400 years B.P. Both studies determined that the onset of anoxia began in the south-east portion of the central basin, where the basin is deepest (> 900 m) and moved outward, asymmetrically, but in all directions (Fig. 4.13). Using the presence of fine laminations as a proxy for oxygen deficiency and establishing the onset of lamination by assignment of age, a ‘lateral’ anoxic spreading rate of 50-80 m yr -1 was calculated (Christensen et al., 1993). Depending on the direction chosen, the rate of anoxia spreading in vertical space varied, from 0.06 m/year up the eastern slope to 0.19 m/year moving in an NNW direction (Fig. 4.13). This asymmetry may be attributed to the major circulation pattern of deep basin water, in which waters from the San Pedro Basin enter SMB from the south-east and travel counter-clockwise. In such a flow, the eastern slope of the SMB would be bathed by overlying waters with slightly more oxygen than waters on the north-north- west side of the basin. The overall expansion of anoxic waters may reflect both a reduction of oxygen in waters entering the basin, as well as increased oxygen consumption within deep basin waters. The latter could arise from either an increase rain-rate of labile carbon, or a reduction in water replacement rates. 146 Figure 4.13: Spreading of the laminated sediments area, defined as a change in depth over time for two transects. Left panel shows the spreading rate of laminated sediments as they progress upslope (shallower depths) moving toward the recent. The figure on the right is modified from Christensen et al. (1994) based on his time scale and shows the growth in areal extent of laminated sediments in the past 300 years ago. The location of three MUC cores obtained in 2016 are also shown. The expansion of laminated sediment accumulation has occurred more rapidly in the NW direction than in the Eastern transect. Only two of the 2016 cores analyzed in the present study showed sedimentary layering in x-radiographs (MUC 9 and MUC 10). The other cores from this study (near the SMB slope) had no laminations and were likely influenced by mixing. For the deepest core, MUC 9, there was clear evidence of finely-laminated sediments in the top 15 cm (Fig. 4.6). MUC 10, which is located in the southern SMB, near the connection to San Pedro Basin, showed a banding (1-2 cm width) of sediments in the upper few cm of the core but no fine lamination, suggesting minimal bioturbation. Given MUC 10’s location in relation to the spread of oxygen deficiency, the absence of finely-laminated sediments in the upper few cm suggests that the spread of oxygen deficiency has not extended to this location. Furthermore, MUC 3 (777 m), which is right at the boundary of the zone of oxygen deficiency defined by Christensen et al. (1993), had no indications of laminations, and 210 Pb clearly showed a mixed zone in the upper 4 cm (Fig. 4.7). 147 These two MUC cores make it tempting to suggest the oxygen deficiency zone is contracting; however, we can conclude with confidence that the position of the laminated zone in SMB has not changed markedly since cores were last obtained and analyzed in the 1980’s. 4.5.3 Changes in mass accumulation rates—A comparison of 210 Pb and 14 C methods Interpretation of 210 Pb and 14 C profiles in terms of sediment accumulation rate rely on assumptions that the delivery of these radiotracers have been consistent and continuous, and that the sediment has not been disturbed via mixing (typically bioturbation). The assumption of consistency is generally assumed to be true in basins that receive sediments via hemipelagic sedimentation, and the assumption of non-disturbance is supported by sediment fabric as revealed by x-radiography (Fig. 4.6). The similarity of 210 Pb profiles in core MUC 9, which shows fine lamination structure in the top 8 cm, and core MUC 10, which shows coarser sediment banding, is evidence that some minor disturbance in the latter core may have obscured lamination structure, but disturbance has been insufficient to change the 210 Pb profile. Both of these cores yield similar sediment accumulation rates, ~17 mg/cm 2 -yr, and show no evidence for a change in sedimentation rate over the lifetime of 210 Pb, which is approximately 80-100 years. Because the Bruland (1974) core does not show any evidence of a change in sedimentation rate through the life of 210 Pb, we can conclude that sedimentation has been constant in SMB since around the late 1800s to early 1900s. We find it striking that sediment accumulation offshore from an urban center has 148 remained constant, even though the region has grown from a small town to the present 15+ million-person megalopolis of Los Angeles. While accumulation rates remained constant during the period of rapid population growth in Los Angeles, the 14 C accumulation rates, not including those horizons that lie within turbidite deposits, define a sedimentation rate for the period ~ about 1500-1900 C.E. that is less than that defined by 210 Pb. In both MUCs 9 and 10 cores, 14 C dated sediment horizons yield sediment accumulation rates of 9-12 mg/cm 2 -yr compared to 17.1 ± 0.6 derived from 210 Pb profiles (Table 4.2). This trend of increasing sedimentation toward the recent is opposite of what might have been predicted due to the trapping of sediment via flood-control engineering of the LA River. However, our data are consistent with the proposal made by Tomašových and Kidwell (2017) noting that sometime in the mid-late 1800s sediment delivery to the coastal zone of the SMB increased. Tomašových and Kidwell (2017) based their interpretation on the change in the SMB shelf ecosystem structure that occurred at that time. From the loss of a filter-feeding ecosystem from the SMB shelf environments, Tomašových and Kidwell (2017) infer an increase in fine sediment delivery to the SMB shelf. The determinations of 14 C sediment accumulation rates could be biased or incorrect if there has been a changing input of particulate organic matter (PO 14 C) to the SMB. In MUC 10 there is an obvious section of core where 14 C age dates are old and d 13 Corg values are light, relative to the trend defined by the other data. However, these two measurements are from a turbidite deposit (Fig. 4.10) and are consistent with the interpretation that such a deposit contains older, terrestrially derived (perhaps more refractory) POC (Meyers, 1994). MUC9 may also 149 show a minor influence from this turbidite, but the effect is subtle. A plot of d 13 Corg versus integrated mass of MUC 9 and MUC 10 show a trend to slightly lighter d 13 Corg near the top, although the change is very small (Fig. 4.10). The 14 C profiles for both cores appear to show slightly older sediments than expected, just above the two mass unit horizon, suggesting a change to additional input of older carbon associated with the modest change in d 13 Corg. The changing trend could record a terrestrial source, but the data are not clear-cut. While there may have been a change in the source of carbon (and sediment) in the late 1800s, the data prior to this time indicates there has been a step-function change in sediment accumulation rate, taking place sometime between the late 1800’s to the early 1900’s. A sensitivity calculation assuming a step- change reduction of 40% in accumulation rate in 1930 (two half-lives before the Bruland (1974) core) shows 210 Pb has the sensitivity to resolve such a change (computed profile not shown). Consequently, the change in accumulation rate must have occurred prior to 1930. It is tempting to suggest that changes in carbon reservoir age or the age of waters upwelling in this region, instead of sedimentation rate, could explain the offset of 14 C values down core. However, if the sedimentation rate determined from the excess 210 Pb profile at MUC 9 is assumed constant down-core to a depth represented by 2-6 mass units, then 236 years would have elapsed during this interval (4 g/cm 2 /0.017 g/cm 2 -yr=236 yr). If the 210 Pb MAR applies through this interval, and the 14 C values record changes in reservoir age and not sedimentation rate, the age for organic carbon (fixed at the surface ocean from DIC) would need to increase by 160 years, at a steady rate, over the time period represented by 2-6 mass units. While this cannot be dismissed, it would imply a higher upwelling rate in the past, and there seems no evidence for this. 150 Another explanation for the lack of 14 C MAR and 210 Pb MAR agreement is that there was a higher proportion of old terrestrial carbon reaching the sediments during the past. However, the lack of a significant change in d 13 Corg through this interval makes this process an unlikely explanation. We think it most likely that an increase in MAR occurred somewhere in the late 1800’s and propose that further 14 C analysis of laminated sediments, preserved under low oxygen conditions, is the best way to find further support for this conclusion. A previous study that considered Holocene sediment accumulation in SMB (Romans et al., 2009) found that the hemipelagic sediment accumulation rate for the late Holocene averaged ~10 mg cm -2 yr -1 (this rate determined from their linear sediment accumulation rate and the extrapolation of our porosity data to a depth of 2 m), although the turbidite accumulation rate was substantially greater. This is a value consistent with the MAR we found from the 14 C dated section of our cores, only 150-300 years before present. Thus, it appears that hemi-pelagic sedimentation in SMB has been very consistent over the past 1000’s of years but has increased by ~70% through a stepwise change about 100-150 years ago. 4.5.4 Biological Activity in Low Oxygen Environments Only three cores analyzed for this study had macrofauna present, these were MUC 12 (508 m), MUC 8 (695 m) and MUC 11 (745 m). All three cores were collected from bottom waters with <20 µM oxygen concentration and the deeper two sites have <10 µM oxygen. The 151 living annelid found in MUC 11 is evidence that macrofauna can be active and hence potentially act to bioturbate at low oxygen levels (<5 µM). A preponderance of sponge spicules was found in replicate cores from the location of MUC 8. This is also a site bathed in waters with <5 µM oxygen. In addition to the core sectioned for biological inspection, a core that was x-radiographed shows the presence of a partially articulated demosponge within the sediment column at ~8 cm depth (Morine, 2017). These sponges are not infaunal, thus the most plausible explanation for the high spicule abundance in these cores is that this sediment zone has been populated by sponges for >100 years. Prior to the work of Christensen et al. (1993), Malouta et al. (1981) mapped out the area of bioturbation throughout the SMB using x-radiographs of basin cores. Using disturbances in laminated sediments as a proxy for different levels of bioturbation, 3 different zones were assigned: completely disturbed laminae, partially disturbed laminae, and fine laminae present. Completely disturbed laminae were cores that showed no laminations or banding and were usually found on the shelf and slopes of the SMB, typically shallower than 750m. Partially disturbed laminae were characterized by some hints of banding and suggested minimal bioturbation. Lastly, finely laminated sediments were zones of no bioturbation and were located in the deep, central basin at depths greater than 900m. The areas to which Malouta et al. (1981) assigned these zones of bioturbation correlate with our cores obtained in 2016, suggesting minimal changes in organism activity vs. depth during the last 40 years. Additionally, our work shows that laminae can be largely obscured and yet a 210 Pb profile from a slightly bioturbated 152 core (MUC 10) can appear nearly indistinguishable from a profile from a well-laminated core (MUC 9). 4.6 Conclusions A suite of cores was collected in 2016 to explore whether changes in the areal extent of laminated sediments and their mass accumulation rates have changed during recent decades. Only one core analyzed in 2016 showed finely laminated sediments in x-radiographs (MUC 9 at 907 m). Other cores showed cm-scale layering of sediments or no layering at all. The absence of finely laminated sediments in MUC 10 (893 m) and MUC 3 (777 m) suggest that the rate of oxygen deficiency-spreading, as noted by Huh et al. (1989) and Christensen et al. (1993) has not increased remarkably since cores were last collected in the 1980’s. It is possible that the rate of anoxic bottom water spreading has declined or even possibly reversed with a slightly shrinking area of laminated sediments. X-radiographs of laminations from cores collected in this study were compared to the different levels of bioturbation mapped out 40 years ago in the SMB. The zones of bioturbation correlate with cores collected in 2016, again suggesting minimal change in macrofaunal activity (assumed a proxy for bottom oxygen concentrations) during the last 40 years. Through a summary of previously published profiles and new measurements of 210 Pb in sediment cores from this study, a comparison of mass accumulation rate records in the central portion of SMB was examined in cores collected over a 42 year span. Mass accumulation rates for the deepest parts of the SMB basin (>900m) have been remarkably consistent since the late 153 1800s, averaging 17.1 ± 0.6 mg/cm 2 -yr. At slightly shallower sites (870-900m), accumulation rates showed a little more variability, but yield the same accumulation rate, averaging 17.9 ± 1.9 mg/cm 2 -yr. Excess 210 Pb near the sediment-water interface was also consistent for all cores deeper than 870 m during the last 4 decades. The consistency of sedimentation rates, both for the past 40 years but also for the lifetime of 210 Pb, ~100 years, is remarkable given the changes that have occurred in the Los Angeles region over the past century. ∆ 14 C values between 7 and 20 cm depths suggest sediment accumulation rates were lower prior to the late 1800s. MUC 9 and MUC 10 reveal sedimentation rates of 8.6 and 12.0 mg/cm 2 - yr prior to the late 1800s, which is 55-75% of the rates determined for younger sediments using the excess 210 Pb profiles. The slower accumulation rate for hemipelagic sediments also occurred during the late Holocene (Roman et al., 2009). The increase in MAR appears to be a step- function change, although the precision of the dating methods can only constrain the transition to somewhere between about 1850 and 1920. A possible explanation, offered by Tomašových and Kidwell (2017), is that sedimentation increased between 1850-1900 due to the rapid rise of cattle grazing and increased erosion. Why this increased rate remained high after urban development, and why it should have remained so constant subsequently, are unanswered questions, particularly following installation of debris basins that trapped a large portion of the sediment flux. Perhaps these basins largely captured coarse debris, while the fine sediment fraction that contributes to hemi-pelagic input has not been captured, but its input was augmented by cattle grazing and subsequent urban development. 154 Evidence of sedimentary change in the SMB during the last 40 years is astonishingly absent. Mass accumulation rates, laminated sediments, extent of bioturbation, and % Corg have changed little during this time. The only parameter that appears to have clearly changed in the last 200 years is the sedimentation rate, which shows a step-function increase in the late 1800s to early 1900s. 155 4.7 References Alexander, C. R. and Lee, H. 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Morine, Laura.: Laminations, Organic Carbon, and Carbonate Content as Indicators of Anoxic Zone Shifting in the Santa Monica Basin, Senior Thesis, University of Southern California, Los Angeles, CA. 2017. Oldfield F., P.G. Appleby.: Empirical testing of 210 Pb-dating models for lake sediments. E.Y. Haworth, J.G. Lund (Eds.), Lake Sediments and Environmental History, Leicester Univ. Press, pp. 93–124., 1984. Quinn, W.H., Zopf, D.O, Short, Kuo Yang, R.T.W.: Historical trends and statistics of the Southern Oscillation and E1 Nifio, and Indonesian droughts. Fishery Bull., 76: 663-678. 1978. Romans, B. W., Normark, W. R., Mcgann, M. M., Covault, J. A. and Graham, S. A.: Coarse- grained sediment delivery and distribution in the Holocene Santa Monica Basin, California: Implications for evaluating source-to-sink flux at millennial time scales, Geological Society of America Bulletin, 121(9-10), 1394–1408, doi:10.1130/b26393.1, 2009. Robbins, J. 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M.: Nineteenth-century collapse of a benthic marine ecosystem on the open continental shelf, Proceedings of the Royal Society B: Biological Sciences, 284(1856), 20170328, doi:10.1098/rspb.2017.0328, 2017. 159 Chapter 5: Summary 5.1 Dissertation Summary The work presented in chapter 2 discusses the geochemical behavior of 227 Ac, 228 Ra, and 226 Ra in deep-sea sediments. The disequilibrium between these radionuclides and their parents provided a framework to study dynamics of their behaviors in Northeast Pacific Ocean sediments. Rates of sedimentation (S) and bioturbation (Db), distribution coefficients (kd), fraction released by parent decay (F), and molecular diffusion (Ds) were all measured or derived from models in order to calculate fluxes of these radionuclide from bottom sediments. These fluxes were then used in chapter 3 to constrain transport rates in the overlying water column and bring insight into mixing rates and circulation pathways along the GP15 transect. This was the first study to measure the source function of 227 Ac along a transect and compare it to the integrated decay of 227 Acex in the water column. Nozaki et al. (1990) was the only previous study to measure 227 Ac in sediments, but it was only done at one location. This study used an approach similar to that of Nozaki et al (1990) and applied it to 5 stations along the northern end of the GEOTRACES GP15 transect. Besides using Nozaki et al. (1990) approach, which used a reaction-transport model to characterize 227 Ac behavior in sediment, this study also used an additional method: core incubation. The core incubation method has been used successfully for measuring Ra isotopes in coastal environments but has never been applied to deep-sea sediments or 227 Ac. The reasons are that the concentrations of 227 Ac in coastal environments are very low and unlikely to produce any signal via RaDeCC. However, the 227 Ac signal in deep-sea 160 sediments is greater and, if sediments incubate long enough, the benthic flux may provide a signal that can be measured by RaDeCC. This dissertation was fortunate enough to be able to incubate 227 Ac for at least 10-14 days on the C-Disk-IV cruise. The 227 Ac flux results for core incubation agree within uncertainty with the reaction-transport model based on measurements of solid phases in sediments. Furthermore, both methods agree with the observed deficiency of 227 Ac in sediments ( 231 Pa – 227 Ac). The agreement of these methods indicates that incubating a multicore can provide a well constrained (20-30%) sediment flux for 227 Ac. The analyses of radioisotope profiles in sediments provided some unexpected results. 230 Th balances for sediments indicated that the majority of the sites analyzed have a smaller inventory than supplied by decay in the water column, indicating that significant winnowing of sediment raining from above must occur. Furthermore, the sites with the greatest winnowing also showed higher 227 Ac activities in the upper sediments than expected, suggesting that some exhumation of material from depth has been occurring. The mechanism causing this behavior is unclear. Chapter 3 discussed 227 Acex in the water column along the GP15 transect in the Eastern Pacific. While benthic inputs of 227 Ac for the northern leg of the GP15 transect was constrained by measurements made in chapter 2, the southern leg had only rough estimates of 227 Ac fluxes, based on measurements of its 231 Pa parent and estimates for other critical variables in core tops. The water column along the GP15 transect shows equilibrium between 227 Ac and 231 Pa from 0 - 3000 meters depth. Below 3000 meters depth, 227 Ac shows excess over its parent. This excess 227 Ac is being sourced from deep-sea sediments, which has been well documented from past 161 studies. The GP15 transect follows this expectation of 227 Ac water column profiles. However, the distribution of 227 Ac in the water column at each station were not all the same and indicated influential variables that controlled its distribution. Three types of 227 Ac water column profiles were seen for the GP15 transect: (1) an expected, exponential decrease of 227 Acex away from the seafloor; (2) a well-mixed bottom layer about 500m thick, with very little 227 Acex above; (3) and lastly, an unexpected, erratic distribution of 227 Acex that has local maxima in 227 Acex overlying bottom waters of lower concentration. The first profile can be considered an “ideal” profile and only a few profiles north of 40˚N showed this type of distribution. A similar profile was found by Nozaki (1984) and that study theorized that 227 Acex profiles could be used to find apparent vertical eddy diffusivity rates (Kz). However, apparent Kz rates obtained in this thesis were very large compared to expectations for diapycnal diffusivity based on estimates of turbulence (Kunze et al., 2006; Hautala, 2018), differing by more than an order of magnitude. The inclined isopycnals indicate that vertical and horizontal diffusivities both affect the large apparent Kz values (Sarmiento and Rooth, 1980), with the isopycnal component dominating the apparent vertical Kz values. The apparent Kz value can be useful for vertical solute transport if tracers have a bottom source function that is proportional to that of Ac, but will overestimate buoyancy fluxes (Sarmiento and Rooth, 1980). At GP15 station’s 12 and 14, a bottom mixed layer was present, reflecting areas with little density stratification. The 227 Acex mixed layer thickness matched closely with the mixed layer defined by neutral density plots, typically a layer about 500 meters thick. 162 The third profile type indicated an erratic distribution of 227 Acex. This was the most common throughout the GP15 transect, particularly in the southern half of the GP15 transect. The maxima appear to be influenced by local topographic highs that provide a localized supply 227 Ac that is then transported laterally with little vertical mixing. These mimic excess 222 Rn profiles that have been described by Chung (Chung and Craig, 1971; Chung and Kim, 1980). A survey of topography vs depth in the surrounding region supports this interpretation. In some cases, mid- depth maxima in 227 Ac are also seen, possibly reflecting hydrothermal sources. However, the depth of these maxima do not perfectly match d 3 He maxima horizons. The mismatch in position might indicate the age of the hydrothermal source reflected by 227 Ac may be only a century, while the effective age of the d 3 He plume may be much longer, as it recirculates or reflects multiple sources with a range of ages. The power of 227 Ac as a tracer for solute transport was demonstrated most noticeably in the Northeast Pacific: by comparing the integrated decay of 227 Acex in the water column to its well- constrained sediment flux, roles of horizontal and vertical transport can be studied. Eastward horizontal transport of 227 Acex is significant between 40˚ - 30˚N, where water column decay is smaller than benthic input as waters from regions with greater depth (and thus lower 227 Ac) replace waters that should have higher 227 Ac. Areas where E-W horizontal transport is not significant is north of 40°N and between 30˚ - 10˚N. In these areas, water column decay is comparable to benthic input. This pattern is consistent with predictions from an inverse model in the Northeast Pacific (Hautala, 2018) that indicates deep-water advection at 150°E is toward the East and strongest between 40˚ - 30˚N. North of 40˚N, the model suggests that circulation is moving in a north-south direction. Near the equator, the minimum in deep-water Ac matches the 163 lower inputs localized at low latitude, in agreement with the interpretation of Talley and Reid (Talley et al., 2003) that abyssal currents in this region largely parallel the equator. However, lateral diffusion appears to homogenize the water column distribution slightly. Chapter 4 used naturally occurring 210 Pb and its disequilibrium between 226 Ra to bring insight into sediment dynamics in the Santa Monica Basin (SMB). The study examined ten sediment cores, collected from different parts of the SMB between spring and summer 2016, and compared them to previously collected cores in order to document changes in sedimentary dynamics during the last 250 years, with an emphasis on the last 40 years. In the deepest part of the Santa Monica Basin, where fine laminations are present up to about 450 years B.P., 210 Pbex mass accumulation rates (MAR) have been remarkedly consistent during the last century (17.1 mg cm -2 yr -1 ). Prior studies have consistently measured MAR in this basin during the last 50 years with the use of 210 Pbex and have found little to no change in sedimentation rates as well during the last century. This is surprising, since the city of Los Angeles is adjacent to the SMB, and a period of massive urbanization during the last century would seem likely to disrupt this ecosystem. However, little change was found in this study. The accumulation rates on millennial time scales can be assessed with a longer-lived isotope. 14 C profiles were also examined in the SMB and indicate that MARs were slower prior to the year 1900 CE (10.5 mg cm -2 yr -1 ). The increase in sedimentation rate towards the recent occurs at about the time that Kidwell (Tomašových and Kidwell, 2017) predicted an increase in siltation that led to the demise of a shelly shelf benthic fauna on the SMB shelf. Her study attributed the siltation increase to changes in land usage in the Los Angeles basin, namely cattle grazing and 164 agriculture that began around the middle of the 19 th century. This study was left with one big question: if the sedimentary dynamics in the SMB change so drastically during the 19 th century, why wasn’t there any changes detected during the 20 th century when mass urbanization began? Thus, the post-1900 CE constancy of sedimentation through a period of massive urbanization in Los Angeles is surprising. 5.2 Future Work Based on the findings from this dissertation, the following recommendations for further research might augment deeper understanding of processes identified by this thesis: (1) Measuring 227 Ac sediment profiles along the entire GP15 transect would help to better constrain 227 Ac F values, which were wide ranging for the C-Disk-IV transect (5 – 94%). Radium’s F values were consistent with prior studies and showed values around 50%, which is the theorized value for F. It is surprising Ac does not have the same F values as Ra. This will need to be investigated by obtaining additional 227 Ac sediment profiles and applying reaction- transport models to see if other areas show a similar range of F values (2) Obtaining additional 227 Ac sediment profiles or core incubations along the remainder of the GP15 transect would better constrain transport rates in the water column, especially in the southern half of the transect where 227 Ac sediment profiles were not measured. 165 (3) Preforming more lab measurements for Ac kd values would help constrain the relationship between Ra and Ac kd values. This study only measured 10 samples and more samples are needed from different deep-sea environment (i.e., CaCO3 rich sediments). (4) The next 227 Ac studies would benefit greatly by measuring higher resolution 227 Ac water column profiles in the deep ocean. The current sampling resolution for 227 Ac makes estimates of vertical transport difficult to constrain. The bottom 200 meters is sampled well, but above this horizon the sampling is sparse. Unfortunately, this is a compromise that must be made, given the limitations of GEOTRACES wire time for understanding the deep ocean. If the bottom 1000 meters were sampled like the surface, higher resolution sampling near the bottom could provide a better framework for the relative roles that density and topography have on the 227 Ac distribution, as well as other deep sea solutes. (5) Deciphering why sedimentation rates have remained constant for the SMB during a time of mass urbanization (last 100 years) would help answer a perplexing question that was left at the end of chapter 4. Why would sedimentation change during the 19 th century when cattle grazing and agriculture increased in the LA basin, but not the 20 th century when mass urbanization took place? This question might be further investigated by searching for the appearance of anthropogenic signals in the sedimentary record that might provide additional constraints on chronologic horizons. Possibilities include Eucalyptus pollen (and pollen from other vegetation changes), as well as industrial chemicals that might appear in the late 19 th or early 20 th centuries. 166 In addition, future studies could measure 210 Pbex profiles in nearby basins, like the San Pedro basin. It might be that wastewater runoff or flood control measures have affected nearby coastal basins, and this might be preserved in the sediment record. 167 5.3 References Chung, Y., and K. Kim. “Excess 222 Rn and the Benthic Boundary Layer in the Western and Southern Indian Ocean.” Earth and Planetary Science Letters 49, no. 2 (1980): 351–59. https://doi.org/10.1016/0012-821x(80)90078-3. Chung, Yu-chia, and Harmon Craig. “Excess-Radon and Temperature Profiles from the Eastern Equatorial Pacific.” Earth and Planetary Science Letters 14, no. 1 (1972): 55–64. https://doi.org/10.1016/0012-821x(72)90079-9. Hautala, Susan L. “The Abyssal and Deep Circulation of the Northeast Pacific Basin.” Progress in Oceanography 160 (2018): 68–82. https://doi.org/10.1016/j.pocean.2017.11.011. Kunze, Eric, Eric Firing, Julia M. Hummon, Teresa K. Chereskin, and Andreas M. Thurnherr. “Global Abyssal Mixing Inferred from Lowered ADCP Shear and CTD Strain Profiles.” Journal of Physical Oceanography 36, no. 8 (2006): 1553–76. https://doi.org/10.1175/jpo2926.1. Nozaki, Yoshiyuki, Masatoshi Yamada, and Hirofumi Nikaido. “The Marine Geochemistry of Actinium-227: Evidence for Its Migration through Sediment Pore Water.” Geophysical Research Letters 17, no. 11 (1990): 1933–36. https://doi.org/10.1029/gl017i011p01933. Nozaki, Yoshiyuki. “Excess 227 Ac in Deep Ocean Water.” Nature 310, no. 5977 (1984): 486–88. https://doi.org/10.1038/310486a0. Sarmiento, J. L., and C. G. Rooth. “A Comparison of Vertical and Isopycnal Mixing Models in the Deep Sea Based on Radon-222 Measurements.” Journal of Geophysical Research 85, no. C3 (1980): 1515. https://doi.org/10.1029/jc085ic03p01515. Talley, Lynne D., Joseph L. Reid, and Paul E. Robbins. “Data-Based Meridional Overturning Stream Functions for the Global Ocean.” Journal of Climate 16, no. 19 (2003): 3213–26. https://doi.org/10.1175/1520-0442(2003)016<3213:dmosft>2.0.co;2. Tomašových, Adam, and Susan M. Kidwell. “Nineteenth-Century Collapse of a Benthic Marine Ecosystem on the Open Continental Shelf.” Proceedings of the Royal Society B: Biological Sciences 284, no. 1856 (2017): 20170328. 168 Appendix: Supplementary Information A.1 Radiochemical procedures for 210 Pb, 227 Ac, 230 Th, and 232 Th for deep-sea sediment samples A.1.1 Sediment Digestion The method from Fuller (1982) to completely digest sediments from a cocktail of acids is followed. The only modification is the addition of a 229 Th spike with the 209 Po (t1/2 = 125 y) spike in the first step. The 229 Th ( 225 Ac) spike is the yield tracer for 227 Ac, 230 Th, and 232 Th, and 209 Po is the yield tracer for 210 Po. (1) First, add 209 Po and 229 Th spikes to a clean Teflon vessel (~50 mL Savillex PFA vessel). Tare vessel on mass balance and add spikes one at a time and record mass. Spike activity should roughly match the 210 Pb and 227 Ac activities in sediments. This can be estimated from HPGe detector results. (2) Weigh out roughly 1.0 gram of finely ground sediments on weighing paper (or aluminum foil) and add to Teflon vessel. Swirl and mix sediments/spike solution together. Good practice is to take a split of the same sediments to measure on HPGe detector to compare 210 Pb and 227 Ac gamma results to alpha measurements. 169 (3) Add 35 mL conc. HCl and 15 mL conc. HNO3 to Teflon vessel. Swirl and mix. Let stand for 30 minutes. (4) Afterwards, transfer the sediments/spike solution from the 50 mL Teflon vessel to a 500 mL Teflon beaker (PTFE beaker). Rinse down the walls of the 50 mL vessel with conc. HNO3 and transfer to 500 mL beaker. This ensures all material is removed from original beaker. If a larger Savillex vessel is used, and it has a screw top, then transferring the solution to a larger beaker is not necessary. (5) Place a Teflon cover on the beaker and heat at 90˚C for at least 3 hours, or until release of brown nitric fumes from beakers has ceased. (6) After 3 hours of heating, remove the cover and take solution to dryness. (7) After dryness, let material cool. After cooling, add 20 mL conc. NHO3 and 3 mL HClO4. Take solution to dryness (no cover). This step ensures all organics have been oxidized. (8) After material has dried, let cool. After cooling, add 20 mL conc. HNO3, 3 mL HClO4, and 10 mL conc. HF. Do not measure conc. HF in graduated glass cylinder! Estimate 10 mL by pouring stock solution directly into beaker or use plastic graduated cylinder. Evaporate to dryness and let cool. Material should be white or reddish brown, with no dark material. If dark material is present, repeat this step. 170 (9) After cooling, dissolve residue by adding 25 mL conc. HCl and 5 mL conc. HNO3. Cover and heat for 2-3 hours. After 2-3 hours, remove cover and take to dryness. If more than 1.0 gram of deep-sea sediments were used, another 10–15 mL conc. HCl would be wise to dissolve material. If not, a lot of metals (and iron) could be present when plating. (10) Re-dry 3-5 additional times with 3-5 mL 8 N HCl. This 8 N HCl solution can be made in a squirt bottle and used by squirting 8 N HCl into beaker. Using 30% H2O2 with the 8 N HCl will also help with the dissolution. Briefly, after the few mL’s of 8 N HCl is evaporated, remove the Teflon beaker from the hotplate and add 0.5-1 mL 30% H2O2 and let the solution effervesce. After the effervescence is done, add a few more mL of H2O2 and repeat. Add a total of 5-6 mL H2O2. After bubbling is complete, place beaker back on hot plate and take solution to dryness (Important: Do not add more than 1 mL of H2O2 or the effervesce will be very violent and spill over the beaker). For deep-sea sediments, which are usually metal rich, this step should be repeated 6 - 8 times. Remaining material after dryness will be clear for coastal sediments and reddish color for deep-sea sediments. Note that reddish color might not go away even after 6 washes with 8 N HCl. A.1.2 Polonium Plating (1) Add 50 mL 1 N HCl to Teflon beaker. Solution should turn yellow or clear. Add 0.1 grams of ascorbic acid for every 1 gram of sediments. Solution should turn clear upon swirling and heating. If not, add 0.1g aliquots of ascorbic acid until solution is clear. Have patience with this 171 step! Swirl for a few minutes before adding more ascorbic acid. Too much ascorbic acid can turn Ag disk black during plating. Ascorbic acid complexes with iron in solution. (2) Before plating, clean a silver disk with Alconox solution and wash with DIW. Make sure to label the back of disk with sample ID and date that Polonium was plated. Drop the plate into the solution label side down by spinning plate when dropping in. Plate for 3-5 hours. If the solution starts to turn dark color or black, remove disk immediately and count on alpha detector (too much ascorbic acid has been added). The silver disk should be dull, and not dark. Remove the disk from solution with plastic tweezers and wash with DIW, then acetone. Let dry for 5 minutes. The plate is ready to alpha count 210 Po and 209 Po. Three Surface Silicon Barrier Detectors (ORTEC, 300 mm; MCB12, MCB14, MCB16) are used to measure 210 Po and 209 Po. Samples were placed 10 mm away from the detector. Background counts and standards are counted regularly on each detector to monitor daughter product buildup and efficiencies with time. Each sample count has a background subtracted. Eq. A.1.1 is used to calculate the 210 Po in the dissolved sample. The average efficiency of each detector is roughly 20%. 𝐴 D. !#O = a !#O a !OP 𝑒 G% DC !#O ∆+H 𝐴 4!bDC (A.1.1) where 210 N and 209 N are the background-subtracted net counts of 210 Po and 209 Po; ∆t is the time that elapsed between the midpoint of plating and midpoint of counting; A209Po is the amount of 209 Po spike added in dpm, calculated at the time of counting; and l is the decay constant for 172 210 Po. It is assumed that 210 Pb and 210 Po (t1/2 = 138 d) are in equilibrium with each other for deep- sea sediments. A.1.3 Purification of Actinides via co-precipitation of PbSO4 A pre-concentration step needs to be performed before adding solution to the chromatography resin (PbSO4 co-precipitation technique, Dulaiova et al., 2013; Martin et al., 1995). After the Polonium plating step is complete, 10 mL of conc. HNO3 is added to the Teflon beaker to destroy ascorbic acid. Heat and take to dryness. This step takes time since a considerable amount of solution needs to evaporate. After evaporation, bring the solution up in 100 mL 0.1 N HNO3 (or 100 mL 0.1 N HCl may be use instead). The solution should be a light yellowish color. Add 1 mL 98% H2SO4 to the 100 mL solution and stir. Then add 2 g of K2SO4 and stir. After it dissolves, continue stirring and add 1 mL 0.24 M Pb(NO3)2 solution, drop-wise. Heat solution and let PbSO4 precipitate age and settle for 30 minutes. After 30 minutes, pour 100 mL solution into 2, 50 mL centrifuge tubes. Centrifuge solution at 3100 rpm for 6 minutes. Decant supernatant and discard. Wash precipitate with 20 mL of 0.1 M K2SO4 with 0.2 M H2SO4 (10 mL in each tube) and centrifuge again for 6 minutes (this step is optional). Decant supernatant and discard again. 173 Dissolve precipitate in about 10 mL of 4 N HCl in each centrifuge tube. It should dissolve easily. If not, add more 4 N HCl and shake tube. It is ok to have up to a 50 mL load solution (for 2 tubes combined), but try to stay around 20 ml. The solution is ready to be added to the chromatography resin. A.1.4 DGA Extraction Chromatography Resins for Separation of Actinides A commercially available, 2 mL pre-packaged extraction chromatography resin (DGA Resin, Normal) can be used to separate Ac, Ra, and Th from batch samples (purchased from Eichrom Inc.). The chromatography resin, DGA, is an extraction chromatographic resin that has a high adsorption capacity for rare earth elements and actinides at varying acidity (DGA Resin, Normal; N, N, N’, N’-tetra-n-octyldiglycolamide). Figure A.1.1 shows a plot of resin capacity factor (k’) vs. molar concentration of HNO3 and HCl. The higher the k’, the higher the fraction of the element in the stationary extractant phase. Figure A.1.2 shows the Dulaiova et al. (2013) DGA resin capacity factor (k’) results for Ac, Fe, Th, and other alkaline earth metals. The load solution is 4 N HCL and is loaded onto the column by pulling into a vacuum (-200 mm Hg from atmospheric). Dulaiova et al. (2013) report that adding 1 mg of Fe solution (FeCl3) to the 4 M HCl load solution helps with Ac uptake on the DGA resin due to a salting out effect. This step can be omitted if there is a high amount of salts or metals in solution, which is likely for deep-sea sediments. The deep-sea sediments measured at USC never needed extra Fe added. The addition of Fe is recommended for water samples (Dulaiova, pers. comm.). 174 Figure A.1.1: k’ for Ac in HNO3 and HCl for DGA resin. The higher the k’, the higher the fraction of the element in the stationary extractant phase. Figure taken from Eichrom Inc.. Figure A.1.2: DGA resin capacity factor (k’) for Ac, Th, Fe, and alkaline earth metals. Figure is taken from Dulaiova et al. (2013). Note that k’ is the resin capacity factor, which indicates the stationary extract phase of a certain metal cation. The higher the k’, the higher the stationary extractant phase is for that metal cation. The 2 mL cartridge is connected to a vacuum and pre-conditioned with 10 mL, 4 N HCl. The average flow rates should be between 1-4 mL per minute. The DGA resin is very hydrophilic, so a vacuum must be used to elute solution through the cartridge. At USC, we 175 created our own vacuum chamber that can hold 6 samples at one time. Figure A.1.3 shows the vacuum setup used at USC. It is a simple rectangular box made of acrylic that has one hole on the bottom for the vacuum to hook up to and has a top that can be removed. The top has a rectangular O-ring on the bottom so it can seal on the box when the vacuum is pulled in the chamber. Figure A.1.3: Vacuum setup for DGA cartridge and funnels. The vacuum box is made of acrylic and can hold a vacuum pressure of 250 mm Hg. Six DGA cartridges can be sampled at one time and the box can hold 50 mL centrifuge tubes. After pre-conditioning of the 2 mL cartridge is complete, add the 4 N HCl load solution from both 50 mL centrifuge tubes, which should have about 10 mL 4 N HCl in each tube. This step retains Ac and Th on the resin, but not Ra. Next, add 10 mL of 3 N HNO3 and discard effluent. This step removes iron and any left-over alkaline earth metals (Dulaiova et al., 2013). Ac is eluted from the cartridge by adding 20 mL of 2 N HCl. This fraction is collected and set aside for a micro-precipitation step. Finally, Th can be eluted from the cartridge by adding 20 mL 0.1 N HF, then 30 mL of 0.01 N HCl, for a total of 50 mL solution needed to subsequently 176 micro-precipitate CeF3. This last step will not remove all Th from the cartridge, but should elute enough to measure 230 Th, 232 Th, 228 Th, and 229 Th by alpha spectroscopy with a reasonable error (~20-60% chemical yield). A more detailed analysis should be done to see how thorium can be more completely eluted from the DGA cartridge. A flow diagram for Ac and Th separation via DGA cartridge is shown in Figure A.1.4. Figure A.1.4: Flow diagram for separation of Ac from DGA resin. Before step #1, the DGA cartridge is pre-conditioned with 25 mL 4 M HCl. Ten mL HNO3 is added in step #2. Finally, step #3 elutes Ac (20 mL 2M HCl). A.1.5 Micro-Precipitation of Actinium and Thorium 177 Alpha-Spectroscopy sources for Ac and Th were prepared by CeF3 micro-precipitation (Dulaiova et al., 2013; Sill, 1987). A solution of 200 µg mL -1 of CeCl3 was prepared. First, the Ac fraction (composed of 25 mL 2 N HCl) had 50 µg of CeCl3 added (0.25 mL of 200 µg mL -1 of CeCl3) to the solution. The solution was stirred episodically for 5 minutes and a pipet was used to add subsequently add 2 mL of concentrated HF while stirring, to form a CeF3 precipitate. The Th fraction (composed of 50 mL 0.1 N HF and 0.01 N HCl ) was treated in a similar manner, adding 30 µg of CeCl3 (0.15 ml of the CeCl3 solution) while stirring, followed by 2.5 mL of HF. After both solutions were allowed to sit for 30 minutes, a 0.1 µm, 25 mm Eichrom Resolve TM Filters (Eichrom Inc.) were set up and prepared for filtration of each solution. The filter is attached to a 50 mL reservoir funnel which is then connected to the box vacuum. Before the solution is added to the funnel reservoir, 2 mLs 80% EtOH (as reagent alcohol) must be passed through the filer to open the micro-pores and subsequently washed with 2 mL DIW. The solution is added to the funnel and passed through the filter once the vacuum was applied. Solutions usually pass through the filter within 5 minutes. After the Ac or Th solution has gone through the filter, DIW can be added to wash the sides of the funnel. Finally, 2 mL 80% EtOH is added to the reservoir and passed through the filter. Discard effluent by disposing in proper hazardous containers (contains HF), but do not add to waste with HNO3. After solution passes through the filter, the filter is removed from the funnel apparatus and placed in the fume hood for 1 hour. A small fan can blow on the filter to speed the drying process. It is important to have the filter completely dried, as not to have HF fumes come off in the alpha detector. After the filter is completely dry, it is mounted on a stainless-steel disk (~2 cm diameter) by gluing the filter and 178 disk together with Elmer’s Glue (very light glue to form a smooth surface for the filter on the disk). The filter is then ready for alpha counting. Figure A.1.5 shows the alpha spectrum for 225 Ac and its daughters. Only 225 Ac and its daughters are present in the figure because 227 Ac is not an alpha emitter, and its daughters have not grown into equilibrium yet. Two measurements are made to calculate the 227 Ac activity: The first measurement is the determination of the net yield from counting 217 At (7.1 MeV), the granddaughter of 225 Ac (5.5-5.8 MeV). The net yield includes both the chemical yield and the detector efficiency and is summarized in eq A.1.2. Two 225 Ac progeny, 221 Fr (6.1-6.3 MeV) and 217 At, have half-lives of 5 min and 32 ms (Table A.1.1). Therefore, 217 At will be in secular equilibrium with 225 Ac within 20 minutes, which is relatively short compared to the alpha source preparation time described above (30 min). 225 Ac and 221 Fr are not used for the yield tracer due to possible interfering isotopes in that energy region (Geibert and Vöge, 2008). Furthermore, virtually all 217 At decays are observed in a single energy (7.1 MeV), and no other isotopes interfere in that energy (Table A.1.1). 𝜀 = L6R !#+(Q B6R !!R() (A.1.2) where: e is the net efficiency (including yield and counting effects), cpm217At is the background- subtracted net counts at 7.1 MeV, and dpm225Ac is the decay corrected activity of 225 Ac added to the sample. After 90 days, 225 Ac and its progeny have decayed, and the activity of 227 Ac is determined by counting its two alpha-emitting progeny (Fig. A.1.6), 227 Th (t1/2=18.7 d) and 223 Ra (t1/2=11.4 d). The activity of 227 Ac is determined by eq. A.1.3 and A.1.4. 179 𝐴 44KIN044[c# = L6R !!/01 0L6R !!+,- M (A.1.3) and 𝐴 44K(L = ( !!+,-. !!/01 Wc !!+,- P !!+,- 0Wc !!/01 P !!/01 (A.1.4) Where A227Ac and A223Ra+227Th are the activity of 227 Ac and 223 Ra + 227 Th in units of dpm/g, BR is the fractional branching ratio, cpm227Th and cpm223Ra are background-subtracted net counts of 223 Ra and 227 Th between 5.5 – 6.04 MeV, and I is the ingrowth factor, which is determined from figure A.1.7 and Bateman equations summarized in section A.1.6. Figure A.1.5: Alpha-spectrum of 225 Ac and its daughters counted immediately after micro- precipitation is completed. The yield of 75% is the chemical yield and is determined by the assuming a detector efficiency of 18% and knowing the amount of 225 Ac added to the sediments (~19 dpm). Note the 230 Th present in the Ac fraction (4.7 MeV). This is due to high amounts of 230 Th present in deep-sea sediments (~60 dpm/g). Note that the energy axis output of the alpha spectrometer has non-linearities. 180 Figure A.1.6: Alpha-spectrum of 227 Ac and its daughters counted 120 days after micro- precipitation. The yield tracer, 225 Ac, has decayed away and only 227 Ac and its daughters remain. The combined peak of 223 Ra and 227 Th is used for computation. 223 Ra and 227 Th have reached 98% of equilibrium with 227 Ac. A small amount of 225 Ac remains within the combined peak and is subtracted from the 223 Ra and 227 Th region, using 217 At for this computation. Note that the energy axis output of the alpha spectrometer has non-linearities. Figure A.1.7: Ingrowth factors for 223 Ra and 227 Th for the determination of 227 Ac activity. At 120 days, 223 Ra and 227 Th are 96% and 98% grown into equilibrium with its parent, 227 Ac. See below for MATLAB script (A.1.6). 181 Figure A.1.8 shows the alpha-spectrum results for Th isotopes, namely, 232 Th, 230 Th, 229 Th, and 228 Th, which can all be calculated at one time. Eq. A.1.3 shows the calculation for 228 Th isotope, which can be extended to 232 Th and 230 Th. 𝑇ℎ 44d (𝑑𝑝𝑚)= a !!S a !!P 𝐴 44bIN (A.1.3) Where 229 N and 228 N are the background-subtracted net counts of 229 Th and 228 Th (or 232 Th & 230 Th) and A229Th is the amount of 229 Th spike added in dpm. A note should be said about using 229 Th as a yield tracer for Th isotopes: The 229 Th peak on the alpha spectrum is very close to the 230 Th peak (Table A.1.1). Thus, the 229 Th energy peak will sometimes overlap into 230 Th energy peak. For ANACONDAS and C-Disk-IV samples, only about 50% of samples were usable for determining Th isotopes using the 229 Th yield tracer. A possible method to avoid this issue would be to use one of 229 Th progenies, 217 At at 7.1 MeV as the yield tracer. However, 3 months must elapse before 217 At builds into secular equilibrium with 229 Th. Figure A.1.8: Alpha-spectrum of 230 Th, 232 Th, 228 Th, and 229 Th counted immediately after micro-precipitation was completed. 229 Th was used as the yield tracer and enough time was allotted to obtain 500 counts in the 229 Th region as to have counting errors less than 5%. Sediments collected during the ANACONDAS cruise were used for calibration of this method. Note that the energy axis output of the alpha spectrometer has non-linearities. 182 Table A.1.1: Energies of alpha particles, branching ratio (BR%), and half-lives for various radioisotopes used in this study. 229 Th and its daughter, 225 Ac were used as the yield tracers for Th and Ac and 209 Po was the yield tracer for 210 Po in deep-sea sediments. Several isotopes have multiple peaks that can be grouped into a region of interest. Isotope Energy Region (MeV) BR% Half-Life 232 Th 3.81 - 4.01 100 1.4E10 y 230 Th 4.44 - 4.69 100 75,380 y 229 Th 4.69 - 5.05 100 7,340 y 209 Po 4.88 100 125 y 228 Th 5.42 – 5.34 99.5 1.91 y 210 Po 5.30 100 138 d 223 Ra 5.50 - 5.75 97.7 11.4 d 225 Ac 5.55 - 5.83 99.2 10 d 221 Fr 6.08 - 6.34 100 4.9 m 227 Th 5.91 - 6.04 54.6 18.7 d 217 At 7.07 100 32 ms 213 Po 8.38 100 4 µs A.1.6 MATLAB Script for Ingrowth Curves (Bateman Equations) %Modified Bateman Equations t1=21.77*365.25; %half-life in days for 227Ac t2=18.72; %half-life in days for 227Th t3=11.44; %half-life in days for 223Ra k1=log(2)/(t1); %decay constant for 227Ac k2=log(2)/(t2); %decay constant for 227Th k3=log(2)/(t3); %decay constant for 223Ra Q10=1.0; %initial dpm for 227Ac (use 1 for fractional ingrowth) Q20=0.00; Q30=0.00; t=[0:1:120]; %amount of days to run model %227Ac activity Q1 = Q10*exp(-k1*t); 183 %227Th activity Q2 = Q10*(k2/(k2-k1))*(exp(-k1*t) - exp(-k2*t)) - Q20*(exp(-k2*t)); %223Ra activity Q3 = ((k2*k3)/(k2-k1))*Q10*[((exp(-k1*t) - exp(-k3*t))/(k3-k1)) - ((exp(- k2*t) - exp(-k3*t))/(k3-k2))] + ((k3/(k3-k2))*Q20*(exp(-k2*t) - exp(-k3*t))) + (Q30*exp(-k3*t)); %Plot 227Ac, 227Th, and 223Ra activity over the days specified above graph1=plot(t,Q1) hold on set(graph1,'LineWidth',1.5) graph1=plot(t,Q2) hold on set(graph1,'LineWidth',1.5) graph1=plot(t,Q3) xlim([0 120]) ylim([0 1.1]) ylabel('Fractional Ingrowth','fontsize',18) xlabel('Time (days)','fontsize',18) legend({'^{227}Ac','^{227}Th','^{223}Ra'},'fontsize',18) 184 A.1.7 References Dulaiova, Henrieta, Kenneth W. Sims, Matthew A. Charette, Julie Prytulak, and Jerzy S. Blusztajn. “A New Method for the Determination of Low-Level Actinium-227 in Geological Samples.” Journal of Radioanalytical and Nuclear Chemistry 296, no. 1 (2012): 279–83. https://doi.org/10.1007/s10967-012-2140-0. Fuller, C. C. “The Use of Pb-210, Th-234 and Cs-137 as Tracers of Sedimentary Processes in San Francisco Bay, California .” M.S. Thesis, University of Southern California, 1982, 251. Geibert, Walter, and Ingrid Vöge. “Progress in the Determination of 227 Ac in Sea Water.” Marine Chemistry 109, no. 3-4 (2008): 238–49. https://doi.org/10.1016/j.marchem.2007.07.012. Martin, P., G.J. Hancock, S. Paulka, and R.A. Akber. “Determination of 227 Ac by α-Particle Spectrometry.” Applied Radiation and Isotopes 46, no. 10 (1995): 1065–70. https://doi.org/10.1016/0969-8043(95)00222-y. Sill, Claude W. “Precipitation of Actinides as Fluorides or Hydroxides for High-Resolution Alpha Spectrometry.” Nuclear and Chemical Waste Management 7, no. 3-4 (1987): 201–15. https://doi.org/10.1016/0191-815x(87)90066-0. 185 A.2 Measurement of 227 Ac Using a High Purity Germanium Well-Type Detector 227 Ac samples measured along the C-Disk-IV transect were also measured by gamma spectroscopy to determine the accuracy and precision of gamma counting 227 Ac. Gamma counting 227 Ac requires no analytical separation, and it is non-destructive, thus making it highly advantageous. However, there are many precautions that must be taken when measuring 227 Ac by gamma spectroscopy. The USC radioisotope lab has two High Purity Germanium (HPGe) well detectors (HPGe ORTEC, 120 cm3 active volume) used for these analyses. Detector 1 (MCB1) has a Full Width Half Maximum resolution of 1.47 eV at 122 keV and 2.06 keV at 1330 keV. Detector 2 (MCB2) has 1.23 eV at 122 keV and 2.10 keV at 1330 keV. Both detectors have a useful energy range between 30 – 2000 keV. However, below 100 keV, some self-absorption may occur, and the sample matrix may need to be considered when measuring different environmental samples. Detector efficiencies for several key isotopes were determined from counting the activities of known standards obtained from EPA (SRM-1 diluted pitchblende and SRM-2 diluted monzonite) and Eckert & Zeigler (WHOI E&Z Actinium-227 CRM). Standards were 3.0 cm high geometry in polyethylene tubes (8 mm ID) and corrections were made to sample results to account for the lower sample heights that were used. Samples were counted for 2-4 days, and Table A.2.1 shows the energies used in both detectors for the determination of 227 Ac and other isotopes in deep-sea marine sediments. The table also indicates efficiencies in MCB2, branching ratios in decays, and factors used to correct for interferences from other isotopes. 186 Table A.2.1: Information for radioisotopes measured on HPGe well-type detector in this study. Branching Ratio (BR%) is the fractional percentage of decays emitting gammas at a particular energy. Detector efficiency was determined from branching ratio and standards with known activities of interfering isotopes, calculated for a sample height of 4.5cm, measured on HPGe detector MCB2 at USC. The comments column includes factors (in parentheses) used to correct observed counts for some isotopes having contributions from an interfering isotope at that energy. For these deep-sea red clays in a 0.5 cm ID plastic tube, the self-absorption amounted to 10% at 46 keV as determined from standard addition experiments. Isotope Energy (keV) Detector Eff. BR% Standard Comments 225 Ra 40.1 0.515 30.0 [1] Low energy: self-absorption 210 Pb 46.5 0.471 4.25 [2] Low energy: self-absorption 234 Th 63.3 0.436 4.8 [2] From 238 U, t1/2 = 24 d 235 U 185.7 0.393 57.2 [1] Calculate from 238 U via 234 Th 226 Ra 186.2 0.393 3.5 [2] 235 U Interference at 185.7 keV (0.50*63keV counts) 221 Fr 218.2 0.322 11.6 [1] From 225 Ac daughter , t1/2 = 5 min 224 Ra 241.0 0.289 3.97 [3] 214 Pb interference at 242 keV (0.40*295keV counts) 212 Pb 238.6 0.289 43.3 [3] Doublet with 224 Ra; Correct 214 Pb interference as above. 223 Ra 269.5 0.247 13.7 [4] 228 Ac interference at 270.2 keV (0.66*911keV counts) 219 Rn 271.2 0.247 10.8 [4] Doublet with 223 Ra; Correct 228 Ac interference as above. 214 Pb 295.2 0.218 18.5 [2] 228 Ac 338.3 0.164 11.3 [3] 223 Ra interference at 338.3 keV (BR = 2.79%, negligible) 214 Pb 351.9 0.176 35.8 [2] Minor Interference from 232 Th series (negligible) 214 Pb 609.3 0.052 44.8 [2] 228 Ac 911.2 0.048 26.6 [3] [1] 229 Th USC #89B; [2] SRM-1 diluted pitchblende; [3] SRM-2 diluted monzonite; [4] E&Z Actinium-227 CRM 227 Ac beta minus decays to 227 Th, with very weak gamma emissions. Therefore, direct measurement of 227 Ac via gamma ray spectroscopy is challenging. However, the 227 Ac progeny, 223 Ra and its daughter 219 Rn, have relatively strong overlapping gammas at 270 and 271 keV that can be quantified if counting time is long enough. 219 Rn can be assumed to be in equilibrium with 223 Ra since its half-life is only 4 seconds, and its leakage can be ignored due to this short half-life. A correction must be used when measuring 223 Ra at 270 keV due to the interference from 228 Ac at 270.2 keV (BR% = 3.4). This interference can be removed if the activity of 228 Ac is 187 known. 228 Ac gives off two reliable gammas at 338.3 and 911.2 keV (BR% = 11.3 and 26.6). However, a problem arises with using 228 Ac at 338 keV: 223 Ra interferes with 228 Ac at 338.3 keV (BR% =2.8). Alternatively, 228 Ac at 911 keV has no interfering isotopes and can be used to correct the 270 keV region. The equations below can be used to measure 223 Ra from the 270 keV energy region. First, the efficiency of counting gammas at 270 keV can be found by measuring a standard with only 227 Ac that has equilibrium activities of its progeny: 𝜀 4K!>ef = ,c !+OTUV ( !!+() (𝐵𝑅 44[c#"4K!>ef +𝐵𝑅 4'bcV"4K!>ef ) (A.2.1) where: BR is the fractional branching ratio, A is activity in dpm, CR is count rate in cpm, and e is the counting efficiency for a specific energy region. Efficiencies and branching ratios used in these equations can be found in Table A.2.1. The count rate from 228 Ac in the 270 keV peak is calculated from the activity observed at 911 keV as: CR 44d(L"4K!>ef =( gh !!S()WP##TUV M P##TUV Wc !!S()WP##TUV )𝜀 4K!>ef 𝐵𝑅 44d(L"4K!>ef (A.2.2) The activity of 227 Ac is then calculated by using eq. A.2.2 to correct the observed count rate at 270 keV: A 44K(L = (,c ,XQW!+OTUV "gh !!S()W!+OTUV ) M !+OTUV (Wc !!/01W!+OTUV 0Wc !#P0YW!+OTUV ) (A.2.3) 188 The activity of 227 Ac for gamma and alpha spectroscopy results is compared in Figure A.2.1. The 227 Ac measurements in figure A.2.1 were samples from this study (CDISK-IV). There is excellent agreement between the two methods, however the errors for gamma spectroscopy were on the order of 10-20 percent. These samples were counted for a minimum of 4 days and weighed a minimum of one gram when counted. If more sediments are used when counting or samples are counted longer, then errors could be lowered. However, many of these samples were counted for over 5 days and the errors were only able to be lowered to about 10% uncertainty. Nevertheless, gamma counting samples for 227 Ac measurements offers an advantageous method for fast and accurate results, although they are less precise than results from isotope dilution alpha spectroscopy. 189 Figure A.2.1: Gamma vs. alpha spectroscopy results for 227 Ac measurements along C-Disk-IV transect. The linear regression line for all measurements is forced through zero. The 1:1 and regression lines are black and dashed red. The slope of the regression line is 0.95 ± 0.03. 190 A.3 Intercalibration of Lead-210 in Marine Sediments Detector calibration for the quantification of radioisotopes is an important and fundamental part of receiving accurate and reliable data. The following procedures were carried out in order to calibrate a High Purity Germanium Detector (HPGe) at the USC radioisotope lab for marine sediments and other environmental samples. Multiple approaches were conducted to compare known standards in different matrixes to determine unknown measurements of marine sediment samples. A.3.1 Intercalibration of 209 Po spike with NIST 210 Pb Standard 210 Pb was standardized using a NIST certified solution (SRM 4337). The nominal activity of the 210 Pb solution was 11,640 ± 300 dpm/g (7/15/2006). The 209 Po (USC #65D) standard was calibrated against the NIST 210 Pb solution in summer of 2018 and had a nominal activity of 24.1 ± 0.3 dpm/g (7/10/2018). The 209 Po standard was corrected for decay to the time of analysis using the recently determined half-life of 125 yr (Colle et al., 2014). A split of each standard solution was added to 40 mL 1 N HCl in a 100 mL Teflon Beaker. A silver disk (18 mm diameter) was also placed in the bottom of the beaker and the solution was heated to 90 ˚C and stirred for 5 hours. After 5 hours, the silver disk was taken out of the beaker, washed with DIW and dried with acetone. After air drying, the disk was placed in an alpha spectrometer, 10 mm below a Surface Barrier Silicon Detector (ORTEC, 300 mm surface) to count the 209 Po and 210 Po activity. After counting for 16 hours, peaks at 4.884 (209Po) and 5.304 MeV ( 210 Po) exceeded 2500 events in both ROIs (Region of Interests), after which 191 counting ceased and the gross counts in both ROIs were tabulated. Backgrounds in each ROI were also determined. Eq.A.3.1 shows the calculations that determined the 210 Pb activity of the NIST solution from the known 209 Po that was added: 𝑃𝑏 4'! (𝑑𝑝𝑚)= a !#O a !OP 𝑒 (% DC !#O ∆+) 𝐴 4!bDC (A.3.1) where 210 N and 209 N are the background-subtracted net counts of 210 Po and 209 Po; ∆t is the time that elapsed from midpoint of plating and midpoint of counting; A209Po is the amount of 209 Po spike added in dpm; and l is the decay constant for 210 Po. It is assumed that 210 Pb and 210 Po (t1/2 = 138 d) in the NIST solution are in equilibrium. This will be true for marine sediments as well. Table A.3.1 shows the results for two replicate measurements using the NIST certified 210 Pb standard and the USC #65D 209 Po spike. The 209 Po spike was used as the yield tracer for the NIST 210 Pb standard. Table A.3.1: 210 Pb activities (SRM 4337) determined from multiple counts on USC alpha detectors MCB12, MCB14, and MCB16 from 209 Po spike (USC #65D). For both experiments, the nominal 210 Pb activity added to the 209 Po spike was 36.1 ± 0.3 dpm. The average of the decay corrected ratio of 210 Po/ 209 Po and 210 Pb activity determined from eq. A.3.1 is shown on the bottom. The sample standard deviation of all 6 sample counts is shown as well. Decay Corr. Ratio 210 Pb Activity Detector Disk ID 210 Po/ 209 Po sig dpm sig MCB 12 Disk A 0.78 0.03 36.56 1.22 MCB 12 Disk B 0.78 0.02 36.27 0.74 MCB 14 Disk A 0.75 0.02 34.90 0.9 MCB 14 Disk B 0.77 0.02 36.15 0.88 MCB 16 Disk A 0.78 0.03 36.33 1.05 MCB 16 Disk B 0.82 0.02 38.13 0.93 Average (± 1sdom) 0.78 ± 0.01 36.4 ± 0.4 Expected 36.1 ± 0.3 192 The 210 Pb activity determined from USC #65D 209 Po spike falls within the uncertainty of the 210 Pb activity stated by NIST. Four of the six measurements differ from the mean by less than 1 standard deviation and the other two differ by less than 2, as expected. The agreement indicates that USC spike #65D 209 Po should be well-calibrated to carry out future measurements on the USC alpha detectors. A.3.2 Intercalibration of HPGe and Alpha Detector using marine sediments Sediments from different environments, including marsh, coastal, and deep-sea sediments were used for intercalibration of HPGe and alpha detectors at USC. These are common sediments that are routinely measured for 210 Pb analysis. Deep-sea and Marsh material are samples of deep-sea sediments (C-Disk-IV), a California marsh (from UCLA) and a Texas marsh (from TAMU). The coastal sediments are samples (MUC) of the Santa Monica Basin (SMB) from sites deep in the basin center (> 900 meters depth). A.2.2.1 Well-Type HPGe Efficiency Geometry corrections The USC radioisotope lab has three high HPGe well-type detectors (HPGe ORTEC, 120 cm 3 active volume). HPGe detector 1 (MCB1) has a Full Width Half Maximum resolution (FWHM) of 1.47 eV at 122 keV and 2.06 at 1330 keV, detector 2 (MCB2) has a FWHM resolution of 1.23 eV at 122 keV and 2.10 keV at 1330 keV, and detector 3 (MCB3) has a 193 FWHM resolution of 1.06 eV at 122 keV and 1.19 keV at 1330 keV. HPGe detector characteristics were last determined by ORTEC/AMETEK on 1/27/2017 (MCB2), 12/9/2002 (MCB1), and 2/18/1997 (MCB3). All detectors have a useful energy range between 30–2000 keV. Detector efficiencies for 210 Pb (46.5 keV; BR%=4.25) were determined by counting a NIST certified 210 Pb liquid standard in the same geometry, but for varying heights in small polypropylene (PP) tubes. We use two different PP tubes to measure samples: large PP tubes, which are 8.5 cm in height and 1.2 cm in diameter (inner diameter) and small PP tubes, which are also 7.5 cm in height, but 0.8 cm in diameter (inner diameter). The heights of the NIST 210 Pb standard varied in each small PP tube, from 0.75-4.4 cm (9 samples total). The 210 Pb efficiency was determined by adding a known amount of 210 Pb solution into the PP tube and dividing the resulting cpm (counts per minute) by the dpm (disintegration per minute), i.e cpm/dpm. Large PP tubes did not undergo this analysis but can be corrected if only one measurement is taken from the big tube (i.e add known amount of 210 Pb to big PP tube and assume same offsets for all heights). Figure A.3.1 shows 210 Pb efficiency vs. height for MCB1, MCB2, and MCB3. A polynomial function was fit to the data to obtain a height-corrected efficiency for 210 Pb at 46.5 keV. 194 Figure A.3.1: 210 Pb efficiency vs. height for HPGe detectors MCB1, MCB2, and MCB3. The 210 Pb efficiency was determined by adding a known amount of 210 Pb solution into a PP tube (8mm ID) and dividing the resulting cpm (counts per minute) by the dpm (disintegration per minute), i.e cpm/dpm. The 210 Pb that was added to the PP tube was a NIST certified 210 Pb standard. A second, third, or fourth order polynomial was fit to the data above until the correlation coefficient (R 2 ) was close to 1.0. 195 A.2.2.2 Self-Adsorption Correction Self-adsorption of the 210 Pb must be considered when measuring on a HPGe well-type detector. This is due to the weak gamma ray energy 210 Pb produces at 46.5 keV, which is weak enough to interact with the surrounding matrix. Usually, any gamma ray energy below 100 keV is weak enough to interact with the surrounding matrix. The self-absorption correction for 210 Pb was determined by two independent methods: spiking and needle experiment. A.2.2.3 210 Pb Spiking Experiment In the first method, the self-absorption correction for 210 Pb was determined from spiking deep-sea sediments with the NIST 210 Pb liquid standard and comparing the efficiency to the liquid NIST 210 Pb standard for different heights. This was only done for the small PP tubes. To accurately homogenize the liquid NIST 210 Pb solution within the sediments, care was taken to add a small amount of NIST solution and sediments sequentially in the PP tubes. Briefly, a small aliquot of NIST solution was placed in tube first and weighed, then a few mg of sediments were placed in the tubes and weighed. The tube was mixed gently to homogenize the sediment and NIST solution into a slurry. This method was applied for two depth intervals at C-Disk-IV station 2: 1-2 cm and 9-10 cm. Each interval had 4 slurry samples varying in height: 1.7, 1.8, 2.2, and 2.8 cm, and each sample was then measured by gamma spectroscopy to calculate the raw cpm. Approximately the same mass of sediment (0.6-0.8 g) was used to make each slurry. Eq. A.3.2 shows how the self-absorption (matrix) factor was calculated using the ratios of efficiencies of the mixture (cpm of slurry/total dpm in slurry) to the NIST solution. 196 𝑀𝑎𝑡𝑟𝑖𝑥 = i<<ULUeVLj ' i<<ULUeVLj = &JkQQj L6R IC+#J B6R ÷ aP&I L6R aP&I B6R (A.3.2) where ‘Slurry cpm’ is the raw cpm for the homogenized sediment mixture with NIST added, ‘Total dpm’ is the expected activity of the homogenized sediments with NIST added, and NIST cpm and dpm are the raw counts and activity of the NIST solution as a liquid. The NIST solution efficiency was calculated based on the height of the slurry for each sample. Table A.3.2 shows the results for each detector and sample matrix. The average self-adsorption correction for deep- sea sediments for small PP tubes was 10% in comparison to the behavior in the liquid used for standardization (average ratio of 0.90 was found between all detectors and all samples). Table A.3.2: Self-absorption Effects for Station 2 C-Disk-IV Sediments in Small PP tubes. Self- absorption for each sample was determined by spiking the sediments with a known amount of 210 Pb solution to make a slurry, taking the resulting efficiency and dividing it by the liquid NIST 210 Pb efficiency in the same geometry. The slurry efficiency was determined by taking the raw 210 Pb cpm counts and dividing it by the total dpm in the PP tube. This total dpm includes NIST solution added and activity of the sediments added. The NIST 210 Pb efficiency is the height- corrected efficiency for the NIST 210 Pb standard. The sediment activity was determined through alpha spectroscopy and is calculated from the sediment mass used. The ratio on the far right of the table is the Efficiency cpm/(total dpm)/Efficiency NIST 210 Pb ratio, which is the self- absorption ratio. MCB1 Height (cm) Slurry (cpm) NIST (dpm) Slurry (dpm) Slurry Eff. cpm/(total dpm) Liquid Eff. NIST 210 Pb Eff. Ratio Sta 2, 1-2cm 1.7 0.40±0.02 0 14 0.0278 0.0327 0.85 ±0.04 1.8 1.89±0.03 55 67 0.0281 0.0324 0.87±0.04 2.2 3.66±0.05 111 125 0.0293 0.0313 0.94±0.04 2.8 4.90±0.05 166 179 0.0273 0.0290 0.94±0.04 Sta. 2, 9-10cm 1.7 0.67±0.04 0 25 0.0264 0.0327 0.81±0.03 1.8 2.29±0.04 57 80 0.0288 0.0324 0.89±0.03 2.2 3.79±0.05 111 134 0.0284 0.0313 0.91±0.03 2.8 5.20±0.07 164 194 0.0268 0.0290 0.92±0.04 Average 0.89±0.05 197 MCB2 Height (cm) Slurry (cpm) NIST (dpm) Slurry (dpm) Slurry Eff. cpm/(total dpm) Liquid Eff. NIST 210 Pb Eff. Ratio Sta. 2, 1-2cm 1.7 0.39±0.02 0 14 0.0272 0.0334 0.82±0.05 1.8 2.02±0.04 55 67 0.0300 0.0333 0.90±0.04 2.2 3.62±0.06 111 125 0.0290 0.0331 0.87±0.04 2.8 5.39±0.13 166 179 0.0301 0.0320 0.94±0.04 Sta. 2, 9-10cm 1.7 0.80±0.03 0 25 0.0315 0.0334 0.94±0.04 1.8 2.40±0.04 57 80 0.0301 0.0333 0.90±0.03 2.2 4.03±0.04 111 134 0.0301 0.0330 0.91±0.04 2.8 5.54±0.06 164 194 0.0285 0.0320 0.89±0.03 Average 0.89±0.05 MCB3 Height (cm) Slurry (cpm) NIST (dpm) Slurry (dpm) Slurry Eff. cpm/(total dpm) Liquid Eff. NIST 210 Pb Eff. Ratio Sta 2, 1-2cm 1.7 0.43±0.03 0 14 0.0295 0.0326 0.91±0.04 1.8 2.08±0.12 55 67 0.0308 0.0323 0.95±0.04 2.2 3.50±0.08 111 125 0.0280 0.0310 0.90±0.04 2.8 4.59±0.07 166 179 0.0256 0.0282 0.91±0.04 Sta. 2, 9-10cm 1.7 0.72±0.04 0 25 0.0283 0.0326 0.87±0.03 1.8 2.41±0.07 57 80 0.0303 0.0323 0.94±0.04 2.2 3.87±0.08 111 134 0.0290 0.0310 0.93±0.04 2.8 4.91±0.08 164 194 0.0252 0.0289 0.87±0.03 Average 0.91±0.03 A.2.2.4 210 Pb Needle Experiment A needle that was spiked with a known amount of 210 Pb (~1200 dpm) at its tip was used in a variety of different matrices to determine the self-adsorption efficiency of 210 Pb at 46.5 keV. The 210 Pb source was supplied by C. Fuller of USGS. The tip, which contained the 210 Pb source, was covered with a section of heat shrink tubing to prevent abrasive loss of the source when it 198 was inserted in sediments or water. The needle was then placed in the center of each tube type (Big and Small PP tubes) with and without sediments and counted on MCB1 and MCB2 gamma detectors. The 210 Pb needle was also placed in each PP tube with DI water to simulate the NIST 210 Pb liquid standard. Table A.3.3 shows the results for each detector and each PP tube. The density of each sample (except the empty tubes) was determined from the graph of volume (cc) vs. height for each tube type (Fig. A.3.2). This graph was created by adding a known volume of DI water to each tube and measuring the resulting height. The self-absorption for each tube is calculated by the ratio of each sample and empty tube’s cpm. The difference between the water and sediment sample is roughly 10% and 5% for big and small tubes. One exception to this analysis is the deep-sea sediments for the small tube, which is 10% different from the DIW. This closely resembles the spike sediment’s adsorption correction seen above. Therefore, whether deep-sea sediments are in small or big tubes, its correction should be 10% adsorption correction. Figure A.3.2: Volume (cc) vs. height for Big and Small PP tubes. These PP tubes are used in USC detector’s MCB1, MCB2, and MCB3 for sediment samples. The density of each sample in the PP tube is determined from the plot above. 199 Table A.3.3: 210 Pb Needle experiment for sediment samples and DI water. Deep-sea and Marsh material are samples of deep-sea sediments (C-Disk-IV), a California marsh (from UCLA) and a Texas marsh (from TAMU). The needle was placed at the bottom of each tube, so the source was within the lowest 1 cm. The ratio is for cps normalized to air. Density was determined from the volume vs. height plot (Fig. A.3.2). The ratio for sediment /water is roughly 0.90 for big tubes and 0.95 for small tubes in the marsh sediments. The ratio for deep sea sediment in large tubes is identical, but in small tubes it is about 0.92. Detector Tube Size matrix Matrix Height (cm) mass (g) density (g/cc) cps sig Ratio (matrix/air) 1 Big air 0.00 0.000 0.000 1.96 0.01 1.00 1 Big water 3.00 3.160 1.000 1.76 0.01 0.90 1 Big Marsh Sed (UCLA) 2.90 2.616 0.819 1.58 0.01 0.81 1 Big Deep-Sea Seds 3.00 3.057 0.922 1.60 0.02 0.81 2 Big air 0.00 0.000 0.000 2.00 0.02 1.00 2 Big water 2.50 2.620 1.000 1.82 0.02 0.91 2 Big Marsh Seds (Texas) 2.40 3.203 1.241 1.57 0.01 0.79 2 Small air 0.00 0.000 0.000 2.01 0.01 1.00 2 Small water 2.60 1.220 1.000 1.88 0.01 0.94 2 Small Marsh Seds (UCLA) 2.40 0.903 0.853 1.80 0.01 0.90 2 Small Deep-Sea Seds 2.50 1.023 0.966 1.73 0.01 0.86 2 Small Coastal Seds 2.50 0.706 0.667 1.81 0.01 0.90 A.2.3 Sediment Analyses using Gamma Spectrometry Samples from the North Pacific and California Borderland were collected with a multicore device. After cores were retrieved on-board ship, cores were sectioned and placed in plastic baggies and shipped to USC. Each sample was sectioned in one-centimeter intervals in the upper 10 cm and two-centimeter intervals after 10 cm. Upon arriving at USC, samples were dried at 55 ˚C, ground, and homogenized. The amount of wet sediments that were dried, ground, and homogenized varied for different samples. Usually, the MUC (CA Borderland: Santa 200 Monica Basin) samples had 4-5 grams of wet sediments, while the C-Disk-IV samples had about 10-15 grams of wet sediments sampled. Marsh sediments from the Texas and California marshes were collected and prepared in a similar manner, except a single core was used to extract the sediments. After drying and homogenizing the sediments for each sample, approximately 1.0 gram of sediments was weighed and placed in PP tubes (Small and Big PP Tubes). HPGe detectors MCB1, MCB2, and MCB3 were used to count the samples in each PP tube A.3.4 Sediment Analyses using Alpha Spectrometry To further verify the height corrected efficiency and self-absorption correction for 210 Pb using the USC HPGe detectors, a multitude of samples from the deep-sea sea and coastal regions were compared against alpha spectroscopy. After sediments were sectioned, dried, and homogenized (as stated earlier), 1.0 - 1.5 grams of sediments were placed in the Teflon beakers and spiked with a known amount of 209 Po spike (USC #65D) usually averaging around 22 dpm. The sediments, along with the 209 Po, underwent a series of acid digestions with HCl, HNO3, HF, and HClO4 or H2O2 outlined by Fuller (1982) and in Appendix A.1.1. After organics and silicates were dissolved, the solution was brought up in 50 mL 1N HCl, and ascorbic acid was added to complex iron. A silver disk was placed in the beaker to plate Po, then counted as described above in Appendix A.1.2. Background counts were done to monitor daughter product buildup with time. Each sample has a background-subtracted count applied and equation A.1 was used to calculate the 210 Po in the dissolved sample. The average counting efficiency of each detector is roughly 18% at the 10 mm distance, and plating yields averaged about 50%, making a combined efficiency of 9%. 201 Ten samples were measured by alpha and gamma spectroscopy. Six samples were deep- sea sediments, and 4 samples were coastal sediments. All samples were prepared by the methods described above for alpha and gamma spectroscopy. All weights were corrected for salt content based on the porosity and density of sediments (2.5 g/cc) and all gamma spectroscopy measurements used a 10% self-adsorption correction for deep-sea (C-Disk-IV) and coastal (MUC) sediments. Table A.3.3 shows the results for all sediments using gamma and alpha spectroscopy. All units are in bq/kg. Results are also plotted in fig. A.3.3. The results of table A.3.4 are then plotted as HPGe vs. alpha for each HPGe detector (Fig. A.3.3). The slopes for MCB1, MCB2, and MCB3 are 0.98 ± 0.01, 0.98 ± 0.01, and 1.02 ± 0.01, which show consistent results for each detector. Finally, ratios for each sample were calculated from the data in table A.3.4 and are shown in table A.3.5 and A.3.6. Deep-sea and coastal sediments are separated to show any discrepancies that might arise from different marine environments. No discrepancies are obvious between the two marine environments. The total averages of each detector ratios against alpha spectroscopy fall within uncertainty of 1.0, indicating that the USC HPGe detectors are well calibrated against alpha spectroscopy. The only mystery in this analysis is why the 210 Pb needle experiment showed a self-adsorption correction of 5% for coastal sediments (MUC samples) and the spiking experiment results showed a 10% self-adsorption correction. Table A.3.6 and Fig. A.3.3 suggest the former self-adsorption correction at 10%. More tests will need to be analyzed to confirm this, but Fig. A.3.3 shows strong evidence that 10% self-adsorption correction might be correct for coastal sediments. 202 Table A.3.4: Specific Activities (bq kg -1 ) for deep-sea and coastal sediments measured by alpha and gamma spectroscopy. All weights are corrected for salt content. C-Disk-IV samples are deep-sea sediments and MUC samples are coastal sediments. MCB1, MCB2, and MCB3 are the 3 USC HPGe detectors available to measure 210 Pb. All gamma spectroscopy measurements used a 10% self-adsorption correction. Counts were done in small PP tubes. Sample ID Alpha sig MCB1 sig MCB2 sig MCB3 sig bq/kg bq/kg bq/kg bq/kg Cdisk_sta.2_0-1cm 500 12 502 22 502 19 477 16 Cdisk_sta.2_1-2cm 360 11 369 12 352 14 368 28 Cdisk_sta.2_2-3cm 447 14 456 14 441 18 450 17 Cdisk_sta.2_3-4cm 544 19 544 21 517 19 514 24 Cdisk_sta.2 4-5cm 536 15 511 26 510 17 495 30 Cdisk_sta.2_9-10cm 631 15 559 30 655 24 602 29 MUC9_0-1cm 1861 87 1835 78 1846 35 1949 51 MUC9_2-3cm 926 23 873 31 871 22 954 62 MUC9 3-4cm 552 15 N/A N/A 544 17 581 20 MUC9 9-11cm 129 5 N/A N/A 134 11 130 17 Table A.3.5: HPGe and Alpha Spectroscopy ratios for coastal and deep-sea sediments. All weights are corrected for salt content. C-Disk-IV samples are deep-sea sediments and MUC samples are coastal sediments. HPGe/Alpha Ratio MCB1 sig MCB2 sig MCB3 sig Cdisk_sta.2_0-1cm 1.00 0.05 1.00 0.04 0.95 0.04 Cdisk_sta.2_1-2cm 1.03 0.05 0.98 0.05 1.02 0.08 Cdisk_sta.2_2-3cm 1.02 0.04 0.99 0.05 1.01 0.05 Cdisk_sta.2_3-4cm 1.00 0.05 0.95 0.05 0.94 0.06 Cdisk_sta.2 4-5cm 0.95 0.06 0.95 0.04 0.92 0.06 Cdisk_sta.2_9-10cm 0.89 0.05 1.04 0.05 0.95 0.05 MUC9_0-1cm 0.99 0.06 0.99 0.05 1.05 0.06 MUC9_2-3cm 0.94 0.04 0.94 0.03 1.03 0.07 MUC9 3-4cm N/A N/A 0.99 0.04 1.05 0.05 MUC9 9-11cm N/A N/A 1.04 0.09 1.01 0.14 203 Table A.3.6: Summary for HPGe and Alpha Spectroscopy ratios for coastal and deep-sea sediments. All weights are corrected for salt content. Sample STDEV is the standard deviation divided by the square root of the number of samples measured (STDEV/SQRT(N)). All HPGe detectors are within uncertainty with alpha measurements. HPGe/Alpha Ratio MCB1 MCB2 MCB3 Coastal Average 0.96 0.99 1.02 Sediments Sample STDEV 0.02 0.02 0.02 Deep-Sea Average 0.98 0.98 0.97 Sediments Sample STDEV 0.02 0.01 0.02 Total Average 0.98 0.99 0.99 Sample STDEV 0.02 0.02 0.02 Figure A.3.3: Specific activities determined from gamma and alpha spectroscopy for deep-sea and coastal marine sediments. The slope of the linear regression line is forced through zero. All activities are corrected for salt content. All gamma spectroscopy measurements used a 10% self- adsorption correction. 204 A.3.5 References Collé, R, R P Fitzgerald, and L Laureano–Perez. “A New Determination of the 209 Po Half-Life.” Journal of Physics G: Nuclear and Particle Physics 41, no. 10 (2014): 105103. https://doi.org/10.1088/0954-3899/41/10/105103. 205 A.4 Calibration of 229 Th Solution: USC #89B The 229 Th solution (USC #89B) was created on October 2, 2018 by Kemnitz, N.J. at USC. This solution was used as a spike for isotope dilution alpha spectrometry analysis of 227 Ac. This appendix describes the calibration of the spike using the WHOI 227 Ac Eckert and Ziegler standard solution and two other reference materials. 229 Th (t1/2=7500yr) and its longest-lived progenitors, 225 Ra (t1/2=15d) and 225 Ac (t1/2=10d), are assumed to be in secular equilibrium. A small aliquot (0.1 mL) was taken from USC #89A ( 229 Th activity: ~ 25,000 dpm/g) and diluted with 250 mL of 3 N HNO3. This solution, now called USC#89B, was calibrated against two certified 227 Ac (WHOI E&Z Actinium-227 CRM) and Thorium Ore (IAEA-RGTh-1) obtained from IAEA. solutions and one previous calibrated solution of 232 U/ 228 Th (Harwell: USC #32) using isotope dilution alpha spectroscopy. Details about extraction and source preparation for Ac and Th can be found in Appendix A.1. Briefly, 2 mL aliquots from each standard solution were added and weighed in Teflon beakers, then evaporated to dryness. The Thorium ore, which was in an oxidized powder form, was dissolved with Aqua Regia and 8 N HCl (#89B was added prior to dissolution) before being evaporated. After dryness, solutions were brought up in 25 mL of 3 N HNO3 and passed through extraction chromatography resins (DGA cartridge, Eichrom Inc.). The cartridges were pre- conditioned with 10 mL 3 N HNO3. The first rinse, with the load solution, removed radium and other earth alkaline metals since these metals have no affinity for the resins at this acid concentration. The DGA cartridge was then rinsed with 25 mL 2 M HCl to remove Ac isotopes 206 ( 225 Ac and 227 Ac), and the eluted solution was set aside for source preparation for Actinium. Finally, the DGA cartridges were rinsed with 20 mL 0.1 N HF + 20 mL 0.01 N HCl, which strips Th from the resins. This last rinse was set aside and prepared for source preparation for Th. Figure A.4.1 shows the alpha spectrum for actinium, counted within 1-2 days of elution. Only 225 Ac and its daughters are present in the figure because 227 Ac is not an alpha emitter and its daughters have not grown into equilibrium yet. The first measurement is the determination of the total yield. The total yield is comprised of the chemical yield and the detector efficiency. The total yield will be used 90 days later when the activity of 227 Ac is determined by counting its two alpha-emitting daughters, 227 Th and 223 Ra. (Fig. A.4.2). Details for the determination of 227Ac activity can be found in Appendix A.1. Figure A.4.1: Alpha-spectrum of 225 Ac and its daughters counted immediately after micro- precipitation was completed. The yield tracer for #89B is 227 Ac, which is not shown on the spectrum because it is not an alpha emitter. In approximately 90 days, 227 Ac daughters will be 90% grown into secular equilibrium and the total yield will be determined at that time. 207 Figure A.4.2: Alpha-spectrum of 227 Ac and its daughters counted 120 days after preparation. The yield tracer for #89B is 227 Ac and its daughters, 223 Ra and 227 Th, which are now 98% grown into equilibrium with 227 Ac. The activity of the #89B solution is believed to be 9.40 ± 0.10 dpm/g, determined from eq. A.1.3, calibrated vs. the Eckert and Ziegler standard. Figure A.4.3: Alpha spectrum for #89B and Harwell #32C solutions using isotope dilution alpha spectroscopy. Enough time has passed (~120 days) that the 229 Th descendant , 217 At, can be used to represent the 229 Th. This isotope is used because of minimal interference in the 217 At energy region. The Harwell (#32C) solution, containing 232 U and its progeny, was calibrated on March 1, 1989, and had a nominal activity of 9.97 ± 0.10 dpm/g. Figure A.4.3 shows the spectrum results for the calibrated solutions against #89B using isotope dilution alpha spectroscopy. Eq. A.4.1 shows the calculation for the #89B solution using the Harwell spike. The BR% and energies of both 228 Th 208 and 229 Th can be found in Table A.4.1. A small correction must be applied to the 228 Th region to correct for 224 Ra interference (BR%=5.0, 5.45 MeV). 𝑇ℎ 44b (𝑑𝑝𝑚)= a !!P a !!S 𝑒 ("% S !/! ∆+) 𝐴 44dIN (A.4.1) Where 229 N and 228 N are the background-subtracted net counts of 229 Th and 228 Th; ∆t is the time that elapsed from the date of last calibration of the Harwell spike (3/1/89) and midpoint of counting; A228Th is the amount of 228 Th spike added in dpm; and l is the decay constant for 232 U (parent of 228 Th for this spike; t1/2=68.9 yr). Table A.4.1: Energies of alpha particles, branching ratio (BR%), and half-lives for various radioisotopes used in this study. Isotope Energy Region (MeV) BR% Half-Life 232 Th 3.81 - 4.01 100 1.4E10 y 230 Th 4.44 - 4.69 100 75,380 y 229 Th 4.69 - 5.05 100 7,340 y 209 Po 4.88 100 125 y 228 Th 5.42 – 5.34 99.5 1.91 y 210 Po 5.30 100 138 d 223 Ra 5.50 - 5.75 97.7 11.4 d 225 Ac 5.55 - 5.83 99.2 10 d 221 Fr 6.08 - 6.34 100 4.9 m 227 Th 5.91 - 6.04 54.6 18.7 d 217 At 7.07 100 32 ms 213 Po 8.38 100 4 µs Lastly, a certified IAEA Thorium Ore (IAEA-RGTh-1) was used to calibrate the #89B spike. Figure A.4.4 shows the alpha-spectrum results for the IAEA Thorium Ore and #89B spike 209 solutions. The Thorium Ore (IAEA-RGTh-1) solution was certified on March 1, 1989 and had a nominal activity of 195.0 ± 10.8 dpm/g (bought from IAEA website). Eq. A.4.3 shows the calculation for the #89B solution using the Thorium Ore spike. 𝑇ℎ 44b (𝑑𝑝𝑚)= a !!P a !!S 𝐴 44dIN (A.4.3) where 229 N and 228 N are the background-subtracted net counts of 229 Th and 228 Th (or 232 Th) and A228Th is the amount of 228 Th (or 232 Th) spike added in dpm. It is assumed that 228 Th is in secular equilibrium with its parent, 232 Th and that both isotopes can be used in eq. 6.4.3. Figure A.4.4: Alpha spectrum for #89B and Thorium Ore (IAEA-RGTh-1) solutions using isotope dilution alpha spectroscopy. Both thorium isotopes, 232 Th and 228 Th, can be used to calibrate the 229 Th activity from #89B. 210 Table A.4.2: Calibration summary for #89B spike solution. The average of all three calibration are shown at the bottom with standard deviation of the mean. The final activity of the #89B spike solution is 9.32 ± 0.12 dpm g -1 . Isotope used as the yield tracer (or isotope used for isotope dilution) for #89B is shown, along with spike activity and reference date. Spike Solution Isotope used for Spike Activity Spike Ref. 229 Th Activity Isotope Dilution (dpm g -1 ) Date (dpm g -1 ) Actinium-227 WHOI 227 Ac 5.40±0.10 4/27/2017 9.40±0.10 HARWELL #32C 228 Th 9.97±0.10 3/1/1989 9.49±0.23 Thorium Ore (IAEA) 232 Th/ 228 Th 195.0 ± 10.8 1/1/1987 9.08±0.50 Average 9.32±0.12 211 A.5 McLane in-situ pump setup for Ra and Ac extraction from seawater The last 5 major US GEOTRACES cruises have collected Ra and Ac from seawater by pumping large volumes of seawater through grooved acrylic cartridges impregnated with MnO2 (called commercial cartridge or CC for short). These cartridges are set up on a McLane in-situ pump (ISP) that is plumbed to allow 1500 L to pass through the cartridges in a 4-hour pump time. Each ISP had two filters in parallel: the higher volume flow path had a 1 µm quartz filter (Whatman QMA) and the lower volume flow path had a 0.8 μm polyethersulfone filter (Pall Supor800). Downstream of the filter heads, flow then passes through two MnO2 cartridges that sit in series. Having two MnO2 filters sit in series during the pumping time allows for a direct adsorption efficiency to be determined and allows about 90% of the Ra and Ac to be captured. The average absorption efficiency for these cartridges during the last 2 US GEOTRACES cruises were 65% and 80% for Ac and Ra. (EPZT: GP16 and PMT: GP15). Figure A.5.1 shows the ISP setup for Ra and Ac extraction from sweater. Filter heads are not present on these pumps; the filter heads are put on right before the ISPs go on the wire casts. Figure A.5.2 shows the schematics for seawater flow through the ISP system. The red arrows are seawater flow, carried by Tygon® tubing (3/4’’ ID) throughout entire system. After seawater flows through each filter head, it travels through a volume counter for each filter (Filter #1 and Filter #2) and then the two flows merge and pass through a one-way valve. After the seawater exits the one-way valve, it enters the first cartridge holder (A), and then immediately enters the second cartridge holder (B), after which the seawater exists cartridge holder B and heads towards the pump. After the pump, the combined seawater flow is recorded and exits the system, directed 212 down and away from the ISP filter intakes. The total volume flow counter should equal the sum of the Filter #1 and Filter #2 volume counters. If it does not, then using the total flow volume counter is likely the most accurate volume for Ra and Ac specific activity (i.e., dpm L -1 ). It is very rare for the volume counters to not be consistent. If there is a mismatch, a leak check must be performed. The MnO2 treated CCs are mounted in a 5” acrylic commercial cartridge holder, with a strong spring below that pushes upward on a PVC disk that forces the CC to seal against the lid of the holder. The A and B holder lids are bolted to a stainless steel (SS) plate that is welded to a 1/2” SS rod that has been covered with electrical tape (to avoid electrochemical junctions) and secured to the ISP frame with hose clamps. As noted above, the system components are connected by Tygon® tubes attached to hose barb fittings (3/4”) using hose clamps (Fig. A.5.1). Electrical tape is also used to secure the tubing to the ISP system. Figures A.5.1 and A.5.2 also show a de-bubbler on cartridge holder A. This de-bubbler consists of an open PVC tube attached to a check valve mounted on a T, with flow passing through the other two openings of the T. The de-bubbler must be oriented vertically so that the flow arrow points up. It allows any air to flow out of the system as it is submerged, and when the pump is turned on, lowering the pressure in the pump stream below ambient pressure, the check ball seats to prevent water flow into the flow stream. Finally, there is a small blue inlet valve on the bottom of the ISP system (before the one-way valve). This inlet allows seawater to be introduced into the system before deployment, to minimize any air bubbles present in the CC housing. It is best practice to fill the system tubing with seawater the best you can before deployment (the blue valve in Fig A.5.1). 213 Figure A.5.1: McLane in-situ pump (ISP) setup for Ra and Ac extraction from seawater. The filter heads are not attached to the ISPs in these pictures. [A] = volume counters , [B] = one-way vale, [C] = cartridge holders , [D] = valve to pump system full of seawater before deployment, [E] = battery pack, and [F] = pump. 214 Figure A.5.2: Schematic for seawater flow through McLane in-situ pump (ISP) system. There are 3 volume counters on the ISP: one for each filter head (Filter#1 and Filter#2 volume counter) and one at the end of the ISP system (total volume counter). The pump is pulling seawater toward it from the end of the system, and seawater exists the system directly after the pump. 215 A.6 Reaction-Transport Model MATLAB Code Below is a MATLAB script for the Cochran and Krishnaswami (1980) solution of the reaction-transport equations defined in eq. 2.3a and 2.3b. C-Disk-IV station 1 is used as an example for the behavior of 227 Ac and 231 Pa in deep-sea sediments. Known variables are first defined, these include sedimentation rate, porosity, density, distribution coefficient, decay constant, bioturbation, molecular diffusion, activity of 231 Pa and 235 U in the bioturbated layer, and the bioturbated layer thickness. The only unknown variable is the fraction released by 231 Pa decay (F) which recoils 227 Ac atoms into porewater (See below for determination of F). It assumes that the fraction released by 235 U decay (50%) is known since it adds negligible 227 Ac into porewaters. The constants needed for solution of the equations (Q, R, D, a, d, e, b, g, µ) are all defined below. These constants are also found in Cochran and Krishnaswami (1980). 𝐷 ! =𝐷 " +𝐾𝐷 # (A.6.1) 𝐷 $ =𝐷 " (A.6.2) 𝛼 ! = %(!'( ) $* ! (A.6.3) 𝛼 $ = %(!'( ) $* " (A.6.4) 𝛽 ! = +, #$ (!'( ) * ! (A.6.5) 𝛽 $ = +, #$ (!'( ) * " (A.6.6) 𝜀 ! =𝛼 ! −) 𝛼 ! $ −𝛽 ! (A.6.7) 𝜀 $ =𝛼 $ −) 𝛼 $ $ −𝛽 $ (A.6.8) 𝛿 ! =𝛼 ! +) 𝛼 ! $ −𝛽 ! (A.6.9) 𝛿 $ =𝛼 $ +) 𝛼 $ $ −𝛽 $ (A.6.10) 216 𝜇= , %& % (A.6.11) 𝛾=(𝐷 ! 𝛿 ! −𝐷 $ 𝜀 $ )𝑒 - " . 𝑒 / " . +(𝐷 $ 𝜀 $ −𝐷 ! 𝜀 ! )𝑒 - ! . 𝑒 - " . (A.6.12) 𝑄 ! = ! 0 12𝐴 12 − 31 %& '41 ' !'( 4(𝐷 ! 𝛿 ! −𝐷 $ 𝛿 $ )𝑒 / ! . 𝑒 - " . −2 31 %& * " - " !'( + , #$ 31 %& (- " '5) 5 " '$6 " 5'7 " 4𝑒 - " . 5 (A.6.13) 𝑅 ! = ! 0 12𝐴 12 − 31 %& '41 ' !'( 4(𝐷 $ 𝜀 $ −𝐷 ! 𝜀 ! )𝑒 - ! . 𝑒 - " . +2 31 %& * " - " !'( + , #$ 31 %& (- " '5) 5 " '$6 " 5'7 " 4𝑒 - " . 5 (A.6.14) 𝑄 ! = " # 01 $ !" %& #$ ' ' % ( !) %'( *% 23𝑒 +&, −𝑒 -&, 6−𝐷 " 1 %& #$ "( . + $ !" %& #$ /% (' % ( !) %'( *% ) 23𝜀 " 𝑒 -&, −𝛿 " 𝑒 +&, 6+𝐷 " 1𝐴 &2 − %& #$ ( 3& ' "( . 2(𝛿 " −𝜀 " )𝑒 -&, 𝑒 +&, < (A.6.15) 𝑅 $ =0 (A.6.16) Where: F = fraction of 231 Pa decay which recoil 227 Ac atoms into porewater f = fraction of 235 U decay which recoil 227 Ac atoms into porewater 𝜆 D# = decay constant for 231 Pa (yr -1 ) 𝜆 (L = decay constant for 227 Ac (yr -1 ) 𝐷 5 = molecular diffusion corrected for tortuosity (cm 2 yr -1 ) 𝐷 . = bioturbation (cm 2 yr -1 ) 𝑆 = sedimentation rate (cm yr -1 ) 𝐿 = length of bioturbation zone (cm) 𝐾 = partition coefficient (dimensionless) 𝐴 (L = initial activity of 227 Ac at sediment-water interface (dpm/cc) 𝐴 D# = activity of 231 Pa in mix layer (dpm/cc) The only parameter that was allowed to vary was F, the fraction released by 231 Pa decay, which recoils 227 Ac atoms into porewaters. Each iteration of F value in the model gave values for 227 Ac in each horizon. These values were then compared to measured 227 Ac values in the upper few cm of sediments. The difference was taken between measured and predicted 227 Ac values and then both were squared and summed: 𝑆𝑆 =∑(𝐴𝑐 Re#5 −𝐴𝑐 RCBeJ ) 4 (A.6.17) 217 The least sum-squares (SS) were then plotted against F and fit with a 2 nd order polynomial function. The polynomial derivative was then taken, and the minimum value of F was determined where the derivative went to zero. Fig. A.6.1 shows C-Disk-IV station 1 SS vs. F plot. Figure A.6.1: Sum-Square (SS) vs. Fraction released (F) for C-Disk-IV station 1. The minimum of the 2 nd order polynomial function for station 1 is 0.47. The data points on the figure refer to the F values chosen for simulation in the reaction-transport model. A.6.1 MATLAB Code %MATLAB code based on Cochran and Krishnaswami (1980) %Equations for 227Ac and 231Pa in sediments: %C-Disk-IV STATION.1 %%Known Variables S=0.40/1000; %sed rate in cm/yr por=0.82; %porosity (no units) average porosity in upper 10 cm rho=2.5; %density of sediments (g/cc) Kd=15120; %distribution coefficient for 227Ac (cc/g) K=Kd*(rho*(1-por))/(por) %partition coefficient (no units) lambdaAc=log(2)/21.77; %decay constant 227Ac (1/yr) lambdaPa=log(2)/32760; %decay constant 231Pa (1/yr) Ds=2.9e-6*(60*60*24*365.25)*por^2; %molecular diffusion of Ac (cm2/yr) 218 Db=0.008; %bioturbation rate (cm2/yr) Pam=3.05; %measured total 231Pa in dpm/g AUm=0.09; %activity of 235U in mix layer (dpm/g); AU = (AUm*rho*(1-por))/por; %activity of 235U (dpm/cc); APal= (Pam*rho*(1-por))/(por); %activity of 231Pa in mix layer(dpm/cc) L=6.5; %mix layer thickness: units, cm AAc0=0.000005; %bottom water concentration of 227Ac %%Unknown variables F=0.45; %Fraction of excess 231Pa decay which recoils Ac atoms into porewater f=0.5; %Fraction of 235U supported 231Pa decay which recoils Ac atoms into porewater %%Constants D1=(Ds+K*Db); %units, cm2/yr D2=(Ds); %units, cm2/yr alpha1=S*(1+K)/(2*D1); %units, 1/cm alpha2=S*(1+K)/(2*D2); %units, 1/cm beta1=-lambdaAc*(1+K)/(D1); %units, 1/cm^2 beta2=-lambdaAc*(1+K)/(D2); %units, 1/cm^2 e1=alpha1-sqrt(alpha1^2-beta1);%units, 1/cm e2=alpha2-sqrt(alpha2^2-beta2);%units, 1/cm q1=alpha1+sqrt(alpha1^2-beta1);%units, 1/cm q2=alpha2+sqrt(alpha2^2-beta2);%units, 1/cm mu=lambdaPa/S; gamma=(D1*q1-D2*e2)*exp(e2*L)*exp(q1*L) + (D2*e2-D1*e1)*exp(e1*L)*exp(e2*L); %units, cm/yr Q1=(1/gamma)*[(AAc0-(F*APal+f*AU)/(1+K))*(D1*q1-D2*e2)*exp(q1*L)*exp(e2*L) - ((F*APal*D2*e2)/(1+K) + (lambdaAc*F*APal*(e2+mu))/(mu^2+2*alpha2*mu+beta2))*exp(e2*L)]; R1=(1/gamma)*[(AAc0-(F*APal+f*AU)/(1+K))*(D2*e2-D1*e1)*exp(e1*L)*exp(e2*L) + ((F*APal*D2*e2)/(1+K) + (lambdaAc*F*APal*(e2+mu))/(mu^2+2*alpha2*mu+beta2))*exp(e2*L)]; Q2=(1/gamma)*[(lambdaAc*F*APal*mu)/(mu^2+2*alpha2*mu+beta2)*(exp(q1*L) - exp(e1*L)) - D1*(F*APal/(1+K) + (lambdaAc*F*APal)/(D2*(mu^2+2*alpha2*mu+beta2)))*(e1*exp(e1*L) - q1*exp(q1*L)) + D1*(AAc0 - (F*APal+f*AU)/(1+K))*(q1-e1)*exp(e1*L)*exp(q1*L)]; R2=0; %constant dx=0.1; %∆x for model %%mix zone 219 %Solutions to Equations from Cochran and Kris (1980) x=[0:dx:L]; %determine 227Ac activity for every interval from 0-L %for 0<x<L A1=Q1*exp(e1*x) + R1*exp(q1*x) + (F*APal+f*AU)/(1+K); %solid phase 227Ac Atot1=Kd*A1+(1-F)*(Pam); %Below mix zone xx=[L:dx:30]; %determine 227Ac activity for every interval from L-30cm %for x>L A2= Q2*exp(e2*xx) - lambdaAc*(F*APal)*exp(-mu*(xx- L))/(D2*(mu^2+2*alpha2*mu+beta2)) + (f*AU)/(1+K); %solid phase 227Ac Atot2=Kd*A2+(1-F)*(Pam)*exp(-mu*(xx-L)); %231Pa decay after mix zone Apa=(Pam)*exp(-mu*(xx-L))+AUm*(1-f); %% plots! figure(1) plot(Atot1,x,'--k'); %plot total 227Ac determined in model in mix layer hold on; plot(Atot2,xx); %plot total 227Ac determined in model below mix layer hold on; plot(Ath,xx,'-.'); %plot total 230Th determined in model below mix layer ylim([0 10]); xlim([0 5]); set(gca, 'YDir','reverse'); title('227Ac vs. depth','fontsize',18); xlabel('227Ac (dpm/g)','fontsize',18); ylabel('Depth (cm)','fontsize',18); A22=[2.33,3.35,3.05]; %measured 227Ac activity in dpm/g x22=[0.5,1.5,2.5]; plot(A22,x22,'o','LineStyle','none','MarkerSize',10); hold on; A33=[3.25,2.94,3.11]; %measured 231Pa activity in dpm/g x33=[0.5,1.5,7.5]; plot(A33,x33,'o','LineStyle','none','MarkerSize',10); hold on; set(gca, 'YDir','reverse'); Er=[0.10,0.16,0.12];%errors 227Ac errorbar(A22,x22,Er,'horizontal','LineStyle','none'); Er1=[0.17,0.07,0.18];%errors 231Pa errorbar(A33,x33,Er1,'horizontal','LineStyle','none'); lgd=legend('Model 0\leqx\leqL','Model x\geqL','Parent Activity','227Ac','231Pa'); lgd.FontSize = 14; %% compare measured and calculated 227Ac sumsquare=0; 220 DP=3; for i=1:1:DP; sumsquare=sumsquare+(A22(i)-Atot1(i))^2; end SS=sumsquare; 221 A.7 Numerical Model MATLAB Code Below is a MATLAB script for Cochran and Krishnaswami (1980) reaction-transport equation defined in eq.2.1. The MATLAB code uses a numerical approach to solve the reaction-transport equation and only uses the upper few cm of sediments to model 227 Ac. Furthermore, the numerical model assumes S=0 since S is very small (<0.5 cm/kyr). Known variables are first defined. These include 227 Ac and 231 Pa decay constants, bioturbation rate (Db), molecular diffusion (Dm), F, 231 Pa activity in mix layer, and distribution coefficient (kd). Next, an array of porosity for every depth modeled was created based on fitting a logarithmic function to the measured porosity vs. depth profile. Next K and Ds values were assigned to every depth since these values depend on porosity. Lastly, initial conditions were applied to the model and the model ran for 100 years, producing 227 Ac values for every 0.1 cm in the top 10 cm. The numerical model results are very similar to the analytical reaction-transport model, differing by only about 10%, much less than the uncertainties. This indicates that rapid changes in porosity in the upper few cm of sediments has little effect on the analytical model. The model is created to change variables and observe how the 227 Ac profiles behaves relative to the change. For this model, it is optimized to change F, Db, and kd. For this thesis, F was changed until the lowest sum square (SS) was achieved. The model takes in the measured 227 Ac values in the upper few cm of sediments (input 227 Ac measured values into variable B1) and compares it to the model results for each cm. 222 A.7.1 MATLAB Code %This is a numerical model for Ac in deep-sea sediments. %The only transport mechanism is diffusion and advection has been ignored %Equation that will be used numerical in this model --> %dC/dt=(Ds+K*Db)*d2C/dx2 - lambda*(1+K)*C + P %% lambdaAc=log(2)/21.77; %decay constant 227Ac in 1/yr lambdaAc1=(log(2)/21.77)/(365.25*24*60); %decay constant 227Ac in 1/min lambdaPa=log(2)/32760; %decay constant 231Pa in 1/yr lambdaPa1=(log(2)/32760)/(365.25*24*60); %decay constant 231Pa in 1/min F=0.49; %fraction released of mobile Ac rho=2.5; %density of sediments in g/cm3 Apa=3.09; %activity of 231Pa in dpm/g Apaa=Apa*(365.25*24*60); %activity of 231Pa in dpy/g Dm=2.9e-6*(60*60*24*365.25) %molecular diffusion(cm2/yr) Db=0.007; %bioturbation rate (cm2/yr) por=0.87; %porosity Kd=15125; %distribution coefficient (cc/g) K=(Kd*rho*(1-por))/(por) %dimensionless coefficient(no units) dx=0.1; %length of interval N=100; %number of Boxes in model %% %Create arrays for porosity and K %porosity array with depth starting at 0.1 cm %equation is the fit to porosity profile x=[1:1:N+1]; xlength=length(x); por1=zeros(xlength,1); for j=1:1:xlength; por1(j)=0.81501 - 0.086889*log10(j*dx); %Fit R value = 0.963 STA.3 end %K array: start by defining Kd in the upper few cm %Kd starts at measured K value from plots: Insert starting K value above K1=zeros(xlength,1); for i=1:1:N+1; K1(i)=(Kd*rho*(1-por1(i)))/(por1(i)); end %create Ds from Dm Ds=Dm*(por1.^2); 223 %% %initial conditions: por=por1(1); porl=por1(N+1); %create porosity for last interval Plast=(F*rho*(1-porl)*Apaa)/(porl); %production term for last interval box %P=[(F*rho*(1-por)*Apaa)/(por)]; %production term units in atoms/cc- yr C=((Plast)/(lambdaAc*(1+K1(N)))*ones(N,1)); %create initial array of all P C(N+1)=((Plast)/(lambdaAc*(1+K1(N)))); %boundary conditions P: (atoms/cc- yr) dt=0.00004; %time step in units of yr sumtime=0; dC=zeros(xlength,1); %% %for loop for timestep=1:2500000; CW00=3;%INITIAL CONCENTRATION AT SWI (UNITS OF dpm/m3) %concentration in water above SWI: 3.0 dpm/m3 = 0.000003 dpm/cc = 49.6 atoms/cc CW0=CW00/1000000;%convert m3 to cc CW=CW0/lambdaAc1; %convert dpm to atoms %initial concentration, %first box dC dC(1)=dt*[(Ds(1)+K1(1)*Db)*por1(1)*((CW - C(1))/(dx^2/2)) + (Ds(1)+K1(1)*Db)*por1(1)*((C(2)-C(1))/(dx^2))- lambdaAc*(1+K1(1))*C(1) + ((F*rho*(1-por1(1))*Apaa)/(por1(1)))]; %because the distance from middle of box 1 to overlying water is 1/2 of a box dimension. for i=2:1:N;%for loop for dC dC(i)=dt*[((Ds(i)+K1(i)*Db)*por1(i)*(C(i-1) - 2*C(i) + C(i+1))/(dx^2)) - lambdaAc*(1+K1(i))*C(i) + ((F*rho*(1-por1(i))*Apaa)/(por1(i)))]; end for i=1:1:N;%increment boxes with dc C(i) = C(i) + dC(i); end sumtime=sumtime+dt; sumdays=sumtime*365.25; end sumetime=sumtime+dt %% Plot 227Ac vs. depth in dpm/cc 224 C1=lambdaAc1*C; % multiple C by 227Ac decay constant in 1/min to convert to dpm (atoms*lambda=Activity) xx=[0:dx:N*dx]; % x array for plotting figure(1);%dpm/cc vs. depth plot(C1,xx); ylim([0.0 10]); set(gca, 'YDir','reverse'); title('227Ac vs. depth','fontsize',18); xlabel('227Ac (dpm/cc)','fontsize',18); ylabel('Depth (cm)','fontsize',18); %% 227Ac Flux Ds(1); %Ds in first interval por1(1); %porosity in first interval Slope=(C1(1)-CW0)/(dx/2); %units=dpm/(g*cm) Flux3=(Slope)*Ds(1)*por1(1)*100^2 %dpm/m2-yr %% Sum Square Values B1=[2.33,3.35,3.05,3.11]; %measured 227Ac values (dpm/g) x1=[0.5,1.5,2.5,7.5]; %depth of 227Ac measured values (cm) Er=[0.09,0.15,0.11,0.18]; %measured 227Ac error values %%mass summation (denominator) mx=rho*(1-por1)*dx; %g/cc*(1-porosity) mxb= reshape(mx(1:100), [], 10); summass = sum(mxb); %sum every 10 intervals. Sum down to 10cm summass1=sum(summass);%total vector sum, all 10cm %%mass summation (numerator) C3=rho*(1-por1).*C1*dx; %g/cc*(1-porosity) C3b= reshape(C3(1:100), [], 10); Actmass = sum(C3b); %sum every 10 intervals. Sum down to 10cm sumActmass1=sum(Actmass);%total vector sum, all 10cm %%take model values and average for every cm, including the mass, final units are in dpm/g for i=1:1:length(summass); Cmodel(i)=Kd*Actmass(i)/summass(i)+ (1-F)*Apa; end %compare measured and calculated 227Ac sumsquare=0; for i=1:1:3; sumsquare=sumsquare+(B1(i)-Cmodel(i))^2; end sumsquare 225 A.8 Santa Monica Basin Supplementary Information Figure A.8.1: 226 Ra values for SMB cores MUC 9, DOE 65, and DOE 25. All three cores are sampled greater than 900 meters depth and within close approximation to each other. MUC 9 was collected in 2016, and the measurement was based on gamma spectroscopy corrected for 10% Rn loss. The other two cores were collected in the late 1980’s and a composite sample was analyzed by Rn ingrowth from the dissolved sediment (Christensen et al., 1993). All 226 Ra values are within uncertainty of each other. 226 Figure A.8.2: Mass Flux vs. Collection Year for all cores greater than 850 meters in the SMB. Open circles and black and white squares are cores collected from >900 meters and 850-900 meters depth regimes. Figure A.8.3: Timeline of Los Angeles basin land usage beginning in 1800 until the present (2020). Note the demise of a shelly benthic community living on the Santa Monica Basin shelf (late 1800: Tomašových and Kidwell, 2017). 227 A.9 Dissolved 231 Pa, 227 Ac, and 227 Acex along GP15 transect Table A.9.1: Activities for total dissolved 231 Pa, 227 Ac, and excess 227 Ac for each depth sampled along GP15 transect. Depth off bottom (DAB) is also shown for each depth along with station bottom depth (B.D.). Cartridge type (cart. type) refers to what cartridges were used on each pump. ‘A’ cartridges were made by Woods Hole Oceanographic Institution (WHOI) group and ‘B’ cartridges were made by University of Southern California (USC) group. 231 Pa data is from R. Anderson, L., Edwards, M. Fleisher, and E. Black (pers. comm.). STA. 4 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 34 5561 0.01±0.00 0.13±0.12 0.13±0.12 A 5595 74 5521 0.01±0.00 0.19±0.10 0.18±0.10 A 104 5491 0.01±0.00 0.26±0.11 0.25±0.12 A 204 5391 0.02±0.00 0.08±0.05 0.07±0.05 A 405 5190 0.04±0.00 0.22±0.07 0.18±0.07 A 806 4789 0.07±0.00 0.04±0.05 -0.03±0.05 A 1205 4390 0.11±0.00 0.17±0.03 0.06±0.03 A+B 1607 3988 0.13±0.01 0.25±0.07 0.12±0.07 A+B 1997 3598 0.21±0.01 0.14±0.07 -0.07±0.08 A 2750 2845 0.23±0.01 0.44±0.05 0.21±0.06 A+B 3603 1992 0.3±0.01 0.66±0.08 0.37±0.09 A+B 4506 1089 0.29±0.01 0.53±0.12 0.24±0.13 A 5005 590 0.02±0.00 1.65±0.18 1.67±0.22 A 5297 298 0.26±0.02 0.79±0.07 0.54±0.09 A+B 5497 98 0.27±0.01 1.40±0.08 1.16±0.13 A+B 5547 48 0.28±0.01 1.36±0.12 1.11±0.16 A+B STA. 5 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 28 4583 0.07±0.02 0.15±0.11 0.08±0.12 A+B 4611 43 4568 0.01±0.00 0.09±0.04 0.08±0.04 A+B 73 4538 0.02±0.01 0.08±0.03 0.06±0.04 A+B 93 4518 0.01±0.01 0.05±0.04 0.04±0.04 A+B 143 4468 0.01±0.01 -0.49±0.24 -0.51±0.24 A+B 303 4308 0.01±0.00 0.07±0.03 0.06±0.03 A+B 453 4158 0.01±0.00 0.05±0.07 0.04±0.07 A+B 803 3808 0.01±0.00 0.14±0.06 0.14±0.06 A+B 984 3627 0.01±0.00 0.13±0.03 0.12±0.03 A+B 1491 3120 0.12±0.01 0.16±0.04 0.04±0.04 A+B 2502 2109 0.17±0.03 0.39±0.05 0.22±0.06 A+B 3517 1094 0.24±0.04 0.83±0.07 0.59±0.09 A+B 4171 440 0.24±0.03 0.72±0.08 0.49±0.10 A+B 4519 92 0.25±0.03 1.49±0.11 1.25±0.16 A+B 4595 16 0.24±0.03 1.50±0.09 1.27±0.14 A+B 228 STA. 6 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 38 5068 0.01±0.00 -0.04±0.08 -0.05±0.08 A 5106 58 5048 0.01±0.00 0.1±0.08 0.09±0.08 A 248 4858 0.02±0.00 0.05±0.08 0.04±0.08 A 549 4557 0.01±0.00 0.25±0.08 0.25±0.08 A 747 4359 0.06±0.00 0.20±0.10 0.15±0.10 A 999 4107 0.09±0.00 0.32±0.15 0.23±0.16 A 1492 3614 0.13±0.00 0.18±0.04 0.05±0.04 A+B 1993 3113 0.16±0.01 0.14±0.03 -0.02±0.03 A+B 2494 2612 0.23±0.01 0.12±0.03 -0.11±0.03 A+B 2995 2111 0.27±0.01 0.56±0.06 0.30±0.07 A+B 3497 1609 0.31±0.01 0.45±0.07 0.14±0.07 A+B 3998 1108 0.29±0.02 0.75±0.08 0.47±0.09 A+B 4497 609 0.31±0.01 0.71±0.21 0.41±0.22 A+B 5064 42 0.33±0.01 1.28±0.09 0.96±0.13 A+B STA. 8 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 41 5097 0.01±0.01 0.15±0.09 0.14±0.10 A 5138 61 5077 0.01±0.01 -0.05±0.05 -0.07±0.05 A 86 5052 0.01±0.01 0.04±0.11 0.03±0.11 A 145 4993 0.02±0.01 0.17±0.09 0.15±0.09 A 195 4944 0.02±0.01 0.28±0.13 0.27±0.14 A 297 4841 0.03±0.01 0.10±0.13 0.07±0.14 A 398 4740 0.03±0.02 0.06±0.08 0.03±0.08 A 401 4737 0.03±0.02 0.11±0.05 0.08±0.06 A 501 4637 0.06±0.01 0.06±0.11 0.00±0.11 A 602 4536 0.07±0.02 0.31±0.92 0.25±0.94 A 802 4336 0.09±0.02 0.08±0.05 -0.01±0.05 A 1002 4136 0.10±0.02 0.04±0.03 -0.07±0.03 A+B 1301 3837 0.12±0.03 0.10±0.03 -0.03±0.03 A+B 1700 3438 0.14±0.02 0.08±0.03 -0.06±0.03 A+B 1891 3247 0.19±0.05 0.13±0.03 -0.06±0.03 A+B 1903 3235 0.19±0.05 0.17±0.04 -0.02±0.04 A+B 2292 2846 0.24±0.05 0.04±0.02 -0.21±0.02 A+B 2994 2144 0.38±0.03 0.27±0.04 -0.11±0.04 A+B 3996 1142 0.28±0.04 0.50±0.08 0.23±0.09 A+B 4497 641 0.38±0.06 2.07±0.11 1.73±0.18 A+B 4996 143 0.47±0.04 1.02±0.10 0.56±0.13 A+B 5047 91 0.32±0.06 2.09±0.18 1.81±0.23 A+B 5088 50 0.17±0.04 2.58±0.15 2.47±0.24 A+B 229 STA. 10 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 22 5078 0.01±0.00 -0.02±0.08 -0.02±0.08 A 5100 57 5043 0.01±0.00 0.13±0.16 0.12±0.16 A 82 5018 0.01±0.00 -0.02±0.05 -0.03±0.06 A 102 4998 0.01±0.00 0.21±0.11 0.21±0.12 A 177 4923 0.01±0.00 0.21±0.20 0.20±0.21 A 421 4679 0.05±0.00 0.13±0.11 0.08±0.11 A 696 4404 0.06±0.00 0.34±0.11 0.29±0.12 A 1094 4007 0.07±0.00 0.23±0.07 0.16±0.07 A 1221 3879 0.09±0.00 0.07±0.03 -0.02±0.03 A+B 2233 2867 0.32±0.01 0.60±0.57 0.29±0.59 A+B 2734 2366 0.32±0.01 0.22±0.07 -0.10±0.07 A+B 3485 1615 0.41±0.02 0.34±0.06 -0.08±0.06 A+B 4885 215 0.35±0.01 2.52±0.15 2.22±0.23 A+B 5003 97 0.36±0.01 3.58±0.13 3.28±0.29 A+B 5044 57 0.35±0.02 4.29±0.17 4.02±0.35 A+B STA. 12 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 22 5568 0.02±0.05 0.17±0.12 0.15±0.13 A 5590 57 5533 0.02±0.01 0.04±0.68 0.03±0.70 A 60 5530 0.02±0.01 0.05±0.04 0.04±0.04 A+B 82 5508 0.02±0.01 0.13±0.18 0.11±0.19 A 102 5488 0.06±0.02 0.12±0.10 0.06±0.10 A 177 5413 0.05±0.03 0.10±0.11 0.05±0.11 A 421 5169 0.07±0.18 0.14±0.11 0.06±0.12 A 696 4894 0.05±0.01 0.18±0.13 0.14±0.13 A 1094 4497 0.10±0.02 0.08±0.03 -0.02±0.03 A+B 1900 3690 0.28±0.03 0.27±0.05 -0.02±0.06 A+B 2750 2840 0.33±0.02 0.25±0.04 -0.08±0.05 A+B 3500 2090 0.43±0.02 0.55±0.17 0.12±0.18 A+B 4500 1090 0.32±0.04 1.18±0.10 0.88±0.14 A+B 5425 165 0.33±0.02 3.21±0.14 2.96±0.27 A+B 5505 85 0.30±0.02 3.17±0.20 2.94±0.31 A+B 5545 45 0.29±0.03 3.19±0.16 2.97±0.28 A+B 230 STA. 14 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 25 5230 0.03±0.00 0.27±0.13 0.25±0.13 A 5255 45 5210 0.03±0.00 0.24±0.11 0.22±0.11 A 75 5180 0.03±0.00 0.21±0.22 0.19±0.22 A 100 5155 0.03±0.00 0.07±0.07 0.04±0.07 A 125 5130 0.03±0.00 -0.07±0.15 -0.10±0.15 A 150 5105 0.04±0.00 -0.03±0.10 -0.07±0.10 A 300 4955 0.04±0.00 0.11±0.15 0.07±0.15 A 500 4755 0.08±0.00 0.08±0.19 0.00±0.19 A 500 4755 0.08±0.00 0.14±0.09 0.07±0.10 A 600 4655 0.08±0.00 0.19±0.15 0.12±0.15 A 800 4455 0.09±0.01 0.06±0.05 -0.03±0.05 A 1000 4255 0.19±0.01 0.24±0.04 0.05±0.05 A+B 1200 4055 0.22±0.01 0.27±0.06 0.05±0.06 A+B 1500 3755 0.30±0.01 0.27±0.04 -0.03±0.05 A+B 1700 3555 0.37±0.01 0.32±0.06 -0.05±0.06 A+B 1900 3355 0.39±0.01 0.33±0.09 -0.06±0.09 A+B 1900 3355 0.39±0.01 0.34±0.05 -0.05±0.06 A+B 2300 2955 0.41±0.01 0.44±0.06 0.03±0.07 A+B 2750 2505 0.47±0.02 0.4±0.07 -0.08±0.08 A+B 3500 1755 0.52±0.01 0.62±0.1 0.10±0.11 A+B 4500 755 0.45±0.02 2.38±0.12 1.96±0.21 A+B 5085 170 0.41±0.01 2.74±0.21 2.38±0.29 A+B 5165 90 0.43±0.01 7.98±0.36 7.68±0.68 A+B 5205 50 0.43±0.01 2.84±0.13 2.46±0.24 A+B STA. 16 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 30 5344 0.03±0.02 0.16±0.13 0.13±0.13 A 5374 65 5309 0.04±0.02 0.1±0.08 0.07±0.09 A 110 5264 0.05±0.02 -0.03±0.01 -0.08±0.02 A 150 5224 0.05±0.02 0.00±0.05 -0.05±0.05 A 200 5174 0.05±0.01 0.00±0.10 -0.05±0.11 A 300 5074 0.07±0.03 0.05±0.06 -0.02±0.06 A 525 4849 0.07±0.14 0.12±0.18 0.05±0.18 A 850 4524 0.19±0.02 0.27±0.08 0.08±0.08 A 1250 4124 0.55±0.05 0.39±0.05 -0.17±0.06 A+B 2250 3124 0.61±0.04 0.74±0.07 0.13±0.09 A+B 3000 2374 0.65±0.05 0.81±0.07 0.17±0.09 A+B 5205 169 0.42±0.06 3.97±0.18 3.64±0.34 A+B 5285 89 0.36±0.04 3.97±0.15 3.71±0.33 A+B 5325 49 0.53±0.04 3.32±0.15 2.87±0.29 A+B 231 STA. 18 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 37 5138 0.02±0 -0.02±0.06 -0.04±0.06 A 5175 76 5099 0.02±0 -0.05±0.09 -0.07±0.09 A 107 5068 0.03±0 -0.01±0.04 -0.04±0.04 A 146 5029 0.03±0 0.02±0.05 0±0.05 A 296 4879 0.06±0 0.06±0.07 0±0.07 A 445 4730 0.07±0 0.06±0.05 -0.01±0.05 A 696 4479 0.18±0.01 0.08±0.04 -0.11±0.04 A 1001 4174 0.32±0.01 0.24±0.06 -0.08±0.07 A+B 1095 4080 0.32±0.02 0.47±0.07 0.16±0.08 A+B 1741 3434 0.53±0.02 0.56±0.05 0.03±0.06 A+B 2242 2933 0.58±0.01 0.83±0.06 0.26±0.09 A+B 3244 1931 0.57±0.02 0.96±0.07 0.41±0.1 A+B 4244 931 0.43±0.01 0.93±0.09 0.52±0.11 A+B 4742 433 0.39±0.01 0.93±0.08 0.56±0.1 A+B STA. 18.3 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 497 1663 0.19±0.01 0.12±0.03 -0.07±0.04 A+B 2160 1098 1062 0.36±0.01 0.34±0.04 -0.01±0.05 A+B 1297 863 0.36±0.01 0.48±0.05 0.12±0.06 A+B 1917 243 0.57±0.02 0.48±0.05 -0.1±0.06 A+B 2040 120 0.57±0.02 1.04±0.11 0.48±0.14 A+B 2101 59 0.57±0.02 0.94±0.09 0.37±0.12 A+B 2132 28 0.57±0.02 1.07±0.07 0.52±0.11 A+B 232 STA. 19 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 37 5100 0.04±0.01 -0.05±0.04 -0.10±0.04 A 5137 76 5061 0.06±0.02 0.3±0.23 0.25±0.24 A 107 5030 0.06±0.02 0.13±0.10 0.07±0.11 A 146 4991 0.06±0.02 0.1±0.10 0.04±0.10 A 296 4841 0.06±0.02 0.17±0.07 0.12±0.07 A 445 4692 0.09±0.04 0.11±0.06 0.02±0.06 A 696 4441 0.19±0.23 0.11±0.05 -0.09±0.05 A+B 1001 4136 0.41±0.05 0.12±0.08 -0.31±0.08 A+B 1500 3637 0.46±0.05 0.28±0.04 -0.19±0.05 A+B 2100 3037 0.51±0.05 0.41±0.05 -0.10±0.06 A+B 2700 2437 0.62±0.05 0.39±0.06 -0.24±0.06 A+B 3500 1637 0.62±0.08 0.89±0.13 0.28±0.15 A+B 4500 637 0.62±0.04 2.19±0.17 1.62±0.23 A+B 5050 87 0.49±0.04 1.90±0.11 1.46±0.18 A+B 5090 47 0.49±0.04 2.02±0.11 1.59±0.18 A+B STA. 21 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 17 5361 0.02±0.00 0.03±0.07 0.01±0.07 A 5378 41 5337 0.01±0.00 0.17±0.09 0.16±0.09 A 96 5282 0.04±0.00 0.13±0.11 0.09±0.11 A 145 5233 0.07±0.00 0.38±0.11 0.31±0.11 A 245 5133 0.11±0.00 0.23±0.09 0.12±0.10 A 395 4983 0.13±0.01 0.20±0.07 0.06±0.07 A 592 4786 0.20±0.01 0.14±0.05 -0.06±0.05 A 900 4479 0.29±0.01 0.22±0.08 -0.07±0.08 A+B 1402 3976 0.40±0.01 0.53±0.07 0.13±0.08 A+B 2006 3372 0.51±0.02 0.87±0.13 0.37±0.14 A+B 2507 2871 0.50±0.02 0.48±0.07 -0.02±0.08 A+B 4256 1122 0.46±0.02 0.95±0.09 0.50±0.11 A+B 5220 158 0.41±0.02 5.12±0.24 4.77±0.43 A+B 5304 74 0.40±0.01 3.27±0.21 2.91±0.32 A+B 5346 32 0.40±0.01 2.97±0.19 2.6±0.28 A+B 233 STA. 23 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 23 5187 0.02±0.01 -0.20±0.17 -0.23±0.18 A 5210 78 5132 0.03±0.01 -0.05±0.12 -0.09±0.13 A 98 5112 0.06±0.03 0.75±0.79 0.71±0.83 A 158 5052 0.08±0.05 0.27±0.10 0.20±0.11 A 198 5012 0.10±0.04 0.59±0.43 0.51±0.45 A 298 4912 0.11±0.01 1.51±1.40 1.46±1.46 A 500 4710 0.17±0.05 0.35±0.22 0.19±0.23 A 792 4418 0.26±0.10 0.19±0.06 -0.08±0.06 A 798 4412 0.26±0.10 0.47±0.12 0.21±0.13 A 1250 3960 0.23±0.08 0.29±0.04 0.06±0.04 A+B 1750 3460 0.29±0.06 0.52±0.04 0.24±0.06 A+B 2150 3060 0.42±0.03 0.38±0.05 -0.04±0.06 A+B 2300 2910 0.39±0.06 0.50±0.07 0.12±0.08 A+B 2450 2760 0.36±0.05 0.56±0.10 0.21±0.12 A+B 2600 2610 0.34±0.08 0.44±0.07 0.11±0.08 A+B 2750 2460 0.32±0.06 0.51±0.05 0.19±0.07 A+B 2900 2310 0.29±0.02 0.44±0.05 0.16±0.06 A+B 3100 2110 0.33±0.03 0.94±0.16 0.64±0.18 A+B 3500 1710 0.36±0.04 0.79±0.07 0.45±0.09 A+B 4000 1210 0.39±0.02 1.43±0.11 1.08±0.15 A+B 4500 710 0.38±0.02 1.45±0.11 1.11±0.16 A+B 4750 460 0.38±0.02 2.73±0.15 2.43±0.25 A+B 5050 160 0.37±0.03 1.91±0.13 1.6±0.19 A+B STA. 25 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 25 5018 0.02±0.00 0.36±0.20 0.36±0.21 A 5043 78 4965 0.02±0.00 0.23±0.16 0.22±0.17 A 123 4920 0.03±0.00 0.00±0.07 -0.03±0.08 A 196 4847 0.07±0.00 0.13±0.13 0.07±0.14 A 490 4553 0.16±0.01 0.06±0.05 -0.10±0.06 A 784 4259 0.23±0.01 0.03±0.05 -0.22±0.06 A 1569 3474 0.37±0.02 0.36±0.08 -0.01±0.09 A+B 1961 3082 0.42±0.01 0.63±0.05 0.22±0.07 A+B 2601 2443 0.39±0.01 0.51±0.05 0.12±0.06 A+B 3000 2044 0.36±0.01 0.57±0.07 0.23±0.09 A+B 3499 1545 0.42±0.01 0.92±0.09 0.53±0.12 A+B 4498 546 0.41±0.01 2.83±0.17 2.57±0.28 A+B 4950 93 0.41±0.01 2.35±0.23 2.06±0.30 A+B 4990 53 0.41±0.01 2.19±0.13 1.89±0.21 A+B 234 STA. 27 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 25 4542 0.04±0.01 0.09±0.10 0.05±0.10 A 4567 60 4507 0.03±0.01 0.12±0.14 0.10±0.15 A 100 4467 0.07±0.03 -0.03±0.06 -0.10±0.06 A 160 4407 0.07±0.02 0.10±0.09 0.03±0.10 A 250 4317 0.08±0.02 0.05±0.06 -0.03±0.06 A 500 4067 0.12±0.01 0.04±0.05 -0.08±0.05 A 800 3767 0.21±0.01 0.15±0.06 -0.07±0.07 A 1400 3167 0.28±0.03 0.99±0.15 0.75±0.18 A+B 2000 2567 0.36±0.02 0.54±0.07 0.19±0.08 A+B 2400 2167 0.37±0.03 0.61±0.06 0.26±0.08 A+B 2800 1767 0.41±0.05 0.61±0.05 0.21±0.07 A+B 3900 667 0.41±0.04 1.63±0.16 1.30±0.20 A+B 4405 162 0.37±0.02 2.11±0.23 1.84±0.29 A+B 4485 82 0.37±0.02 1.66±0.14 1.36±0.19 A+B 4525 42 0.33±0.03 2.15±0.15 1.92±0.23 A+B STA. 29 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 25 4355 0.03±0.00 0.19±0.15 0.17±0.16 A 4380 75 4305 0.03±0.00 0.15±0.2 0.13±0.21 A 115 4265 0.04±0.00 0.16±0.08 0.13±0.08 A 130 4250 0.04±0.00 -0.04±0.06 -0.08±0.06 A 155 4225 0.05±0.00 -0.01±0.04 -0.06±0.05 A 200 4180 0.08±0.00 0.14±0.08 0.06±0.09 A 250 4130 0.09±0.00 0.05±0.06 -0.04±0.06 A 350 4030 0.08±0.00 0.45±0.11 0.39±0.12 A 400 3980 0.11±0.01 0.31±0.09 0.22±0.10 A 500 3880 0.13±0.01 0.10±0.06 -0.03±0.07 A 600 3780 0.16±0.01 0.16±0.06 0.00±0.07 A 700 3680 0.18±0.01 -0.02±0.01 -0.21±0.01 A 800 3580 0.22±0.01 0.61±0.11 0.41±0.13 A 1000 3380 0.25±0.01 0.26±0.06 0.01±0.07 A 1200 3180 0.28±0.01 0.34±0.05 0.07±0.06 A+B 1400 2980 0.31±0.01 0.28±0.11 -0.04±0.11 A+B 1800 2580 0.36±0.01 0.58±0.08 0.23±0.10 A+B 2200 2180 0.40±0.01 0.13±0.05 -0.28±0.05 A+B 2800 1580 0.44±0.02 0.46±0.09 0.02±0.10 A+B 3600 780 0.44±0.02 0.95±0.12 0.54±0.14 A+B 4000 380 0.43±0.02 2.32±0.36 1.99±0.41 A+B 4215 165 0.42±0.01 2.19±0.26 1.87±0.32 A+B 4295 85 0.41±0.01 1.36±0.11 1.00±0.16 A+B 4335 45 0.53±0.02 2.46±0.14 2.03±0.23 A+B 235 STA. 31 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 30 4608 0.05±0.01 0.02±0.12 -0.03±0.12 A 4638 85 4553 0.02±0.01 -0.18±0.11 -0.20±0.12 A 150 4488 0.05±0.02 0.03±0.09 -0.02±0.09 A 260 4378 0.08±0.02 0.09±0.05 0.01±0.05 A 400 4238 0.06±0.02 0.00±0.03 -0.06±0.04 A 600 4038 0.15±0.02 0.09±0.11 -0.07±0.11 A 900 3738 0.26±0.02 0.00±0.09 -0.27±0.11 A 1400 3238 0.32±0.02 0.37±0.06 0.04±0.07 A+B 2000 2638 0.39±0.03 0.38±0.11 -0.02±0.12 A+B 2500 2138 0.43±0.02 0.53±0.14 0.11±0.15 A+B 3000 1638 0.46±0.03 0.63±0.10 0.17±0.12 A+B 4200 438 0.43±0.03 2.45±0.19 2.11±0.27 A+B 4470 168 0.43±0.03 1.77±0.19 1.40±0.24 A+B 4550 88 0.4±0.03 2.27±0.16 1.96±0.23 A+B 4590 48 0.4±0.03 2.65±0.15 2.37±0.25 A+B STA. 33 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 25 5311 0.03±0.00 -0.01±0.07 -0.04±0.08 A 5336 80 5256 0.03±0.00 0.06±0.09 0.03±0.10 A 125 5211 0.03±0.00 -0.01±0.12 -0.04±0.12 A 175 5161 0.04±0.00 -0.06±0.02 -0.11±0.03 A 250 5086 0.07±0.00 0.06±0.05 -0.02±0.06 A 400 4936 0.09±0.00 0.09±0.07 0.00±0.07 A 600 4736 0.17±0.01 0.17±0.08 0.00±0.09 A 900 4436 0.27±0.01 0.28±0.09 0.01±0.10 A 1400 3936 0.41±0.01 0.46±0.07 0.05±0.08 A+B 2000 3336 0.42±0.01 0.20±0.08 -0.23±0.09 A 2500 2836 0.46±0.02 0.45±0.06 -0.01±0.07 A+B 4000 1336 0.41±0.01 1.25±0.11 0.88±0.15 A+B 4500 836 0.38±0.04 1.70±0.16 1.38±0.21 A+B 5175 161 0.36±0.01 2.94±0.21 2.70±0.31 A+B 5295 41 0.35±0.01 2.92±0.17 2.68±0.28 A+B 236 STA. 35 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 25 5111 0.02±0.00 0.02±0.04 0.00±0.04 A 5136 75 5061 0.02±0.00 0.02±0.04 0.00±0.04 A 100 5036 0.03±0.00 0.04±0.04 0.02±0.05 A 150 4986 0.03±0.00 -0.02±0.03 -0.05±0.03 A 225 4911 0.06±0.00 0.07±0.03 0.01±0.04 A 350 4786 0.09±0.00 0.11±0.06 0.02±0.06 A 550 4586 0.19±0.00 0.09±0.06 -0.10±0.06 A 800 4336 0.28±0.01 0.35±0.09 0.08±0.10 A 1400 3736 0.41±0.01 0.68±0.07 0.28±0.09 A+B 2000 3136 0.44±0.03 0.43±0.05 -0.01±0.06 A+B 2200 2936 0.42±0.04 0.91±0.12 0.51±0.14 A+B 2400 2736 0.42±0.04 0.52±0.07 0.11±0.08 A+B 2600 2536 0.42±0.04 0.32±0.05 -0.10±0.06 A+B 2800 2336 0.38±0.02 0.54±0.07 0.17±0.08 A+B 3000 2136 0.38±0.02 0.79±0.12 0.42±0.14 A+B 3200 1936 0.39±0.02 0.82±0.13 0.45±0.14 A+B 3600 1536 0.39±0.02 1.44±0.09 1.09±0.14 A+B 4000 1136 0.41±0.01 1.95±0.12 1.61±0.19 A+B 4750 386 0.35±0.01 4.10±0.21 3.91±0.37 A+B 4970 166 0.34±0.01 4.07±0.16 3.88±0.34 A+B 5050 86 0.33±0.01 1.51±0.11 1.23±0.16 A+B 5090 46 0.33±0.01 2.95±0.10 2.73±0.24 A+B STA. 37 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 25 4763 0.04±0.01 0.01±0.03 -0.04±0.03 A 4788 60 4728 0.03±0.01 0.01±0.02 -0.02±0.02 A 135 4653 0.04±0.02 0.07±0.02 0.03±0.02 A 250 4538 0.05±0.02 0.04±0.03 0.00±0.03 A 450 4338 0.11±0.00 0.03±0.01 -0.08±0.01 A 800 3988 0.21±0.02 0.14±0.02 -0.08±0.03 A 1200 3588 0.39±0.07 0.09±0.02 -0.31±0.02 A 1800 2988 0.48±0.05 0.29±0.03 -0.20±0.04 A+B 2400 2388 0.42±0.05 0.93±0.06 0.52±0.09 A+B 2600 2188 0.40±0.06 0.92±0.07 0.54±0.10 A+B 2800 1988 0.40±0.06 1.50±0.10 1.14±0.15 A+B 3000 1788 0.38±0.04 1.63±0.07 1.28±0.14 A+B 3200 1588 0.38±0.04 1.30±0.07 0.95±0.12 A+B 4620 168 0.35±0.04 5.05±0.12 4.83±0.38 A+B 4740 48 0.35±0.04 4.25±0.13 4.01±0.33 A+B 237 STA. 39 Depth DAB 231 Pa Total 227 Ac Excess 227 Ac cartridge m m dpm m -3 dpm m -3 dpm m -3 B.D. (m) 25 4276 0.03±0.00 0.07±0.03 0.04±0.03 A 4301 45 4256 0.03±0.00 0.05±0.03 0.03±0.03 A 120 4181 0.03±0.00 0.07±0.03 0.05±0.04 A 150 4151 0.04±0.00 0.10±0.04 0.06±0.04 A 200 4101 0.05±0.00 0.00±0.02 -0.05±0.02 A 300 4001 0.07±0.00 0.04±0.03 -0.03±0.03 A 400 3901 0.11±0.00 0.09±0.03 -0.02±0.03 A 500 3801 0.11±0.00 0.18±0.04 0.07±0.04 A 600 3701 0.19±0.01 0.05±0.02 -0.14±0.02 A 800 3501 0.28±0.01 0.30±0.04 0.02±0.05 A 1000 3301 0.36±0.01 0.42±0.05 0.06±0.06 A 1200 3101 0.42±0.02 0.39±0.05 -0.03±0.06 A 1400 2901 0.47±0.02 0.67±0.06 0.20±0.08 A 1600 2701 0.48±0.01 0.60±0.05 0.11±0.06 A 1600 2701 0.48±0.01 0.72±0.07 0.23±0.09 A 1800 2501 0.50±0.01 0.67±0.05 0.17±0.07 A 2400 1901 0.48±0.02 1.36±0.20 0.89±0.22 A 3000 1301 0.45±0.01 1.06±0.10 0.61±0.12 A+B 3800 501 0.41±0.02 2.82±0.22 2.43±0.29 A+B 4060 241 0.38±0.02 2.69±0.13 2.31±0.23 A+B 4140 161 0.38±0.02 3.11±0.18 2.75±0.28 A+B 4180 121 0.38±0.02 2.73±0.21 2.37±0.29 A+B

Abstract (if available)

Abstract The disequilibrium that occurs within the U and Th-decay series in marine environments can be exploited to give rates of transport of dynamic processes. To provide the most accurate transport rates, the radiotracer should have the following characteristics: (1) the radiotracer should be easily sampled and measured; (2) the source function and geochemical behavior of the radiotracer should be well understood; (3) the radiotracer’s half-life should be on the same order as the process being investigated. This dissertation will utilize the distribution of 227Ac on the basin-scale range in the North- and Southeast Pacific in order to provide rates of transport of other solutes. The half-life of 227Ac (t1/2=21.8 y) is well suited for this scale and its geochemical behavior has been well-studied throughout the last few decades. The GEOTRACES program protocol for sampling and measuring Ac and Ra in the water column has led to multiple 227Ac profiles throughout the world’s oceans. This dissertation will utilize U and Th-decay series equilibria to establish 227Ac bottom fluxes that can provide insight into transport rates and circulation pathways in the Northeastern Pacific Ocean.
227Ac is directly produced by decay of 231Pa (t1/2=32.8 ky). 231Pa is scavenged from the water column by falling particulates that carry it to sediments, where it decays to its more soluble daughter. A fraction of this 227Ac diffuses out of deep-sea sediments and is transported vertically and horizontally as it decays in the water column. The water column distribution of excess 227Ac (227Acex) was measured along the U.S. GEOTRACES Meridional Transect (GP15) from Alaska to Tahiti in fall 2018. To constrain its benthic input, cores from 5 stations near the northern half of the GP15 transect were collected in the summer of 2017 (C-Disk-IV transect stations 23-50°N). The GP15 transect parallels the C-Disk-IV cruise track in the Northeast Pacific, offset by a few hundred km.
Five sediment cores along the C-Disk-IV transect were measured and modeled with the objective of characterizing the behavior of 227Ac, 228Ra, and 226Ra and their fluxes into the overlying water column. Solid phase profiles of these isotopes were measured, and reaction-transport models were applied that incorporate effects of molecular diffusion, bioturbation, sedimentation, distribution coefficient (kd), and fraction of each isotope released to pore water by parent decay (called F). Good fits to the 226Ra profiles showed F values of 57-83% and sedimentation rates of 0.10 – 0.40 cm kyr-1 for C-Disk-IV sediments. The 228Ra profiles were difficult to measure due to high counting uncertainties, but F values obtained from 228Ra profiles were similar to 226Ra values. Most solid phase 226Ra profiles showed a large deficiency compared to 230Th in the upper 15cm of sediments, while the 228Ra profiles showed a modest deficiency relative to its 232Th parent in the top 3cm of sediments. F values for 227Ac had a much larger range than the Ra isotopes, ranging from 5 - 94% for C-Disk-IV sediments. About half of the 227Ac profiles showed a large deficiency relative to 231Pa in the upper few cm of sediments, while the other half showed a very small deficiency. Sediment composition, loss of surficial material, non-steady state behavior, and non-local bioturbation transport of sediments might explain the discrepancy between the two types of 227Ac profiles. It is noteworthy that 230Th budgets indicate significant sediment winnowing at sites with low F values, perhaps indicating that exhumation of formerly buried sediment plays some role.
Two independent approaches were used to quantify the source function of 227Ac and 228Ra in the Northeast Pacific: (1) use of solid phase profiles with a reaction-transport model, as well as integrated downcore daughter-parent deficiency; and (2) direct measurement of fluxes based on core incubation. The two independent methods agree within uncertainty, and the average 227Ac and 228Ra sediment fluxes for the Northeast Pacific are 90 ± 20 and 600 ± 200 dpm m-2 yr-1. The 226Ra sediment flux was only determined by the former approach, and the flux calculated in this study is similar to previous work in the North Pacific. The average sediment flux for 226Ra along the C-Disk-IV cruise is 1300 ± 10 dpm m-2-yr-1, which is over 2x higher than the water column inventory of 226Ra in this region (600 dpm m-2-yr-1). 227Ac fluxes for the southern half of the GP15 transect were calculated by estimating F and using 231Pa measurements in the upper few cm of sediments.
Profiles of 227Ac and 231Pa were measured and modeled in the water column along the GP15 transect. Along the GP15 transect, 227Ac and 231Pa are typically near equilibrium between 0-3000m depths, and below this horizon, 227Ac is often in excess over its parent. Excess 227Ac (227Acex) generally increases in activity with increasing depth and the highest concentrations of 227Acex are contained within the bottom 1000m. The highest concentrations of 227Acex in the Eastern Pacific are found near the center of the Northeast Pacific Basin (NEPB) and south of 10˚S. These areas are dominated by low sedimentation and high 231Pa activity in sediments. Along the southern leg of the GP15 transect (Sta.19 - 37), some elevated activities of 227Acex are found at mid-depths (~2600m). These areas appear to be influenced by hydrothermal activity from the East Pacific Rise (EPR), due to the proximity of 3He anomalies and 227Acex activities at those depths, although maxima in the two tracers are not perfectly coincident.
Three types of 227Acex profiles were found in the water column along the GP15 transect: (1) an expected, exponential decrease of 227Acex away from the seafloor; (2) a well-mixed 500m thick bottom layer with very little 227Acex above; (3) and lastly, an unexpected, erratic distribution of 227Acex that has local maxima in 227Acex overlying bottom waters of lower concentration. The first type of 227Acex profile is found toward the northern end of the GP15 transect (Sta.6 – 10). These profiles can be generated if water circulation is flowing along a constant depth, where the 227Ac bottom source is constant. If vertical diffusion of 227Acex is constant, it should produce an 227Acex distribution that is exponentially decreasing away from the seafloor due to the combination of diffusion and radioactive decay. This is the most ideal profile and can be used to find apparent vertical eddy diffusivity rates (Kz), if the bottom is flat, or inclined with a constant slope. The apparent vertical diffusivity will be affected by both diapycnal and isopycnal transport, in situations where the isopycnals and/or the bottom topography are inclined.
The second type of 227Acex profile found along the GP15 transect shows nearly constant 227Acex activities within the bottom few hundred meters. These profiles are found in the middle of the Northeast Pacific (30˚ - 40˚N) and indicate that the bottom ~500 meters are rapidly mixed, reflecting the density structure. Above this benthic layer, little 227Acex is found, indicating low vertical transport. Profiles in the southern half of the GP15 transect show the 3rd type of 227Acex profile: an erratic distribution of 227Acex in the bottom few hundred meters that correlates with the depth distribution of regional topographic features. The irregularity appears to reflect high density stratification, coupled with inputs from irregular topography in the Southeast Pacific that produces localized maxima in the 227Acex source function at multiple depths, which then travels horizontally along isopycnals, mimicking the complex source function.
Horizontal advection of 227Acex is significant in some parts of the transect, as shown by comparing the integrated decay of 227Acex in the water column to the benthic source: between 40˚ - 30˚N and 10˚ - 0˚N, water column decay is smaller than benthic input and south of 10˚S, it is larger than benthic input. Areas where horizontal advection does not produce a significant effect are: north of 40°N, between 30˚ - 10˚N, and near the equator. In these areas, water column decay is comparable to benthic input. This pattern is consistent with predictions from an inverse model in the Northeast Pacific (Hautala, 2018) that indicates deep-water advection is strongest between 40˚ - 30˚N and circulation is moving in a west-east direction (along 150˚W). North of 40˚N, the model suggests that circulation is moving in a north-south direction.
The last chapter of thesis focuses on sedimentary dynamics in the Santa Monica Basin (SMB) during the last 250 years, with an emphasis on the last 40 years. Mass accumulation rates (MAR) for the deepest and lowest oxygen-containing parts of the SMB basin have been remarkably consistent during the past century, averaging 17.1 ± 0.6 mg cm-2 yr-1. However, MAR were slower prior to ~ 1900 CE (~10.5 mg/cm2-yr). The increase in sedimentation rate towards the recent occurs at about the time previous studies predicted an increase in siltation and the demise of a shelly shelf benthic fauna on the SMB shelf. The post-1900 CE constancy of sedimentation through a period of massive urbanization in Los Angeles is surprising.

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Asset Metadata

Creator Kemnitz, Nathaniel (author)

Core Title Modeling deep ocean water and sediment dynamics in the eastern Pacific Ocean using actinium-227 and other naturally occurring radioisotopes

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Geological Sciences

Degree Conferral Date 2022-12

Publication Date 09/09/2022

Defense Date 06/08/2022

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag ²²⁷Ac,benthic fluxes,core incubation,distribution coefficient,Marine sediments,OAI-PMH Harvest,Ra isotopes,radionuclides

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Hammond, Douglas E. (committee chair), Berelson, William (committee member), John, Seth (committee member), Moffett, James (committee member)

Creator Email kemnitz@usc.edu,nathanielkemnitz@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC111939445

Unique identifier UC111939445

Legacy Identifier etd-KemnitzNat-11198

Document Type Dissertation

Format application/pdf (imt)

Rights Kemnitz, Nathaniel

Type texts

Source 20220909-usctheses-batch-979 (batch), University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

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