University of Southern California Dissertations and Theses

Comparative behavior and distribution of biologically relevant trace metals - iron, manganese, and copper in four representative oxygen deficient regimes of the world's oceans

(USC Thesis Other)

Comparative behavior and distribution of biologically relevant trace metals - iron, manganese, and copper in four representative oxygen deficient regimes of the world's oceans

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content 1 COMPARATIVE BEHAVIOR AND DISTRIBUTION OF BIOLOGICALLY RELEVANT TRACE METALS – IRON, MANGANESE, AND COPPER IN FOUR REPRESENTATIVE OXYGEN DEFICIENT REGIMES OF THE WORLD’S OCEANS by Jagruti Vedamati ________________________________________________________________________ A DISSERTATION 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 (OCEAN SCIENCES) August 2013 Copyright 2013 Jagruti Vedamati 2 This thesis is dedicated to my parents. For instilling in me the courage to always dream big and for being my greatest source of encouragement and support throughout. 3 Acknowledgments This journey through the years working on my dissertation wouldn’t have been as memorable without the support and assistance of many people I would like to thank here. First and foremost, I would like to extend my sincerest and heartfelt thanks to my advisor James W. Moffett for being a constant source of support and encouragement through all these years. Thanks for always believing in me and knowing when to be there. It’s because of his excellent advice; my doctoral degree has been a fulfilling experience, filled with exciting research and fabulous field sites. Thanks for giving me this great opportunity Jim! I would also like to express my gratitude to my dissertation committee members – Doug Hammond, Doug Capone, Sergio Sanudo-Wilhelmy and Dave McKemy along with my guidance committee member- Joshua West for all their helpful suggestions and critiques. Thanks for all your help and insight. Special thanks to Linda Bazilian, Don Bingham and Adolfo at the Biology front office along with Cindy and Vardui at the Earth Sciences front office for their immense will to help me navigate the logistics and the nitty-gritty of graduate school with ease. Thanks to all my past and present Moffett lab members for keeping work interesting and assisting me through the usual ups and downs of graduate school. Thanks Daniel Ohnemus and Tyler Goepfert for helping me traverse the complicated hallows of the lab with ease during my initial years at USC. Thanks Yoshiko Kondo for being a great mentor and friend through rough times and I will always cherish the lunch time memories of our sharing lunch together. Thanks Jeremy Jacquot for sailing in the same boat with me and helping me navigate it together. Huge thanks to my labmate- little sister Hantten Han for always being there for me during times when I was in need of a family. Thanks to Catherine Chan for all her hard work and help during 4 analysis in the lab along with being a great friend and confidant all through. Thanks Jessica Tsay for all your help during the Costa Rica Cruise and sharing innumerous bowls of cup noodles thereby. Lastly, a huge bunch of thanks to my close bunch of friends and family who have been with me throughout. My family has been my strongest support system even though we were thousands of miles away. Cheering me up during rough times has been one of their best tricks - be it by the innumerous hours on the phone or by just being there virtually at any hour of the night or day. Thank you - Mama, Bapa, Nani and Sonu for making this all worth it. Huge thanks to my extended family (Maa, Debu Mamu, Bibhu Mamu, Main and Om) for being my virtual support here so far away from home. I feel really lucky to be a part of such loving and caring family. In the absence of my family here at times, I am blessed to have a loving bunch of friends who have held me during times of stress, cajoled me to keep going and given me strength to never ever give up. Thanks so much Manish for being there always and being my strongest ever cheer leader/support system/bestest friend ever. I am indeed very lucky to have found my dearest friend in you! Thanks Sushree and Tanmay for all your support throughout the really tough times and being there at many important ones. From my first year at USC to the last year, you have seen me through it all. Finally thanks to all my friends I have met during my journey through graduate school for imprinting their impression on this journey through graduate school. Lastly, I received funding towards my graduate research from the Tyler Environmental Fellowship, Marine and Environmental Biology Fellowship and Chemical Oceanography Program of the National Science Foundation. 5 Table of Contents Dedication 2 Acknowledgments 3 List of Figures 8 List of Tables 12 Abstract 13 INTRODUCTION 15 CHAPTER 1 Distribution of manganese in the eastern tropical South Pacific during different austral seasons 29 1.1 Abstract. 30 1.2 Introduction 31 1.3 Methods 34 1.4 Results 37 1.5 Discussion 43 1.6 References 51 CHAPTER 2 Iron speciation in the Eastern Tropical South Pacific oxygen minimum zone off of Peru 81 2.1 Abstract 82 2.2 Introduction 83 6 2.3 Methods 86 2.4 Results 90 2.5 Discussion 97 2.6 References 103 CHAPTER 3 Iron speciation at the Costa Rica Upwelling Dome 120 3.1 Abstract 121 3.2 Introduction 122 3.3 Methods 125 3.4 Results 129 3.5 Discussion 132 3.6 References 135 CHAPTER 4 Comparative behavior and distribution of copper in the Arabian Sea oxygen minimum zone and in the Eastern Tropical South Pacific oxygen 148 4.1 Abstract 149 4.2 Introduction 150 4.3 Methods 153 4.4 Results 157 4.5 Discussion 163 4.6 References 167 7 CHAPTER 5 Distributions of total dissolved iron, manganese, zinc and copper along Line-P in the North eastern Pacific Ocean 184 5.1 Abstract 185 5.2 Introduction 186 5.3 Methods 188 5.4 Results 193 5.5 Discussion 204 5.6 References 209 SUMMARY 232 8 List of Figures Fig.1-1 Map of the cruise track off coastal Peru in the Eastern Tropical South Pacific Ocean sampled in October-November 2005 aboard the R/V Knorr. 66 Fig.1-2 Depth profiles of oxygen for stations along Transect 1 of KN-182-09 off of Peru. 67 Fig.1-3 Depth profiles of oxygen for stations along Transect 2 of KN-182-09 off of Peru. 68 Fig.1-4 Depth profiles of oxygen for stations along Transect 3 of KN-182-09 off of Peru. 69 Fig.1-5 Depth profiles of temperature and salinity for stations along Transect 1 of KN-182-09 off of Peru. 70 Fig.1-6 Depth profiles of temperature and salinity for stations along Transect 2 of KN-182-09 off of Peru. 71 Fig.1-7 Depth profiles of temperature and salinity for stations along Transect 3 of KN-182-09 off of Peru. 72 Fig.1-8 Depth profiles of total dissolved Mn and nitrite for stations along Transect 1 of KN-182-09 off of Peru. 73 Fig.1-9 Depth profiles of total dissolved Mn and nitrite for stations along Transect 2 of KN-182-09 off of Peru. 74 Fig.1-10 Depth profiles of total dissolved Mn and nitrite for stations along Transect 3 of KN-182-09 off of Peru. 75 Fig.1-11 Map of the cruise track in the eastern tropical South Pacific Ocean that was sampled from 19 January to 3 March 2010 aboard the R/V Atlantis. 76 Fig.1-12 Upper 300 m oceanographic sections of temperature along the 10 o S and 20 o S zonal transect in the ETSP. 77 Fig.1-13 Upper 1000 m oceanographic sections of salinity along the 10 o S and 20 o S zonal transect in the ETSP. 78 Fig.1-14 Upper 1000 m oceanographic sections of oxygen concentrations along the 10 o S and 20 o S zonal transect. 79 9 Fig.1-15 Upper 1000 m oceanographic sections of nitrite and total dissolved Mn concentrations along the10 o S and 20 o S zonal transect in the ETSP. 80 Fig.2-1 Map of the cruise track off coastal Peru in the Eastern Tropical South Pacific Ocean that was sampled in October-November 2005 aboard the R/V Knorr. 108 Fig.2-2 Surface distributions of salinity; temperature and oxygen off the coast of Peru along three transects sampled in October-November 2005. 109 Fig.2-3 Depth profiles of temperature and salinity for stations along Transect 1 off of Peru. 110 Fig.2-4 Depth profiles of temperature and salinity for stations along Transect 2 off of Peru. 111 Fig.2-5 Depth profiles of temperature and salinity for stations along Transect 3 off of Peru. 112 Fig.2-6 Depth profiles of total dissolved Fe, Fe (II), nitrite and total dissolved oxygen for station 24, and station 26 along Transect 1 off of Peru. 113 Fig.2-7 Depth profiles of total dissolved Fe, Fe (II), nitrite and total dissolved oxygen for station 27, and station 29 in Transect 1 off of Peru. 114 Fig.2-8 Depth profiles of total dissolved Fe, Fe (II), nitrite and total dissolved oxygen for station 19 along Transect 2 off of Peru. 115 Fig.2-9 Depth profiles of total dissolved Fe, Fe (II), nitrite and total dissolved oxygen for station 22 and station 23 along Transect 2 off of Peru. 116 Fig.2-10. Depth profiles of total dissolved Fe, Fe (II), nitrite and total dissolved oxygen for stations for station 11 and station 12 along Transect 3 off Peru. 117 Fig.2-11 Depth profiles of total dissolved Fe, Fe (II), nitrite and total dissolved oxygen for stations 9, and station 10 along Transect 3 off Peru. 118 Fig.2-12 Upper 1000m oceanographic sections of total dissolved Fe and Fe (II) for Transects 1, 2 and 3 during the KN-182-09 cruise. 119 Fig.3-1 Map of the cruise track in the Costa Rica Upwelling Dome that was sampled during June – July 2010 aboard the R/V Melville. 139 Fig.3-2 Depth profiles of temperature and salinity for Cycles 1, 2, 3, 4, & 5 during the Costa Rica Upwelling Dome cruise in 2010. 140 10 Fig.3-3 Depth profiles of total dissolved Fe, Fe (II), nitrite & oxygen during Cycle 1 in the Costa Rica Upwelling dome. 141 Fig.3-4 Depth profiles of total dissolved Fe, Fe (II), nitrite & oxygen during Cycle 2 in the Costa Rica Upwelling dome. 142 Fig.3-5 Depth profiles of total dissolved Fe, Fe (II), nitrite & oxygen during Cycle 3 in the Costa Rica Upwelling dome. 143 Fig.3-6 Depth profiles of total dissolved Fe, Fe (II), nitrite & oxygen during Cycle 4 in the Costa Rica Upwelling dome. 144 Fig.3-7 Depth profiles of total dissolved Fe, Fe (II), nitrite & oxygen during Cycle 5 in the Costa Rica Upwelling dome. 145 Fig.3-8 Depth profiles of total dissolved Fe, Fe (II) and nitrite for Station 11 in the Costa Rica Upwelling dome sampled in June 2005. 146 Fig.3-9 Depth profiles of total dissolved Fe, Fe (II) and nitrite for Station 39 in the Costa Rica Upwelling dome sampled in November 2005. 147 Fig.4-1 Map of the cruise track in the Arabian Sea that was sampled during August – September 2007 aboard the R/V Roger Revelle. 171 Fig.4-2 Map of the cruise track off coastal Peru in the Eastern tropical South Pacific that was sampled in October-November 2005 aboard the R/V Knorr. 172 Fig.4-3 Upper 1200 m oceanographic sections of nitrite, oxygen, nitrate; and upper 150 m oceanographic section of Chl A concentrations in the Arabian Sea. 173 Fig.4-4 Upper 1200 m oceanographic sections of salinity; and temperature; and surface distribution of salinity; and temperature in the Arabian Sea. 174 Fig.4-5 Upper 1200 m oceanographic section of total dissolved Cu in the Arabian Sea. 175 Fig.4-6 Depth profiles of total dissolved Cu, oxygen and nitrite for stations 3, 4, 5, and 6 in the Arabian Sea. 176 Fig.4-7 Depth profiles of total dissolved Cu, oxygen and nitrite for stations 7, 8, 22, and 23 in the Arabian Sea. 177 Fig.4-8 Depth profiles of total dissolved Cu, oxygen and nitrite for stations 20, 21, 11, and 18 in the Arabian Sea. 178 11 Fig.4-9 Depth profiles of total dissolved Cu, oxygen and nitrite for stations 15 and 16 in the Arabian Sea. 179 Fig.4-10 Surface oceanographic distributions of salinity; temperature; and oxygen during the KN-182-09 cruise off the Peruvian coast. 180 Fig.4-11 Depth profiles of total dissolved Cu oxygen and nitrite for stations 24, 26, 27, and 29 along Transect 1 off of Peru. 181 Fig.4-12 Depth profiles of total dissolved Cu oxygen and nitrite for stations 19, 21, 22, and 23 along Transect 2 off of Peru. 182 Fig.4-13 Depth profiles of total dissolved Cu oxygen and nitrite for stations 9, 10, 11, and 12 along Transect 3 off of Peru. 183 Fig.5-1 Map of the cruise track along Line P in the North East Pacific off of Seattle that was sampled in May 2012 aboard R/V Thompson. 223 Fig.5-2 Currents in the upper 350m during the transect along Line P. Data collected by Acoustic Doppler Current Profiler (ADCP). 224 Fig.5-3 Upper 2500 m oceanographic sections of temperature and salinity along Line P. 225 Fig.5-4 Upper 2500 m oceanographic sections of oxygen and nitrite along Line P. 226 Fig.5-5 Upper 2500 m oceanographic sections of nitrate; phosphate and silicate along Line P. 227 Fig.5-6 Oceanographic sections of total dissolved Fe; total dissolved Cu; total dissolved Mn and total dissolved Zn along Line P. 228 Fig.5-7 Oceanographic sections of particulate Fe, particulate Cu, particulate Mn and particulate Zn along Line P. 229 Fig.5-8 Total dissolved Zn vs. silicate from all stations along Line P. 230 Fig.5-9 Total dissolved Zn vs. phosphate from all stations along Line P. 231 12 List of Tables Table 1-1 Mn concentrations in nmol L -1 for KN-182-09 cruise in October 2005. 57 Table 1-2 Mn concentrations in nmol L -1 for AT-15-61 cruise in February 2010. 61 Table 5.1 Total dissolved Fe concentrations in nmol L -1 215 Table 5.2 Total dissolved Cu concentrations in nmol L -1 217 Table 5.3 Total dissolved Mn concentrations in nmol L -1 219 Table 5.4 Total dissolved Zn concentrations in nmol L -1 221 13 Comparative behavior and distribution of biologically relevant trace metals – Iron, Manganese and Copper in four representative oxygen deficient regimes of the world’s oceans. Abstract by Jagruti Vedamati This thesis explores the behavior and distribution of key redox sensitive elements - Fe and Mn under spatially varied suboxic conditions along eastern boundary upwelling regions as compared to that of the non-redox sensitive, bioactive trace metal - Cu. The response of these metals was then analyzed in a different non-oxygen minimum zone (OMZ) setting along Line P to compare and analyze the differences in distribution, if any caused by the suboxic conditions. Fe, Cu and Mn were investigated in the three major OMZs of the world’s oceans – namely, the eastern tropical south Pacific off the coasts of Peru and Chile, the Arabian Sea and in the Costa Rica Upwelling Dome in eastern tropical north Pacific. For the non-OMZ sampling site, we sampled across a dynamic, high productivity region in the North East sub-arctic Pacific along Line P. Total dissolved Fe, Cu and Mn concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS). Fe (II) concentrations were determined using an automated flow injection analysis system. Results from the Peruvian OMZ indicate that Mn is largely decoupled from Fe. While Fe concentrations were very high on the shelf, it decreased drastically offshore and was coupled to redox conditions. In contrast, Mn concentrations were lower over the shelf and were often higher offshore, especially in surface waters. Results suggest that Mn is efficiently transported away from the highly reducing conditions of the shelf because of slow oxidation kinetics – in contrast to Fe. In nearshore stations, off the broad continental shelf along the northern and central transects off Peru, exceedingly high Fe were measured with most of the 14 dissolved Fe present as Fe(II) below the oxycline. Along the narrower southern Peruvian shelf, dissolved Fe concentrations were 10-fold lower. Cu distribution in the OMZs showed some interesting features observed for the first time. In transects through the Arabian Sea OMZ and off of Peru, a distinct draw down in Cu concentrations was observed at mid-depths coincident with the secondary nitrite maximum (SNM) while no such feature was present in stations outside the denitrification zone. Distributions along Line P suggest that one of the most striking differences in Fe & Mn distribution was the presence of high Mn: Fe ratios off the continental shelf along Line P as compared to those obtained in transects off the Peruvian coast. Complex redox cycling of Fe and Mn in the reducing sediments along the continental margin underlying the Peruvian OMZ results in the “Fe trapping” while Mn diffuses off shore into the water column, thereby resulting in lower DMn values along the Peruvian continental shelf. No distinct subsurface Fe plume was present throughout the transect along Line P. Cu depth profiles along Line P exhibit general features of a nutrient like element and agree with previous data from the central North Pacific. However, in the absence of a SNM along Line P, no draw down of Cu was observed at mid- depths similar to Cu distributions within the Arabian Sea OMZ. Overall, this thesis adds to our understanding of the effect of redox conditions within the suboxic zones on redox sensitive elements – Fe and Mn. It also furthers the knowledge of the behavior and distribution of important, biologically relevant trace metals-Fe, Mn and Cu in the world’s oceans. 15 Introduction Trace metals such as Fe, Mn and Cu play a critical role in the biological and chemical functioning of the oceans. In turn, these biologically relevant metals are affected by the changes in redox gradient and oxygen availability within subsurface oceanic regimes known as the oxygen minimum zones. Oxygen Minimum Zones (OMZs) - Definition Oxygen minimum zones (OMZs) are subsurface oceanic zones (e.g., at 50–100 m depth in the Arabian Sea;(Morrison et al. 1999)) identified by the presence of ultra-low O 2 concentrations as low as <2 nmol L -1 (Revsbech et al. 2009) and a secondary nitrite maximum (SNM) coinciding with low oxygen concentrations, formed due to the dissimilatory reduction of NO 3 - occurring within the OMZ (Cline and Richards 1972; Codispoti and Christensen 1985). Sluggish ventilation combined with oxygen consumption via remineralisation of sinking organic matter is the most likely cause of formation of these oxygen deficient regions at mid depths (~100-1000 m) (Kamykowski and Zentara 1990; Wyrtki 1962). Although the OMZs (O 2 < 20 μmol L -1 ) occupy only 8% and 7% of the ocean area and volume respectively (Paulmier and Ruiz-Pino 2009), these are regions of active denitrification and are major sinks of fixed nitrogen leading to an oceanic nitrate deficit of 14.7 (Tyrrell 1999). The presence of a SNM denotes intense denitrification by denitrifying microbial assemblages thereby, representing a very bioactive region where any change in the chemical behavior of bioactive trace metals could have huge impacts on the N-cycle. The shift from oxygen to nitrate as the terminal electron acceptor during denitrification in suboxic regimes results in a change in the redox environment. Now since most of the processes involved in the overall cycling of nitrogen in seawater involve metalloenzymes, it is of prime interest to understand the impact of 16 reducing conditions within the OMZ and on the sediments underlying the OMZ, on the trace metal distribution in these suboxic regimes. Previous studies in the OMZs have hinted towards a change in the behavior and speciation of redox sensitive trace metals such as Fe, Mn, Cr, Se etc. (Nameroff et al. 2002; Rue et al. 1997), where each element displays an individual response to the varying redox conditions. OMZs as eastern boundary sources of metals Upwelling along eastern continental boundaries accounts for almost 11% of the global ocean primary production (Chavez et al. 1995) and 50% of the global fish production (Ryther 1969). Since no such corresponding subsurface, off shore source of dissolved bio-essential trace metals in upwelled waters is known, resuspension of particles into the benthic boundary layer during upwelling must account for the large inputs of these metals that sustain increased rates of primary productivity (Johnson et al. 1999; Johnson et al. 1997). The importance of eastern upwelling regimes as boundary sources of metals has been very poorly constrained even though these oceanic OMZs currently impinge on over 1 million km 2 of permanently hypoxic shelves and continental margins worldwide (Helly and Levin 2004), and coastal shelf hypoxia, or even anoxia, has been increasing in frequency and intensity (Diaz and Rosenberg 2008; Naqvi et al. 2000). Several studies have indicated the importance of the organic rich, eastern boundary continental shelf sediments as a primary source of trace element inputs to the OMZs (Bruland et al. 2001; Chase et al. 2005; Elrod et al. 2004; Johnson et al. 1999), thereby indicating that reducing continental shelf sediments might be a dominant external source of iron as significant as aerosols. Although much of the work done in continental margin upwelling settings highlight the role of sediments supplying iron to the water column by mechanisms of resuspension (Hutchins et al. 1998) and reductive mobilization (Johnson et al. 1999), relatively little is known 17 about the influence of upwelling conditions and reducing sediments underlying the oxygen minimum zones on the behavior and distribution of bioactive trace metals such as Mn and Cu. The benthic –pelagic coupling between sediments underlying the oxygen deficient upwelled waters and the influx of redox sensitive trace metals could possibly have huge impacts on the cycling of these metals, especially in regions like the Peruvian upwelling region characterized by low atmospheric inputs. Hence, any potential impacts of global warming or increasing anthropogenic pressures on biogeochemical cycling in either the OMZ waters or the continental margins would likely be magnified by the effects on the other. Suboxic zones and metals In this study, we examine the behavior and distribution of key redox sensitive elements such as Fe and Mn under spatially varied suboxic conditions along eastern boundary upwelling regions and compare to that of the non-redox sensitive, bioactive trace metal - Cu. The response of these metals is then analyzed in a different non-OMZ oceanic setting to compare and analyze the differences in distribution, if any caused by the suboxic conditions. The goal was to better characterize the effect of redox conditions within the suboxic zones on these elements and to create an understanding of both equilibrium and kinetic controls on the behavior of these trace elements, which could then allow them to serve as indicators of spatial and temporal variation in redox conditions. It will also better equip us to predict future changes in trace metal distribution in response to changing oxygen concentrations within the water column due to global warming and climate change. Iron speciation and distribution in the OMZs Iron, the fourth most abundant element in the Earth’s crust, is an important biologically relevant trace metal that is primarily used in electron transport and redox catalysis and is important in controlling phytoplankton productivity, community species composition, trophic structure and nitrogen fixation in the oceans (Brand 1991; Coale et al. 1996; Mills et al. 2004). 18 High requirement of iron during nitrogen fixation and denitrification has been previously noted (Berman-Frank et al. 2001; Milligan and Harrison 2000; Morel et al. 2003). Although in oxygenated seawater, Fe (III) is the thermodynamically favored form of Fe, at pH 8.6, Fe (III) is strongly hydrolyzed as compared to Fe(II) and oxidation rates of Fe (II) are high (Millero et al. 1987). Fe (III) in seawater can be reduced to Fe (II), which is more soluble and kinetically labile (Moffett 2001; Waite 2001) and is weakly bound to known Fe (III) chelators such as the siderophores thereby making it more bioavailable (Sunda 2001). Since the presence of iron is critical to the growth and metabolism of marine phytoplankton, even the presence of small amounts of Fe in the system creates a lot of interest. Thus, considerable interest is exhibited to decipher non-equilibrium reactions that work on this large pool of Fe leading to accumulation of the dissolved, reduced, more bioavailable form of Fe – Fe (II). Only a few studies have studied the speciation and distribution of iron in the OMZs (Hong and Kester 1986; Landing and Bruland 1987; Moffett et al. 2007; Rue et al. 1997; Witter et al. 2000), although it is a cofactor in vital enzymes required during denitrification (Morel et al. 2006). Recent studies have reported the presence of up to about ~ 50% of dissolved Fe as Fe(II) in the Arabian Sea during the SW Monsoon (Moffett et al. 2007). Metabolic remineralisation processes (Hopkinson and Barbeau 2007; Moffett et al. 2007) and advective transport from hypoxic shelf waters and sediments (Hong and Kester 1986; Landing and Bruland 1987; Lohan and Bruland 2008) have been reported to serve as primary Fe(II) sources in the OMZs. Significant boundary inputs from shelf sediments have also been reported, thereby hinting at the importance of eastern boundary upwelling regimes as important Fe sources to the ocean. Thus, we attempt to study the impact and influence of upwelling regimes along eastern continental boundaries and the suboxic OMZ core on the behavior and distribution of Fe. 19 Mn: A comparative element Like Fe, bioactive trace metal, Mn plays a vital role in the redox buffering of living cells and is responsible for the terminal photo-oxidation of water during the light reactions of photosynthesis (Da Silva and Williams 2001). However, in contrast to Fe, Mn concentrations are controlled by scavenging processes and its distribution is mainly influenced by external sources of inputs into the ocean (Landing and Bruland 1980). Fe and Mn are similar in several aspects - both the elements are soluble in their reduced forms, they undergo photoreduction in surface waters and are known to have atmospheric and benthic sources. In spite of these similarities; there are several dissimilarities in their chemistry - Mn redox reaction is microbially catalyzed and is a slow reaction whereas the redox reactions in case of Fe are fast. The oxidized state of Mn is insoluble as it forms MnO2 particles whereas Fe(III) complexes are soluble. In suboxic oceanic regimes, similar to Fe, a typical Mn profile exhibits a distinct Mn (II) maximum (Klinkhammer and Bender 1980; Landing and Bruland 1980; Landing and Bruland 1987; Martin and Knauer 1984; Saager et al. 1989) that coincides with the SNM and is also known to be associated with a corresponding minima in p-Mn and in p-Mn/Al and HAc/REF Mn ratios (Landing and Bruland 1987; Martin and Knauer 1984). Lateral advection from continental margins (Chaigneau et al. 2008; Czeschel et al. 2011), remineralisation of organic matter and decrease in the oxidation rate of Mn (II) within the oxygen minimum layer along with reduction in the scavenging rate within the OMZ (Johnson et al. 1996) collectively account for the Mn maxima in the OMZ which also depends on the distance from the shore, local hydrodynamics, and shelf width (Chase et al. 2005). Lewis and Luther (2000) also observed two d-Mn peaks in the Arabian Sea during the 1995 Spring Intermonsoon period. The observed mid-depth Mn maxima was associated with the low oxygen core of the OMZ similar to the Fe(II) maxima observed by Moffett et al. (2007) and appeared to be maintained by a southward horizontal advective diffusive flux of d-Mn 20 from highly reducing Pakistan margin sediments. In spite of somewhat similar yet different chemistries, the observance of similar peaks in the OMZ regions poses an important question as to if they are in fact related. Cu – a non-redox active but biologically relevant trace metal Although large horizontal and vertical gradients in the concentrations of redox reactive metals like Fe and Mn in subsurface low oxygen regions have been previously reported (Lewis and Luther 2000; Moffett et al. 2007), few data about the behavior and distribution Cu in these redox active, upwelling regions (Boyle et al. 1977; Saager et al. 1992) is available, resulting in very limited understanding of the impact of OMZs on Cu distribution. Copper (Cu) performs a vital role in the nitrogen cycle, acting as a cofactor for the enzymes associated with nitrous oxide reduction (nitrous oxide reductase; NoSZ), nitrite reduction (nitrite reductase; NiRK), and ammonia oxidation (ammonium monooxidase; AMO)(Francis et al. 2007; Philippot 2002). Recent studies indicate that there could be some linkage between copper availability and the nitrogen cycle as there is a substantial Cu requirement for microbes catalyzing these processes (J. Jacquot and J.W Moffett; in press). Unlike Fe and Mn, Cu is not redox sensitive and hence, is not expected to be affected by the presence of a reducing, low oxygen regime. The behavior of this non-redox reactive, yet biologically relevant element- Cu was thus, studied in an OMZ and was then compared to our non-OMZ site in the NE Pacific. Areas of study We have investigated the behavior and distribution of Fe, Cu and Mn in the three major OMZs of the world’s oceans – namely, the eastern tropical south Pacific (ETSP) off the coasts of Peru and Chile, the Arabian Sea (AS) and in the Costa Rica Upwelling Dome in eastern tropical north Pacific. The Arabian Sea contains the thickest and largest permanent oxygen minimum zone (Helly and Levin 2004) of the three OMZs while the area off northern Chile and Peru 21 experiences quasi-permanent coastal upwelling and contains one of the shallowest and most intense oceanic OMZs. The ETNP OMZ is the deepest OMZ that also corresponds to the less intense (Mean O 2 = 18 µmol L -1 ) and deeper (below 850 m) OMZ core (Paulmier and Ruiz-Pino 2009). Even though each of these OMZs is formed due to a unique interplay of biogeochemical cycling and physical ocean ventilation at their respective geographical locations, a combination of suboxic conditions along with impact of reducing sediments underlying the OMZ may still be a deciding factor for trace metal distribution in these regimes. Line P – A comparative regime In order to compare the behavior of redox sensitive trace metals – Fe and Mn in the OMZs to that of an non-OMZ setting, we chose a dynamic, high productivity region in the NE sub-arctic Pacific along Line P between the coast of British Columbia and Ocean Station Papa (Pena and Bograd 2007). The site of comparison is a well-defined gradient in biogeochemical properties in the northeast Pacific along Line P where high nutrient, low chlorophyll waters limited by iron meet low nutrient, iron-replete waters (Ribalet et al. 2010). The resulting transition zone is a surrogate for a geochemical province boundary and is a seasonally persistent feature characterized by a gradient in nutrients, salinity and density at the shelf break (Whitney et al. 1998). This hotspot of phytoplankton diversity and productivity holds special significance as declining oxygen concentrations at mid-depths (100-400 m) over a span of 50 years (1956 – 2006) due to ocean warming have been recently reported (Whitney et al. 2007). This decline in oxygen concentrations at mid-depths in an otherwise well oxygenated regime could be likely to cause a shift in redox potential at mid-depths and eventually, alter the distribution of these redox sensitive metals. This thesis explores the potential of eastern boundaries as sizeable sources of bioactive trace metals to the suboxic oceanic regions where differences in metal source strength could 22 significantly impact the biological response to upwelling conditions and consequent changes in OMZ depths and expanse in the future. We also examine the influence of redox-active, suboxic regions on the distribution and preservation of these elements. The effect of suboxic conditions on these elements is of primary importance especially since recent observational analysis conducted by Stramma et al. (2008), supports climate model predictions of dissolved oxygen declines in the tropical ocean and an expansion of the tropical OMZs due to a contribution of thermal, dynamical, and biogeochemical factors. These predicted trends could have a strong impact on the elemental cycles and alter the distribution of these redox sensitive elements thereby either positively or negatively affecting them. Thus, it is important to have an in depth knowledge of the distribution of these elements in both OMZ and non-OMZ conditions so that changes in their availability and distribution can be rightly predicted in the future. Overall, we intend to further the knowledge of the behavior and distribution of important, biologically relevant trace metals-Fe, Mn and Cu in the world’s oceans. 23 References Berman-Frank, I., J. T. Cullen, Y. Shaked, R. M. Sherrell, and P. G. Falkowski. 2001. Iron availability, cellular iron quotas, and nitrogen fixation in Trichodesmium. Limnol. Oceanogr. 46: 1249-1260. Boyle, E. A., F. R. Sclater, and J. M. Edmond. 1977. DISTRIBUTION OF DISSOLVED COPPER IN PACIFIC. Earth Planet. Sci. Lett. 37: 38-54. Brand, L. E. 1991. MINIMUM IRON REQUIREMENTS OF MARINE-PHYTOPLANKTON AND THE IMPLICATIONS FOR THE BIOGEOCHEMICAL CONTROL OF NEW PRODUCTION. Limnol. Oceanogr. 36: 1756-1771. Bruland, K. W., E. L. Rue, and G. J. Smith. 2001. Iron and macronutrients in California coastal upwelling regimes: Implications for diatom blooms. Limnol. Oceanogr. 46: 1661-1674. Chaigneau, A., A. Gizolme, and C. Grados. 2008. Mesoscale eddies off Peru in altimeter records: Identification algorithms and eddy spatio-temporal patterns. Prog. Oceanogr. 79: 106- 119. Chase, Z. and others 2005. Manganese and iron distributions off central California influenced by upwelling and shelf width. Mar. Chem. 95: 235-254. Chavez, F. and others 1995. Upwelling in the ocean: modern processes and ancient records. Physical Estimates of Global New Production: the Upwelling Contribution. Chichester: Wiley: 313-320. Cline, J. D., and F. A. Richards. 1972. OXYGEN DEFICIENT CONDITIONS AND NITRATE REDUCTION IN EASTERN TROPICAL NORTH-PACIFIC OCEAN. Limnol. Oceanogr. 17: 885-900. 24 Coale, K. H., S. E. Fitzwater, R. M. Gordon, K. S. Johnson, and R. T. Barber. 1996. Control of community growth and export production by upwelled iron in the equatorial Pacific Ocean. Nature 379: 621-624. Codispoti, L. A., and J. P. Christensen. 1985. NITRIFICATION, DENITRIFICATION AND NITROUS-OXIDE CYCLING IN THE EASTERN TROPICAL SOUTH-PACIFIC OCEAN. Mar. Chem. 16: 277-300. Czeschel, R., L. Stramma, F. U. Schwarzkopf, B. S. Giese, A. Funk, and J. Karstensen. 2011. Middepth circulation of the eastern tropical South Pacific and its link to the oxygen minimum zone. J. Geophys. Res.-Oceans 116. Da Silva, J. F., and R. J. P. Williams. 2001. The biological chemistry of the elements: the inorganic chemistry of life. Oxford University Press. Diaz, R. J., and R. Rosenberg. 2008. Spreading dead zones and consequences for marine ecosystems. Science 321: 926-929. Elrod, V. A., W. M. Berelson, K. H. Coale, and K. S. Johnson. 2004. The flux of iron from continental shelf sediments: A missing source for global budgets. Geophys. Res. Lett. 31. Francis, C. A., J. M. Beman, and M. M. M. Kuypers. 2007. New processes and players in the nitrogen cycle: the microbial ecology of anaerobic and archaeal ammonia oxidation. Isme Journal 1: 19-27. Helly, J. J., and L. A. Levin. 2004. Global distribution of naturally occurring marine hypoxia on continental margins. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 51: 1159-1168. Hong, H. S., and D. R. Kester. 1986. REDOX STATE OF IRON IN THE OFFSHORE WATERS OF PERU. Limnol. Oceanogr. 31: 512-524. 25 Hopkinson, B. M., and K. A. Barbeau. 2007. Organic and redox speciation of iron in the eastern tropical North Pacific suboxic zone. Mar. Chem. 106: 2-17. Hutchins, D. A., G. R. Ditullio, Y. Zhang, and K. W. Bruland. 1998. An iron limitation mosaic in the California upwelling regime. Limnol. Oceanogr. 43: 1037-1054. Johnson, K. S., F. P. Chavez, and G. E. Friederich. 1999. Continental-shelf sediment as a primary source of iron for coastal phytoplankton. Nature 398: 697-700. Johnson, K. S., K. H. Coale, W. M. Berelson, and R. M. Gordon. 1996. On the formation of the manganese maximum in the oxygen minimum. Geochim. Cosmochim. Acta 60: 1291- 1299. Johnson, K. S., R. M. Gordon, and K. H. Coale. 1997. What controls dissolved iron concentrations in the world ocean? Mar. Chem. 57: 137-161. Kamykowski, D., and S. J. Zentara. 1990. HYPOXIA IN THE WORLD OCEAN AS RECORDED IN THE HISTORICAL DATA SET. Deep-Sea Research Part a- Oceanographic Research Papers 37: 1861-1874. Klinkhammer, G. P., and M. L. Bender. 1980. THE DISTRIBUTION OF MANGANESE IN THE PACIFIC-OCEAN. Earth Planet. Sci. Lett. 46: 361-384. Landing, W. M., and K. W. Bruland. 1980. MANGANESE IN THE NORTH PACIFIC. Earth Planet. Sci. Lett. 49: 45-56. ---. 1987. THE CONTRASTING BIOGEOCHEMISTRY OF IRON AND MANGANESE IN THE PACIFIC-OCEAN. Geochim. Cosmochim. Acta 51: 29-43. Lewis, B. L., and G. W. Luther. 2000. Processes controlling the distribution and cycling of manganese in the oxygen minimum zone of the Arabian Sea. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 47: 1541-1561. 26 Lohan, M. C., and K. W. Bruland. 2008. Elevated Fe(II) and dissolved Fe in hypoxic shelf waters off Oregon and Washington: An enhanced source of iron to coastal upwelling regimes. Environ. Sci. Technol. 42: 6462-6468. Martin, J. H., and G. A. Knauer. 1984. VERTEX - MANGANESE TRANSPORT THROUGH OXYGEN MINIMA. Earth Planet. Sci. Lett. 67: 35-47. Millero, F. J., S. Sotolongo, and M. Izaguirre. 1987. THE OXIDATION-KINETICS OF FE(II) IN SEAWATER. Geochim. Cosmochim. Acta 51: 793-801. Milligan, A. J., and P. J. Harrison. 2000. Effects of non-steady-state iron limitation on nitrogen assimilatory enzymes in the marine diatom Thalassiosira weissflogii (Bacillariophyceae). J. Phycol. 36: 78-86. Mills, M. M., C. Ridame, M. Davey, J. La Roche, and R. J. Geider. 2004. Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic. Nature 429: 292-294. Moffett, J. W. 2001. Transformations among different forms of iron in the ocean. IUPAC series on analytical and physical chemistry of environmental systems 7: 343-372. Moffett, J. W., T. J. Goeffert, and S. W. A. Naqvi. 2007. Reduced iron associated with secondary nitrite maxima in the Arabian Sea. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 54: 1341- 1349. Morel, F., A. Milligan, and M. Saito. 2006. Marine Bioinorganic Chemistry: The. The Oceans and Marine Geochemistry 6: 113. Morel, F., A. Milligan, M. Saito, and H. Elderfield. 2003. Treatise on Geochemistry. Oxford: Pergamon. Morrison, J. M. and others 1999. The oxygen minimum zone in the Arabian Sea during 1995. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 46: 1903-1931. 27 Nameroff, T. J., L. S. Balistrieri, and J. W. Murray. 2002. Suboxic trace metal geochemistry in the eastern tropical North Pacific. Geochim. Cosmochim. Acta 66: 1139-1158. Naqvi, S. W. A. and others 2000. Increased marine production of N2O due to intensifying anoxia on the Indian continental shelf. Nature 408: 346-349. Paulmier, A., and D. Ruiz-Pino. 2009. Oxygen minimum zones (OMZs) in the modern ocean. Prog. Oceanogr. 80: 113-128. Pena, M. A., and S. J. Bograd. 2007. Time series of the northeast Pacific. Prog. Oceanogr. 75: 115-119. Philippot, L. 2002. Denitrifying genes in bacterial and Archaeal genomes. Biochimica Et Biophysica Acta-Gene Structure and Expression 1577: 355-376. Revsbech, N. P., L. H. Larsen, J. Gundersen, T. Dalsgaard, O. Ulloa, and B. Thamdrup. 2009. Determination of ultra-low oxygen concentrations in oxygen minimum zones by the STOX sensor. Limnol. Oceanogr. Meth. 7: 371-381. Ribalet, F. and others 2010. Unveiling a phytoplankton hotspot at a narrow boundary between coastal and offshore waters. Proc. Natl. Acad. Sci. U. S. A. 107: 16571-16576. Rue, E. L., G. J. Smith, G. A. Cutter, and K. W. Bruland. 1997. The response of trace element redox couples to suboxic conditions in the water column. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 44: 113-134. Ryther, J. H. 1969. Photosynthesis and fish production in the sea. Science 166: 72-76. Saager, P. M., H. J. W. Debaar, and P. H. Burkill. 1989. MANGANESE AND IRON IN INDIAN-OCEAN WATERS. Geochim. Cosmochim. Acta 53: 2259-2267. Saager, P. M., H. J. W. Debaar, and R. J. Howland. 1992. CD, ZN, NI AND CU IN THE INDIAN-OCEAN. Deep-Sea Research Part a-Oceanographic Research Papers 39: 9-35. 28 Stramma, L., G. C. Johnson, J. Sprintall, and V. Mohrholz. 2008. Expanding oxygen-minimum zones in the tropical oceans. Science 320: 655-658. Sunda, W. G. 2001. Bioavailability and bioaccumulation of iron in the sea. IUPAC series on analytical and physical chemistry of environmental systems 7: 41-84. Tyrrell, T. 1999. The relative influences of nitrogen and phosphorus on oceanic primary production. Nature 400: 525-531. Waite, T. D. 2001. Thermodynamics of the iron system in seawater. IUPAC series on analytical and physical chemistry of environmental systems 7: 291-342. Whitney, F. A., H. J. Freeland, and M. Robert. 2007. Persistently declining oxygen levels in the interior waters of the eastern subarctic Pacific. Prog. Oceanogr. 75: 179-199. Whitney, F. A., C. S. Wong, and P. W. Boyd. 1998. Interannual variability in nitrate supply to surface waters of the Northeast Pacific Ocean. Mar. Ecol.-Prog. Ser. 170: 15-23. Witter, A. E., B. L. Lewis, and G. W. Luther. 2000. Iron speciation in the Arabian Sea. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 47: 1517-1539. Wyrtki, K. 1962. The oxygen minima in relation to ocean circulation. Deep Sea Research and Oceanographic Abstracts 9: 11-23. 29 Chapter 1 Distribution of manganese in the eastern tropical South Pacific during different austral seasons. 30 1.1. Abstract The Eastern Tropical South Pacific, characterized by a highly productive upwelling region and a large oxygen minimum zone, is also accompanied by strong gradients of redox active metals such as Fe and Mn. Since reducing conditions in the OMZ could lead to large boundary sources of Mn, vertical profiles for total dissolved Mn (DMn), nitrite and hydrographic data were obtained in the eastern tropical South Pacific in October 2005 and February 2010. DMn concentrations were determined using simple dilution and matrix-matched external standardization inductively coupled mass spectrometry (ICP-MS). Transects in October 2005 sampled the shelf slope interaction (75 o W–79 o W) within the Peruvian EEZ during the highest upwelling period while transects in February 2010 sampled the region of active denitrification (80 o W–100 o W) outside of the Peruvian EEZ during period of highest primary productivity. Results indicate that Mn is largely decoupled from Fe. Fe concentrations are very high on the shelf, decrease drastically offshore and are coupled to redox conditions. In contrast, DMn concentrations were lower over the shelf and are often higher offshore, especially in surface waters. Along the Peruvian coast, surface Mn concentrations were lower over the northern broad shelf (1.6 nmol L -1 ) as compared to that over the southern narrow shelf (3.4 nmol L -1 ) – in contrast to Fe. Results suggest that Mn is efficiently transported away from the highly reducing conditions of the shelf because of slow oxidation kinetics – in contrast to Fe. Atmospheric deposition from arid regions in southern Peru and northern Chile is the most plausible explanation for the elevated concentrations found offshore. There is also evidence for biological uptake and remineralisation processes in the upper water column, particularly in the highly productive northern section of our study area. 31 1.2. Introduction Manganese plays a vital role in the terminal photo-oxidation of water during the light reactions of photosynthesis (Da Silva and Williams 2001) and is also used as a co-factor for catalases, superoxide dismutases (SODs) and peroxidases that defend against reactive oxygen species that are toxic to living cells (Horsburgh et al. 2002; Yocum and Pecoraro 1999). In natural waters, Mn exists as both insoluble Mn (III and IV) oxides and as soluble Mn(II) ions while the relative balance between oxidation of soluble Mn (II) and reduction of insoluble Mn (III and IV) oxides is controlled by the removal of seawater Mn via particle scavenging processes. Even though Mn (II) is thermodynamically unstable with respect to oxidation by O 2 , the majority of Mn in oxygenated seawater exists as soluble Mn (II) due to slow oxidation kinetics combined with oxide reduction by organics and other reductants (Stone and Morgan 1984). Previous studies have established that absolute chemical rates of oxidation are exceedingly slow under seawater conditions and marine Mn(II) oxidation is mostly bacterially mediated (Sunda and Huntsman 1987; Sunda and Huntsman 1988; Tebo and Emerson 1985; Vojak et al. 1985). A typical seawater Mn profile resembles that of a scavenged type element typified by relatively short residence times and removal via particle scavenging. It exhibits a distinct maximum in surface waters (Landing and Bruland 1980) due to atmospheric input (Aguilar-Islas and Bruland 2006; Baker et al. 2006; Landing and Bruland 1980) or fluvial input (Aguilar-Islas and Bruland 2006). The photoreduction of insoluble Mn(IV) oxides in the presence of sunlight to produce soluble Mn(II) contributes to the maintenance of this feature (Sunda and Huntsman 1988; Sunda et al. 1983; Sunda and Kieber 1994). This maximum is associated with a minimum in the concentration of Mn oxides, suggesting that it results partially from a decrease in 32 particulate scavenging rates. Mn can also reach the open ocean via other pathways - reductive dissolution from (anoxic or suboxic) sediments (Froelich et al. 1979; Johnson et al. 1992) lateral transportation from the marginal sediments (Johnson et al. 1992; Landing and Bruland 1980; Martin and Knauer 1984) or hydrothermal input (Bender et al. 1977; Klinkhammer and Bender 1980). However, Mn profiles in oxygen minimum zones frequently exhibit subsurface maxima (Klinkhammer and Bender 1980; Landing and Bruland 1980; Landing and Bruland 1987; Lewis and Luther 2000; Martin and Knauer 1984; Saager et al. 1989) that coincides with oxygen minima. This feature has been associated with corresponding minima in p-Mn and in p-Mn/Al and HAc/REF Mn ratios (Landing and Bruland 1987; Martin and Knauer 1984). Lateral advection from continental margins (Chaigneau et al. 2008; Czeschel et al. 2011), remineralisation of sinking organic matter and decrease in oxidative scavenging within the OMZ (Johnson et al. 1996) collectively account for the Mn maxima in the OMZ which also depends on the distance from the shore, local hydrodynamics, and shelf width (Chase et al. 2005). Klinkhammer and Bender (1980) were the first to report total dissolved Mn concentrations in 13 stations throughout the Pacific specifying the unique structure of the profiles with a maximum at the surface and a minimum at the top of the thermocline. In regions with an OMZ, there was a peak in Mn concentration and was believed to originate from reduction of Mn oxides in near shore sediments and subsequent advection or by seawater equilibrium with a metastable oxide similar to hausmannite. Landing and Bruland (1980) reported similar features of the dissolved Mn profile in their data from the NE Pacific where they observed an increased surface signal indicating its sources- riverine and atmospheric, accompanied by a decrease at 300m and then an enhanced Mn signal coinciding with the OMZ. Martin and Knauer (1984) also reported dissolved 33 and particulate Mn values from a cruise off of Central Mexico where they observed a similar structure in Mn profile and noted an increase in dissolved Mn values coinciding with a decrease in particulate Mn values. 1.2.1. Study Objective This study focuses on the distribution of Mn in the Eastern Tropical South Pacific region using samples obtained on two cruises - one being off the Peruvian continental shelf sampled in October-November 2005 (KN-182-09) while AT-15-61 cruise was outside of the Peruvian EEZ along 10 o S and 20 o S in Feb-March 2010. During KN-182-09 (October – November 2005), samples were obtained in stations between (11 o S and 16 o S) over the continental shelf and focused mainly on the shelf slope interactions of Mn. KN-182-05 took place during the austral winter which is characterized by high upwelling rates (Pennington et al. 2006). In order to investigate Mn geochemistry further offshore, we obtained samples from two zonal transects at t at 10 o S and 20 o S from 80 o W to 100 o W on AT-15-61 (February -March 2010). Samples were collected during months that historically have some of the highest levels of chlorophyll and primary production (Pennington et al. 2006). These transects extended across the western and southern boundaries of the OMZ, enabling us to study the behavior of Mn along these redox gradients. 1.2.2. Study area The Peruvian upwelling region is one of the most productive regions of the world’s oceans characterized by the largest tonnage fishery in the world (Pennington et al. 2006). It is shallowest towards the coast with the OMZ extending well into the euphotic zone in some regions and is characterized by perennial upwelling favored by constant alongshore winds that cause the Ekman transport of surface water (Strub et al. 1998). Upwelling brings nutrient laden waters to the 34 surface and fuels high primary productivity of up to 3.6 g Cm -2 d -1 (Pennington et al. 2006). Fueled by high primary productivity, Eastern Tropical South Pacific has one of the shallowest and intense oxygen minimum zones. The Peruvian oxygen minimum zone is thickest (>600m) between 5 o S and 13 o S and extends to about 1000km off shore (Fuenzalida et al. 2009). The core of the OMZ is characterized by the presence of a distinct secondary nitrite minimum and oxygen values are lower than 10nM, the detection limit of STOX sensors (Revsbech et al. 2009; Thamdrup et al. 2012). 1.3. Methods 1.3.1. Sample collection Sampling on the KN-182-09 cruise was done during October-November 2005 aboard the R/V Knorr and was concentrated on three transects perpendicular to the Peruvian shelf, at 11 o S, 13 o S and 16 o S (Fig. 1-1). Seawater samples for total dissolved manganese analysis were collected in 10L Niskin bottles with Teflon coated interior surfaces (Ocean Test Equipment) mounted on a trace metal clean rosette (Sea-Bird Electronics) and the samples were immediately acidified to pH ~1.7 by adding trace metal grade HCl (Fisher Optima) for storage and analysis in the lab. All hydrographic data during this cruise were obtained using a CTD-O 2 probe (Seabird). Samples during the AT-15-61 cruise in January – March 2010 were collected aboard the R/V Atlantis using 5 L Teflon coated external spring Niskin-type bottles (Ocean Test Equipment) mounted on a trace metal clean rosette (Sea-Bird Electronics). A Kevlar line was used for sampling and the Niskin bottles on the rosette were preprogrammed to trip on the downcast at pre-specified depths. The bottles were pressurized with filtered compressed nitrogen gas and the seawater samples were collected using Teflon tubing, through acid-cleaned 0.2 m Acropak capsules (Pall Corporation) into 250 mL acid cleaned, sample rinsed, low-density polyethylene 35 bottles (LDPE, Nalgene). Samples for total dissolved metal analysis were then acidified to pH ~ 1.7 by adding trace metal grade HCl (Fisher Optima). All critical ship board manipulations such as sample acidification and loading of the sampling filters were carried out in a laminar flow bench inside a positive pressure clean room enclosure. 1.3.2. Cleaning protocol 500 ml LDPE (Nalgene) sampling bottles were thoroughly cleaned in a four-step process that consisted of soaking them in a 5% Citranox acid detergent bath (Alconox) for at least a day followed by another overnight soak in a 10% hydrochloric acid bath (VWR). They were then filled with 10% HCl and baked at 60°C for at least 2 days and finally, filled with 0.1% trace metal grade HCl (Optima, Fisher) and baked at 60°C again for another 2 days. The insides and outsides of the bottles were thoroughly rinsed at least five times with Milli-Q water (18.2 MΩ; Millipore) in between each step. All samples were prepared in 15 mL polypropylene centrifuge tubes (VWR) which were first cleaned in a two-step process by soaking them in 10% HCl at 60°C for 48 hours and then, rinsing each tube at least five times with Milli-Q water. After the rinses, the tubes were filled to a positive meniscus with 0.5% trace metal grade HCl, capped and then baked at 60°C overnight. After retrieving them from the oven, the tubes were left capped & stored until further use. Upon analysis, the tubes were emptied and rinsed three times with Milli-Q water and at least once with the sample. 1.3.3. Nitrite measurements Nitrite concentrations in samples obtained during the KN-182-09 cruise were analyzed using an Alpkem autoanalyser (equipped with a Winflow V4 operating system). When measured with nitrate, nitrite was determined through reduction by cadmium metal followed by colorimetric 36 detection (Grasshoff et al. 1983). When measured by itself, it was measured following the same procedure mentioned above without the cadmium column. During AT-15-61 cruise, reactive nitrite was measured spectrophotometrically after (Strickland and Parsons 1968). Small Acropak-filtered sample aliquots (15 mL or less) were each made to react with an acidified sulfanilamide solution to form a diazonium compound. The compound was then mixed with N-(1-Naphthyl)-ethylenediamine dihydrochloride to produce a colored azo-dye, whose extinction was measured on a Shimadzu UV-1700 UV-VIS spectrophotometer to determine the NO 2 - concentration of each sample. 1.3.4. Total dissolved Manganese (DMn) Total dissolved Mn concentrations were determined by seawater dilution using inductively coupled plasma mass spectrometry (ICP-MS) method adapted from (Field et al. 1999). All samples were analyzed in triplicate using the Finnegan Element 2 (Thermo Scientific) Inductively Coupled Plasma Mass Spectrophotometer (ICP-MS) on a medium resolution mode. The standardization of samples was done by a matrix-matched external calibration curve with variations in sensitivity corrected by normalizing to an added internal standard, In. 0.5 mL of sample was pipetted into the 15 ml pre-cleaned vials and 4.5 ml of 5% trace metal grade HNO 3 (Optima) was added to dilute each sample by 10 fold. The samples were then spiked with 1ppb of In, shaken and left to equilibrate for an hour. Each sample was analyzed in triplicates and the total Mn counts (cps) were determined on the instrument in medium resolution mode. Seawater Mn standards for external standardization were prepared the same way using low Mn deep seawater from the SAFe D2 reference samples ([Mn] = 0.35 ± 0.06 nmol L -1 (n=3)). The accuracy of the method was evaluated by measuring 2008 SAFe reference standards S1 and D2 (Johnson et al. 2007). The Mn values obtained by this method for S1 and D2 were 0.83 ± 0.01 37 nmol L -1 and 0.42 ± 0.01 nmol L -1 , respectively and were in agreement with certified consensus values of 0.79 ± 0.06 nmol L -1 Mn (S1) and 0.35 ± 0.06 nmol L -1 Mn (D2). 1.4. Results 1.4.1. "Dissolved" Mn off the coast of Peru during KN-182-09 cruise. Transects 1 and 2 were off a broader shelf while the southernmost Transect 3 was off of a relatively narrower shelf (Fig.1-1). All three transects were characterized by an intense oxygen deficient zone over the shelf, which deepens on moving offshore. The OMZ characterized by dissolved oxygen values < 5 µmol L -1 extended from 70-600 m in Transect 1 (Fig. 1-2), 100- 500 m in Transect 2 (Fig. 1-3) and from 150-450 m in Transect 3 (Fig. 1-4). However, the OMZ extended deeper from 200-500 m along the 10 o S zonal transect during AT-15-61 cruise. Overall nitrite concentrations during the KN-182-09 were around 3.5 – 8 µM while along the 10 o S zonal transect, nitrite concentrations were as high as 2.5 µM. The salient features common to most of the profiles in the transect was a subsurface Mn maximum in the upper water column, ranging from 20 m at station 22 (a shelf station in the north), to 200m at station 14 (an offshore station at the southernmost end of KN-182-09). The maxima coincided with the secondary nitrite maxima in some cases, but otherwise tended to be above it, whilst still in the oxycline. The other salient feature was a strong enrichment in Mn in surface waters, particularly offshore. Indeed, on KN-182-09, surface Mn typically increased on going from east to west, in contrast to nutrients. Stations 19 and 21 displayed surface maxima. Stations on the broad shelf in northern Peru (22, 23 and 29) showed fairly uniform and low concentrations (Table 1-1). Although only a partial data set was available for station 23, the data indicate that bottom waters on the shelf are not particularly high in Mn, in agreement with 38 stations 22 and 29. Interestingly, both Stations 22 and 29 have subsurface maxima at 20m and 35-50m, respectively, but these do not coincide with the SNM or any other feature. The offshore data from the 10 o S transect (Table 1-2) also revealed a surface maximum that extended westward to at least 90 o W, before diminishing. This feature was much less pronounced at 20 o S. There was no significant enrichment of Mn in the core of the OMZ, at least in comparison to the strong signal in the overlying water. One of the most surprising features we observed was the variability in Mn concentrations below 500m. For example, at Station 19, DMn at 600m was only 0.5 nmol L -1 , whereas at station 24 there was a massive subsurface maximum, with Mn approaching 5 nmol L -1 . Elsewhere, concentrations at 600m were about 1 nmol L -1 , except at station 12, where a large subsurface feature led to a maxima at 700m of 6 nmol L -1 , the highest concentration we measured. Yet at the adjacent station (14), the feature was absent. Further offshore, the AT-15-61 data showed fewer excursions, with concentrations between 500m and 800m around 1 nmol L -1 in the east, decreasing to less than 0.5 nmol L -1 in the west. The exception was the 20 o S zonal transect, which showed a surprising minimum in Mn between 500m and 800m. DMn along Transect 1 The surface mixed layer at this transect was shallow ranging from ~ 60m at the off shore station to ~45 m at the station closest to the shore (Fig. 1-5). Surface DMn concentrations at this transect ranged between ~1 - 3 nmol L -1 (Fig. 1-8) with highest surface values of ~ 3 nmol L -1 observed at the farthest offshore station, Station 24 (Fig. 1-8A). At each station in this transect, there was a subsurface Mn maximum coinciding with the upper oxygen deficient zone centered around 70m which gradually became deeper in the offshore stations. Surprisingly, Mn 39 concentrations decrease below the subsurface maxima, even though oxygen remains low. Station 24 was an exception as Mn concentrations were highest at the surface and declined with depth, showing no increase at the boundary of the OMZ. The oxygen minimum zone in this transect spanned from ~70 m to 500 m (Fig. 1-2). The westernmost station 24 (Fig. 1-8A) is characterized by maximum Mn concentrations (2.81 nmol L -1 ) in the OMZ corresponding to high nitrite (5.55 µmol L -1 ) at the SNM. Stations 26 and 27 (Fig. 1-8B & C) are characterized by a prominent DMn peak that correlates well with the nitrite maxima profile. In Station 26, DMn concentration ranges from 0.9 – 2.8 nmol L -1 coinciding with a broad nitrite maxima ranging from 5 – 7µmol L -1 at its peak. Station 27 (Fig. 1-8C) is characterized by the presence of two DMn maxima (1.1-1.7 nmol L -1 ) coinciding with two nitrite peaks in the OMZ. Deep water DMn values for Station 29 (Fig. 1-8D; 1-2 nmol L -1 ) was higher than in Stations 26 and 27 (Fig. 1-8B & C; 0.5 -0.6 nmol L -1 ) that were offshore. Below the OMZ, deep water DMn values at Station 26 and 27 decreases to relatively constant levels of ≤ 0.6 nmol L -1 . Station 24 exhibited unusually high DMn values for this transect below 500m which ranged from 3 - 5 nmol L -1 . However, Station 29, the shallowest station closest to shore, had high DMn throughout all depths. DMn along Transect 2 Surface DMn concentrations in Transect 2 ranged from 1.7 – 4 nmol L -1 (Fig. 1-9). Highest surface DMn concentration was measured at Station 21. This transect had higher surface values as compared to Transect 1. This transect is located off the northern broad shelf with the westernmost station (Station 19) being only about 1500 m deep. The OMZ is as shallow as 50 m at this transect; however, due to its proximity to the coast, DMn values seem to be influenced by 40 advection from sediments. Nitrite accumulates in the core of the OMZ below 50m which is associated with a slight increase in DMn concentrations. No distinct DMn peak coincides with the nitrite peak. The deep water values for the westernmost Station 19 (Fig. 1-9A) were lower than those previously noted and were around 0.3 nmol L -1 . DMn along Transect 3 Surface DMn concentrations in Transect 3 ranged between 2 – 4 nmol L -1 (Fig. 1-10). Higher surface concentrations were noted at all stations for this transect. In stations 9, 10 & 14 (Fig. 1- 10 A, B & E), surface DMn concentrations were lower than DMn values at the depth below them. There seemed to be a plume extending offshore from the surface which extends to about Station 10. The OMZ in this transect is deeper than the previous two transects (~ 80-100m). Station 14 (Fig. 1-10E) is the farthest station characterized by deeper and narrower OMZ. However, the SNM in this station at 180 m with 6 µmol L -1 of nitrite is characterized by a DMn peak with a concentration of 2.8 nmol L -1 . At station 12 (Fig. 1-10 D), the OMZ is shallower and narrow, spanning about 200 m deep. This station is characterized by higher DMn concentrations throughout the water column with the lowest values being ~2.5 nmol L -1 . Station 11 (Fig. 1-10C) has a strong nitrite peak at 150 m. There is no corresponding DMn peak at this Station but it could have been missed due to low sampling frequency. Station 9 (Fig. 1-10A) displays a DMn peak of 4.2 nmol L -1 that coincides with the 6 µmol L -1 nitrite peak at 60m. Deep water DMn values for deeper Stations 11 and 14 ranged between 0.7- 0.8 nmol L -1 while Station 12 had higher DMn values below 600m. Shallower stations 9 & 10 had DMn values ~ 1.5 nmol L -1 . 41 1.4.2. Total dissolved nitrite and Mn off shore during AT-15-61 cruise (2010) Nitrite DMn concentrations was also analyzed in samples collected on two transects in February 2010 along 10 0 S and 20 0 S from 80 0 W to 100 0 W (Fig. 1-11). Oceanographic sections of temperature, salinity and oxygen are represented in Fig 1-12, 1-13 and 1-14, respectively. Nitrite concentrations in the 10 0 S zonal transect regularly exceeded 1 µmol L -1 , reaching values as high as around 4 µmol L -1 at station 11, at the heart of the OMZ (Fig. 1-15B). Secondary nitrite maxima (SNM) were also observed at stations 9 through 11, coincident with the depth of the OMZ while No SNM was observed in the southern transect. DMn in the ETSP (2010) The 10 0 S zonal transect was marked by a distinct OMZ (Fig. 1-6B) while along 20 0 S zonal transect was more oxygenated (Fig. 1-15D). The north and south transects of the AT-15-61 cruise were also characterized by a distinct difference in mixed layer depths. The mixed layer depth was ~ 60m at the station closest to the shore (Station 11) while it was deeper in the other stations in that transect (~ 100m). The 20 o S zonal transect, which was outside the part of the OMZ that exhibits a SNM, was in turn characterized by a deeper mixed layer depth of ~ 150 m in the stations closer to the shore while the off shore stations were as deep as ~ 200-300 m. For the stations in the 10 o S zonal transect of the AT-15-61 cruise, dissolved oxygen values < 5 µmol L -1 extend from 225 to 550 m. In contrast, dissolved oxygen values in the 20 o S zonal transect never decreased < 10 µmol L -1 at any of the stations. Mn along 10 0 S zonal transect The surface mixed layer in the 10 0 S transect was about 100m deep, closer to the shore and about 150 m deep at the farthermost station at 100 0 W. The surface mixed layer was shallower 42 than the 20 o S zonal transect. Surface DMn concentrations ranged from 0.8 nmol L -1 to 2.8 nmol L -1 at all stations along the 10 o S zonal transect (Fig. 1-15A). A plume of high concentration DMn in the surface waters extends off shore till about 95 0 W. This plume seems parallel to the oxygen minimum zone below the surface waters which also tapers off at 95 0 W. The 10 0 S zonal transect was characterized by the presence of an intense oxygen minimum zone associated with a SNM which extends to about 95 0 W. The OMZ spans from 200 - 800m deep while the core extends from 200-500 m (Fig. 1-15B). DMn values ranged from 0.9 – 1.1 nmol L -1 in Stations 9, 10 and 11, which were closest to the shore. The off shore stations 7 and 8 had DMn concentrations ranging between 0.4 – 0.7 nmol L -1 . There was no distinct DMn peak coinciding with the SNM, but elevated DMn concentrations were observed throughout the subsurface OMZ at stations 9, 10 and 11 which were closest to the shelf. A high concentration DMn plume seems to extend from the continental shelf coinciding with the OMZ to about 95 0 W but this feature was dispersed throughout the water column. This subsurface DMn plume extends parallel to the high concentration surface DMn plume. West of 95 0 W, the DMn concentrations at the mid depths of Stations 7 and 8 decrease to ~ 0.4 – 0.7 nmol L -1 . Below the OMZ, concentrations decrease to relatively constant values in the 800-1000 m depth interval with most values being in the range of 0.3- 0.7 nmol L -1 . These were the lowest DMn values observed throughout the cruise. Mn along the 20 0 S zonal transect The surface mixed layer in the 20 o S zonal transect is deeper than that of the 10 o S zonal transect. The surface mixed layer extends to about 200 m depth at all stations. Surface DMn concentrations range from 1 – 2.5 nmol L -1 with high DMn values extending as deep as 150m at all stations (Fig. 1-15C). No distinct surface DMn plume is visible unlike in the 10 o S zonal 43 transect. DMn concentrations were relatively constant in between 100-170 m at all stations in the 20 o S zonal transect. The 20 o S zonal transect is characterized by a subsurface “moderately” low oxygen layer at 400 m which is not associated with a coinciding SNM. This moderately low oxygen layer extends to about 100 0 W and has oxygen values as low as 50 µmol L -1 . Coinciding with this low oxygen zone, there was a distinct DMn maximum that extended to 90 0 W, decreasing in concentration towards the west. The subsurface DMn plume indicates that a SNM is not a prerequisite for sharp subsurface maxima in Mn, in sharp contrast to Fe. DMn concentrations ranged from 0.9 nmol L -1 in stations 12, 2 and 3 (closest to the shore) to 0.3-0.6 nmol L -1 in off shore stations 5 and 6. In this transect, a DMn peak is observed at all stations coinciding with the low oxygen layer which is sharp contrast to the 10 o S zonal transect, where there was no such distinct and sharp DMn peak associated with the OMZ. Interestingly, subsurface DMn concentrations in the OMZ were significantly higher here than in the 10 o S zonal transect (Fig. 1- 15A), in contrast to Fe (Y. Kondo and J.W. Moffett; in prep). Deep water DMn values for the 20 o S zonal transect ranged from 0.5 nmol L -1 – 0.8 nmol L -1 (Fig. 1-15B). Higher DMn values at Station 3 (~ 0.8 nmol L -1 ) coincide with higher total dissolved Fe values (Y. Kondo and J.W. Moffett; in prep) for those particular depths. This could indicate an external source such as hydrothermal vents or a deep sea rise or resuspension of sediments from continental margin. 1.5. Discussion 1.5.1. Total dissolved Mn concentrations at the surface. Distinct differences in DMn distribution were noted along the three transects during the KN- 182-09 cruise. Higher surface DMn concentrations followed the general trends observed 44 previously by several researchers (Landing and Bruland 1987; Sunda and Huntsman 1988). Overall surface DMn concentrations were higher in the southern transect (Fig. 1-10; Transect 3) followed by Transect 2 and Transect 1 (Figs. 1-8 & 1-9). This was unexpected since Transect 1 and 2 were over a broader shelf, whereas Transect 3 was off of a narrower shelf. A similar study of Mn and Fe distributions off of Central California (Chase et al. 2005) also reported variations in Mn concentrations with upwelling strength. They observed that although Mn oxide concentrations were highest over a broader shelf, concentrations of DMn were lowest in recently upwelled waters and increased with time following upwelling and theorized that even though Mn oxides are brought to the surface during upwelling, they had to be reduced for an increase in DMn concentrations which happens following upwelling with exposure to sunlight (Sunda and Huntsman 1988; Sunda and Huntsman 1994; Sunda et al. 1983). However, their explanation may not be applicable to the Peru upwelling, with its highly reducing water column, where photochemical reduction is unlikely to be a predominant influence. Our results are even more surprising in light of the very high concentrations of dissolved Fe found in the same samples during the KN-182-09 cruise (J.Vedamati, in prep). Unlike Fe, there was no significant increase in DMn concentrations towards the bottom of the water column closer to the sediments in all three transects of KN-182-09. Interestingly, Chase et al. (2005) also did not observe a pronounced increase in DMn concentrations towards the seafloor at all shallow stations (< 400m depth) closer to the coast. This reflects the Mn-depleted condition of the benthic fluxes, consistent with sediment flux measurements of Scholz et al. (2011) and Noffke et al. (2012), who arrived at similar conclusions about the preferential trapping of iron. In their study of trace metal concentrations in sediments deposited at the upper edge, within and below the Peruvian OMZ (Boning et al. 2004), Mn was found to be significantly depleted in sediments 45 within and below the OMZ while Mn diffusion from reducing sediments only seemed to occur in near coastal sediments at shallow depth . This Mn depletion was attributed to the Mn 2+ mobilization from particles via reduction within the suboxic water column when settling through the OMZ. Indeed, the Mn/Al ratio in andesite (Boning et al. 2004), the detrital background and principal source of Fe to the sediments in this region, is strongly depleted on the entire continental margin (Scholz et al. 2011). If the source of Fe and Mn in the shelf arises from andesite diagenesis, then clearly, Mn is being preferentially removed from the shelf waters while Fe is reductively mobilized from the sediments and is rapidly oxidized at the boundaries of the oxygen-free zone (surface waters and along the western boundary). Such “Fe trapping” is more efficient than for Mn, which is oxidized more slowly. While this is a very different mechanism than the one proposed by Chase et al. (2005), it is also likely to be enhanced with greater upwelling strength resulting in lower Mn values in the northern stations marked with highest rates of upwelling based on temperature and salinity measurements. This is also similar to recent measurements of total dissolved Fe in samples from the AT-15- 81 cruise (Y. Kondo and J.W. Moffett, in prep) where high dissolved Fe concentrations were observed in stations closer to the shore in the OMZ and a distinct reduced Fe plume coinciding with the OMZ. However, in the 20 o S zonal transect (not an OMZ) there was no Fe plume in mid- depths whereas we did observe a distinct DMn plume in the mid depths of the transect. Thus, effective "trapping" of Fe in the sediments underlying the OMZ, results in the distinct differences in the distribution patterns of redox sensitive elements, Fe and Mn, which were previously thought to behave similarly under low oxygen conditions. Elevated concentrations of DMn observed offshore suggest slow scavenging and removal processes and could hint towards Fe limitation of microbial Mn oxidation (Moffett 1997). 46 Moffett (1997) reported that microbial Mn oxidation rates in the euphotic zone of the equatorial Pacific were negligible, in contrast to the Sargasso Sea, where rates were significant, and hypothesized that microbial Mn oxidizers are Fe limited in the equatorial Pacific. Previously , microbial Mn oxidizers have been shown to be Fe limited in culture (Adams and Ghiorse 1985). This Fe limitation of microbial Mn oxidation could be a possible reason for the observed unexpected trend in DMn for the three transects after Hutchins et al. (2002) concluded that iron limitation was a major constraint on phytoplankton growth along the South Pacific Eastern Boundary current. One interesting feature at stations 9, 22 & 29 during the KN-182-09 cruise was that the DMn values at some stations increased sharply with depth in the upper 50m and the concentrations were higher than the surface most values. This feature can be explained by the observations of Schoemann et al. (1998) in their analysis of coastal water samples from the Southern North Sea during spring bloom where they stated that decreased concentrations of Fe, Al and Mn during the spring bloom could be due to biological uptake, adsorption of abiotic particles onto settling phytoplankton cells, or ingestion of abiotic particles by zooplankton during grazing, with subsequent settling of fecal pellets. This fact is further corroborated with data for primary production from the same cruise (KN-182-09) in October 2005 (Fernández et al. 2009). Higher primary production values were obtained in the northern part of the study area (2.4 ± 2.4 g m -2 d -1 ) compared to southern Peru (0.9 ± 0.6 g m -2 d -1 ). They also reported higher primary production rates in stations closer to the coast which decreased with distance from the shore which indicated the dynamics of biogeochemical processes in the region. In the northern stations, the obtained carbon fixation rates reflected higher biological consumption compared to the southern transect (Fernández et al. 2009). Based on previous studies, it can be stated that both 47 high productivity and subsequent utilization of dissolved Mn in the surface as well as microbial oxidation of Mn (II) could be the reason for the observed lower values on the surface. However, in some stations the oxycline is as shallow as 70m and it coincides with the sub-surface DMn peak. Decrease in oxygen concentrations could lead to microbial reduction of Mn oxides especially in stations exhibiting high primary productivity. Station 24 in Transect 1 had highest surface DMn values throughout the section and was also characterized by abnormal high DMn concentrations (~5 nmol L -1 ) below 600m. These values could indicate the presence of a Mn source closer to the station which results from Mn transported along isopycnals from the continental shelf to that station. During the AT-15-61 cruise, high DMn surface plumes extending off shore were observed in both the 10 o S and 20 o S zonal transects. These plumes might depict the production of soluble Mn via photoreduction of high concentration of Mn oxides (Sunda and Huntsman 1988) advected from the continental shelf by the photo-activated reduction of dissolved organic matter derived directly or indirectly from photosynthesis in the surface waters due to higher primary productivity during the austral summer (Chavez 1995; Sunda et al. 1983). The maximum riverine discharge of precipitation and snowmelt along the western flank of Andes also occurs in the austral summer months of January to April (Scheidegger and Krissek 1982), thereby forming an additional source of Mn to the region. These high DMn surface plumes extend as deep as 100 m which also depicts that Mn gets well mixed in the surface mixed layer especially since the eastern Pacific between 8 o S to 10 o S is also a region with high eddy occurrence which moves westward from their region of formation on the shelf bringing along with them plumes of high DMn. 48 Atmospheric inputs to the region have been studied in the VOCALS program and are derived from arid regions in northern Chile and southern Peru (Hawkins et al. 2010). This eolian deposition of Mn could also account for the increase in DMn concentrations observed on the surface where the strong southeasterly surface winds transport aerosol and aerosol precursor components from developed cities such as Santiago, Chile, to the marine boundary layer (Hawkins et al. 2010). 1.5.2. DMn distribution in the oxygen minimum zone (OMZ) The Mn distribution in the Peruvian OMZ seems to be due to a combination of both advective processes from continental margins and in-situ processes. In most stations, the DMn concentrations are elevated throughout the water column with a slight increase in the OMZ. Near shore values seem to be highly influenced by advection from coastal sediments which lead to higher values throughout the water column while the more off shore values seem to be a result of both in-situ and advection processes. Czeschel et al. (2011) studied mid depth circulation in the ETSP and noted that the overall OMZ circulation at 400m depth was zero. In a separate study by Chaigneau et al. (2008), this region was found to be characterized by eddies which move westward from the formation region near the shelf, thereby maintaining the core of the OMZ by stagnant flow supplied with low DO water from the near shelf region by westward propagating eddies. These observations support the fact that DMn in stations closer to the shore could be advected from the shelf leading to higher values throughout the water column whereas higher concentrations in the core of OMZ could be due to advected Mn from the shelf along with in-situ reduction of Mn oxides in the low oxygen core. In the 10 o S zonal transect of AT-15-61 (Fig. 1-15A), stations closest to the shelf exhibit high DMn concentrations at all depths, reflecting lateral advection from the margin and partially 49 obscuring the feature present within the secondary nitrite maximum. Further offshore, DMn decreases, presumably reflecting slow oxidative scavenging. Johnson et al. (1996) also noticed the absence of a distinct secondary maximum at the FeLine Station 7 (3 o S, 140 o W) in the equatorial Pacific where the oxygen concentrations dropped to 32 µmol L -1 . They explained the absence of a distinct secondary Mn maximum to the absence of strong Mn minimum formation between the surface and subsurface Mn maxima. Since the oxygen minimum was much shallower than other stations, the shallow OMZ did not allow regeneration rates to decrease sufficiently to form a Mn minimum before concentrations begin to rise again as scavenging rates decreased in the OMZ. This could well explain the absence of a distinct secondary Mn maximum in our data since similarly to FeLine station 7 in the North Pacific, the oxygen minimum layer is shallow owing to the shallow thermocline in the ETSP. This gives rise to higher DMn concentration throughout the water column and an absence of a distinct secondary maximum in any of the stations in the 10 o S zonal transect. During another study by Johnson in 1992, the Mn flux from the continental shelves and slopes of the central California coast was studied using benthic chambers and it was observed that highest Mn efflux occurred over the continental margins and the lowest values occurred in the OMZ. They concluded that the formation of the DMn plume in the OMZ was mainly due to processes acting within the water column. Indeed, the fundamental difference is probably that there is little redox cycling occurring in the water column, so reduced Mn is not restricted to a narrow zone. Instead, reduced Mn (II) is advected from the boundary without oxidation over a range of depths, a process that is much more limited for Fe (II). Noffke et al. (2012) measured benthic fluxes of Fe and P along 11 o S and reported highest benthic fluxes among similar, oxygen-depleted environments and emphasized on the importance 50 of sediments underlying anoxic water bodies as nutrient sources to the ocean. This could also be the case for Mn, and the continental shelves adjacent to the OMZ in the ETSP could in fact be an important source of DMn to the region. Interestingly, a distinct “DMn plume” is evident in the 20 o S zonal transect (AT-15-61), indicating that a secondary nitrite maximum is not a prerequisite for sharp subsurface maxima in DMn. This is in sharp contrast to the presence of Fe(II) and Mn peaks coincident with the SNM in the Arabian Sea (Lewis and Luther 2000; Moffett et al. 2007). Even though, there is no SNM in the 20 o S zonal transect, there’s the presence of a low oxygen plume that coincides with the DMn plume. Thus, the DMn plume along the 20 o S zonal transect may arise because of slower oxidation rates rather than in situ reduction of Mn oxides. Even though there’s not a distinct presence of SNM in the 20 o S zonal transect, the oxygen values are lower than 50µmol L -1 which could be the reason for the re-equilibration of Mn oxides. 1.4.3. Deep water values of DMn Below the OMZ, DMn concentrations decrease to relatively constant levels (with most values ≤ 0.6 nmol L -1 generally in good agreement with values reported by (Klinkhammer and Bender 1980) for the region. Station 24 in Transect 1 exhibits unusually high DMn values (~ 4-5 nmol L - 1 ) below 600m depth which could be attributed to hydrothermal input (Bennett et al. 2008). At deeper stations of Transect 1 (Stations 26 & 27), deep water DMn values agree with values reported by Klinkhammer and Bender (1980). They state that the deep water Mn concentrations are a result of in-situ processes within the ocean (away from spreading centers) rather than advective processes. The deep water values of DMn in KN-182-09 cruise were in the same range as previously reported by Klinkhammer and Bender (1980). 51 1.6. References Adams, L. F., and W. C. Ghiorse. 1985. MECHANISM OF MANGANESE OXIDE DEPOSITION BY LEPTOTHRIX-DISCOPHORA. Abstr. Pap. Am. Chem. Soc. 190: 85-GEC. Aguilar-Islas, A. M., and K. W. Bruland. 2006. Dissolved manganese and silicic acid in the Columbia River plume: A major source to the California current and coastal waters off Washington and Oregon. Mar. Chem. 101: 233-247. Baker, A. R., T. D. Jickells, M. Witt, and K. L. Linge. 2006. Trends in the solubility of iron, aluminium, manganese and phosphorus in aerosol collected over the Atlantic Ocean. Mar. Chem. 98: 43-58. Bender, M. L., G. P. Klinkhammer, and D. W. Spencer. 1977. MANGANESE IN SEAWATER AND MARINE MANGANESE BALANCE. Deep-Sea Research 24: 799-812. Bennett, S. A., E. P. Achterberg, D. P. Connelly, P. J. Statham, G. R. Fones, and C. R. German. 2008. The distribution and stabilisation of dissolved Fe in deep-sea hydrothermal plumes. Earth Planet. Sci. Lett. 270: 157-167. Boning, P. and others 2004. Geochemistry of Peruvian near-surface sediments. Geochim. Cosmochim. Acta 68: 4429-4451. Chaigneau, A., A. Gizolme, and C. Grados. 2008. Mesoscale eddies off Peru in altimeter records: Identification algorithms and eddy spatio-temporal patterns. Prog. Oceanogr. 79: 106- 119. Chase, Z. and others 2005. Manganese and iron distributions off central California influenced by upwelling and shelf width. Mar. Chem. 95: 235-254. 52 Chavez, F. P. 1995. A comparison of ship and satellite chlorophyll from California and Peru. J. Geophys. Res.-Oceans 100: 24855-24862. Czeschel, R., L. Stramma, F. U. Schwarzkopf, B. S. Giese, A. Funk, and J. Karstensen. 2011. Middepth circulation of the eastern tropical South Pacific and its link to the oxygen minimum zone. J. Geophys. Res.-Oceans 116. Da Silva, J. F., and R. J. P. Williams. 2001. The biological chemistry of the elements: the inorganic chemistry of life. Oxford University Press. Fernández, C., L. Farías, and M. E. Alcaman. 2009. Primary production and nitrogen regeneration processes in surface waters of the Peruvian upwelling system. Prog. Oceanogr. 83: 159-168. Field, M. P., J. T. Cullen, and R. M. Sherrell. 1999. Direct determination of 10 trace metals in 50 mu L samples of coastal seawater using desolvating micronebulization sector field ICP- MS. J. Anal. At. Spectrom. 14: 1425-1431. Froelich, P. N. and others 1979. EARLY OXIDATION OF ORGANIC-MATTER IN PELAGIC SEDIMENTS OF THE EASTERN EQUATORIAL ATLANTIC - SUBOXIC DIAGENESIS. Geochim. Cosmochim. Acta 43: 1075-1090. Fuenzalida, R., W. Schneider, J. Garces-Vargas, L. Bravo, and C. Lange. 2009. Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 56: 1027-1038. Grasshoff, K., M. Ehrhardt, and K. Kremling. 1983. Determination of nutrients. Methods of seawater analysis, 2nd ed. Verlag Chemie: 125-188. 53 Hawkins, L. N., L. M. Russell, D. S. Covert, P. K. Quinn, and T. S. Bates. 2010. Carboxylic acids, sulfates, and organosulfates in processed continental organic aerosol over the southeast Pacific Ocean during VOCALS-REx 2008. J. Geophys. Res.-Atmos. 115. Horsburgh, M. J., S. J. Wharton, M. Karavolos, and S. J. Foster. 2002. Manganese: elemental defence for a life with oxygen? Trends Microbiol. 10: 496-501. Hutchins, D. A. and others 2002. Phytoplankton iron limitation in the Humboldt Current and Peru Upwelling. Limnol. Oceanogr. 47: 997-1011. Johnson, K. S. and others 1992. MANGANESE FLUX FROM CONTINENTAL-MARGIN SEDIMENTS IN A TRANSECT THROUGH THE OXYGEN MINIMUM. Science 257: 1242-1245. Johnson, K. S., K. H. Coale, W. M. Berelson, and R. M. Gordon. 1996. On the formation of the manganese maximum in the oxygen minimum. Geochim. Cosmochim. Acta 60: 1291- 1299. Johnson, K. S. and others 2007. Developing standards for dissolved iron in seawater. Eos, Transactions American Geophysical Union 88: 131-132. Klinkhammer, G. P., and M. L. Bender. 1980. THE DISTRIBUTION OF MANGANESE IN THE PACIFIC-OCEAN. Earth Planet. Sci. Lett. 46: 361-384. Landing, W. M., and K. W. Bruland. 1980. MANGANESE IN THE NORTH PACIFIC. Earth Planet. Sci. Lett. 49: 45-56. ---. 1987. THE CONTRASTING BIOGEOCHEMISTRY OF IRON AND MANGANESE IN THE PACIFIC-OCEAN. Geochim. Cosmochim. Acta 51: 29-43. 54 Lewis, B. L., and G. W. Luther. 2000. Processes controlling the distribution and cycling of manganese in the oxygen minimum zone of the Arabian Sea. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 47: 1541-1561. Martin, J. H., and G. A. Knauer. 1984. VERTEX - MANGANESE TRANSPORT THROUGH OXYGEN MINIMA. Earth Planet. Sci. Lett. 67: 35-47. Moffett, J. W. 1997. The importance of microbial Mn oxidation in the upper ocean: a comparison of the Sargasso Sea and equatorial Pacific. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 44: 1277-1291. Moffett, J. W., T. J. Goeffert, and S. W. A. Naqvi. 2007. Reduced iron associated with secondary nitrite maxima in the Arabian Sea. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 54: 1341- 1349. Noffke, A. and others 2012. Benthic iron and phosphorus fluxes across the Peruvian oxygen minimum zone. Limnol. Oceanogr. 57: 851-867. Pennington, J. T., K. L. Mahoney, V. S. Kuwahara, D. D. Kolber, R. Calienes, and F. P. Chavez. 2006. Primary production in the eastern tropical Pacific: A review. Prog. Oceanogr. 69: 285-317. Revsbech, N. P., L. H. Larsen, J. Gundersen, T. Dalsgaard, O. Ulloa, and B. Thamdrup. 2009. Determination of ultra-low oxygen concentrations in oxygen minimum zones by the STOX sensor. Limnol. Oceanogr. Meth. 7: 371-381. Saager, P. M., H. J. W. Debaar, and P. H. Burkill. 1989. MANGANESE AND IRON IN INDIAN-OCEAN WATERS. Geochim. Cosmochim. Acta 53: 2259-2267. 55 Scheidegger, K. F., and L. A. Krissek. 1982. DISPERSAL AND DEPOSITION OF EOLIAN AND FLUVIAL SEDIMENTS OFF PERU AND NORTHERN CHILE. Geol. Soc. Am. Bull. 93: 150-162. Schoemann, V., H. De Baar, J. De Jong, and C. Lancelot. 1998. Effects of phytoplankton blooms on the cycling of manganese and iron in coastal waters. Limnol. Oceanogr. 43: 1427- 1441. Scholz, F., C. Hensen, A. Noffke, A. Rohde, V. Liebetrau, and K. Wallmann. 2011. Early diagenesis of redox-sensitive trace metals in the Peru upwelling area - response to ENSO- related oxygen fluctuations in the water column. Geochim. Cosmochim. Acta 75: 7257- 7276. Stone, A. T., and J. J. Morgan. 1984. REDUCTION AND DISSOLUTION OF MANGANESE(III) AND MANGANESE(IV) OXIDES BY ORGANICS .2. SURVEY OF THE REACTIVITY OF ORGANICS. Environ. Sci. Technol. 18: 617-624. Strickland, J. D., and T. R. Parsons. 1968. A practical handbook of seawater analysis. Fisheries Research Board of Canada Ottawa. Strub, P. T., F. A. Shillington, C. James, and S. J. Weeks. 1998. Satellite comparison of the seasonal circulation in the Benguela and California current systems. South Afr. J. Mar. Sci.-Suid-Afr. Tydsk. Seewetens. 19: 99-112. Sunda, W. G., and S. A. Huntsman. 1987. MICROBIAL OXIDATION OF MANGANESE IN A NORTH-CAROLINA ESTUARY. Limnol. Oceanogr. 32: 552-564. ---. 1988. EFFECT OF SUNLIGHT ON REDOX CYCLES OF MANGANESE IN THE SOUTHWESTERN SARGASSO SEA. Deep-Sea Research Part a-Oceanographic Research Papers 35: 1297-1317. 56 ---. 1994. PHOTOREDUCTION OF MANGANESE OXIDES IN SEAWATER. Mar. Chem. 46: 133-152. Sunda, W. G., S. A. Huntsman, and G. R. Harvey. 1983. PHOTO-REDUCTION OF MANGANESE OXIDES IN SEAWATER AND ITS GEOCHEMICAL AND BIOLOGICAL IMPLICATIONS. Nature 301: 234-236. Sunda, W. G., and D. J. Kieber. 1994. Oxidation of humic substances by manganese oxides yields low-molecular-weight organic substrates. Tebo, B. M., and S. Emerson. 1985. EFFECT OF OXYGEN-TENSION, MN(II) CONCENTRATION, AND TEMPERATURE ON THE MICROBIALLY CATALYZED MN(II) OXIDATION RATE IN A MARINE FIORD. Appl. Environ. Microbiol. 50: 1268-1273. Thamdrup, B., T. Dalsgaard, and N. P. Revsbech. 2012. Widespread functional anoxia in the oxygen minimum zone of the Eastern South Pacific. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 65: 36-45. Vojak, P. W. L., C. Edwards, and M. V. Jones. 1985. EVIDENCE FOR MICROBIOLOGICAL MANGANESE OXIDATION IN THE RIVER TAMAR ESTUARY, SOUTH-WEST ENGLAND. Estuar. Coast. Shelf Sci. 20: 661-671. Yocum, C. F., and V. L. Pecoraro. 1999. Recent advances in the understanding of the biological chemistry of manganese. Curr. Opin. Chem. Biol. 3: 182-187. 57 Table 1-1: Mn concentrations in nmol L -1 for KN-182-09 cruise in October 2005 Station Depth (m) [Mn] (nmol L -1 ) 9 30 3.45 ± 0.02 55 4.26 ± 0.07 80 3.36 ± 0.05 125 2.61 ± 0.04 150 2.36 ± 0.01 225 1.45 ± 0.05 250 1.64 ± 0.05 300 1.76 ± 0.02 400 1.40 ± 0.08 600 1.19 ± 0.04 10 8 2.30 ± 0.01 30 4.36 ± 0.02 100 3.27 ± 0.06 150 1.63 ± 0.02 200 1.72 ± 0.01 250 1.78 ± 0.09 350 1.47 ± 0.09 11 8 2.62 ± 0.02 50 2.51 ± 0.01 200 1.68 ± 0.05 600 1.17 ± 0.05 800 1.13 ± 0.09 1000 0.86 ± 0.03 12 65 2.59 ± 0.08 100 2.91 ± 0.04 150 2.56 ± 0.10 300 2.35 ± 0.24 400 2.35 ± 0.42 600 4.16 ± 0.06 700 6.06 ± 0.12 800 5.18 ± 0.01 1000 4.57 ± 0.14 14 50 1.94 ± 0.04 100 2.32 ± 0.04 200 2.79 ± 0.08 250 2.00 ± 0.01 300 1.57 ± 0.04 400 1.22 ± 0.02 550 0.96 ± 0.02 600 0.87 ± 0.04 800 0.72 ± 0.00 1000 0.71 ± 0.01 58 Station Depth (m) [Mn] (nmol L -1 ) 19 8 2.20 ± 0.08 50 2.04 ± 0.02 100 1.29 ± 0.00 150 1.37 ± 0.01 200 1.15 ± 0.00 300 1.10 ± 0.04 400 0.97 ± 0.07 500 0.47 ± 0.02 600 0.48 ± 0.04 900 0.37 ± 0.00 1000 0.32 ± 0.01 20 8 1.77 ± 0.01 30 3.25 ± 0.01 50 2.42 ± 0.03 80 1.91 ± 0.00 120 1.76 ± 0.03 200 1.78 ± 0.02 230 1.37 ± 0.04 270 1.41 ± 0.00 300 1.23 ± 0.02 400 1.29 ± 0.02 550 1.15 ± 0.01 800 1.12 ± 0.02 21 8 4.14 ± 0.09 50 3.47 ± 0.10 80 2.35 ± 0.03 120 2.01 ± 0.06 160 2.01 ± 0.07 200 1.71 ± 0.03 230 1.82 ± 0.00 270 1.54 ± 0.00 300 1.45 ± 0.01 350 1.49 ± 0.03 400 1.46 ± 0.01 22 8 1.91 ± 0.00 20 3.10 ± 0.04 40 2.18 ± 0.03 55 1.84 ± 0.03 60 1.56 ± 0.02 65 1.51 ± 0.01 90 1.59 ± 0.04 120 1.29 ± 0.02 140 1.20 ± 0.01 180 1.27 ± 0.01 59 Station Depth (m) [Mn] (nmol L -1 ) 23 60 2.15 ± 0.04 80 2.07 ± 0.02 24 8 2.58 ± 0.08 50 2.90 ± 0.12 60 2.65 ± 0.07 85 2.81 ± 0.01 100 2.19 ± 0.02 160 2.02 ± 0.08 180 1.50 ± 0.03 200 1.52 ± 0.02 240 1.39 ± 0.03 280 1.21 ± 0.00 300 1.73 ± 0.12 350 1.29± 0.02 400 1.06 ± 0.04 500 1.02 ± 0.02 600 4.97 ± 0.29 700 4.84 ± 0.07 800 3.26 ± 0.22 1000 3.15 ± 0.09 26 8 1.64 ± 0.11 30 2.02 ± 0.09 70 2.56 ± 0.05 95 2.83 ± 0.02 120 1.96 ± 0.05 160 2.16 ± 0.07 200 2.07 ± 0.04 225 1.65 ± 0.06 250 1.46 ± 0.05 275 1.96 ± 0.07 300 1.42 ± 0.07 350 0.99 ± 0.04 400 1.01 ± 0.04 500 0.98 ± 0.01 600 0.94 ± 0.01 800 0.89 ± 0.02 1000 0.58 ± 0.01 27 30 0.98 ± 0.02 70 1.70 ± 0.02 90 1.53 ± 0.01 110 1.44 ± 0.01 120 1.36 ± 0.00 150 1.01 ± 0.00 175 0.90 ± 0.00 200 0.89 ± 0.01 60 Station Depth (m) [Mn] (nmol L -1 ) 225 0.69 ± 0.01 250 0.62 ± 0.01 300 1.13 ± 0.03 350 1.02 ± 0.01 400 0.97 ± 0.04 450 1.07 ± 0.01 600 1.07 ± 0.02 900 0.61 ± 0.01 28 8 1.90 ± 0.01 30 2.10 ± 0.05 50 2.44 ± 0.06 100 1.56 ± 0.01 135 1.48 ± 0.01 150 1.38 ± 0.04 175 1.35 ± 0.01 200 1.63 ± 0.07 225 1.23 ± 0.00 250 1.37 ± 0.01 275 1.35 ± 0.05 300 1.25 ± 0.01 325 1.31 ± 0.09 350 1.19 ± 0.02 400 1.07 ± 0.03 500 1.21 ± 0.05 580 1.18 ± 0.04 29 8 1.63 ± 0.16 30 1.84 ± 0.04 35 2.56 ± 0.05 50 2.56 ± 0.00 60 1.74 ± 0.03 85 2.01 ± 0.05 100 1.86 ± 0.04 125 1.63 ± 0.03 130 1.98 ± 0.05 150 1.76 ± 0.04 170 1.65 ± 0.06 180 2.11 ± 0.00 61 Table 1-2: Mn concentrations in nmol L -1 for AT-15-61 cruise in February 2010 Station Depth (m) [Mn] (nmol L -1 ) 2 20 1.41 ± 0.02 40 1.52 ± 0.09 60 1.44 ± 0.02 80 1.39 ± 0.01 100 1.40 ± 0.02 110 1.41 ± 0.00 120 1.17 ± 0.03 140 0.80 ± 0.01 200 0.55 ± 0.02 250 0.65 ± 0.03 275 0.88 ± 0.00 300 0.88 ± 0.05 325 0.85 ± 0.03 350 0.92 ± 0.02 400 0.92 ± 0.00 500 0.61 ± 0.02 3 20 2.43 ± 0.05 40 2.11 ± 0.09 60 1.66 ± 0.00 80 1.24 ± 0.06 100 1.15 ± 0.02 120 1.40 ± 0.01 140 1.72 ± 0.07 145 1.52 ± 0.00 150 1.18 ± 0.03 160 0.87 ± 0.00 180 0.93 ± 0.06 200 0.84 ± 0.01 225 0.53 ± 0.03 250 0.59 ± 0.03 275 0.85 ± 0.00 300 0.97 ± 0.01 325 0.91 ± 0.04 350 0.88 ± 0.01 400 0.97 ± 0.06 500 0.75 ± 0.00 600 0.76 ± 0.01 700 0.86 ± 0.03 800 0.98 ± 0.07 1000 0.82 ± 0.27 4 10 1.25 ± 0.00 20 1.22 ± 0.03 40 1.41 ± 0.05 62 60 1.47 ± 0.02 90 0.95 ± 0.04 125 1.14 ± 0.00 140 0.97 ± 0.01 200 0.48 ± 0.00 250 0.67 ± 0.01 300 0.48 ± 0.02 340 1.10 ± 0.16 375 1.09 ± 0.03 400 0.86 ± 0.02 475 0.78 ± 0.05 600 0.56 ± 0.02 800 0.69 ± 0.05 5 10 1.88 ± 0.07 20 1.76 ± 0.01 30 1.86 ± 0.06 40 1.88 ± 0.18 60 1.86 ± 0.14 70 1.74 ± 0.01 80 1.74 ± 0.00 100 1.72 ± 0.01 140 1.33 ± 0.07 160 1.14 ± 0.19 200 0.71 ± 0.05 250 0.44 ± 0.04 300 0.37 ± 0.06 350 0.33 ± 0.05 400 0.44 ± 0.00 450 0.34 ± 0.04 475 0.68 ± 0.08 550 0.49 ± 0.08 600 0.46 ± 0.04 700 0.56 ± 0.01 800 0.43 ± 0.01 900 0.51 ± 0.00 1000 0.49 ± 0.05 6 20 2.55 ± 0.08 40 2.46 ± 0.00 65 2.09 ± 0.03 100 1.79 ± 0.00 120 1.78 ± 0.00 150 1.47 ± 0.01 170 1.08 ± 0.02 200 0.81 ± 0.02 250 0.60 ± 0.04 63 300 0.80 ± 0.00 350 0.84 ± 0.02 400 0.63 ± 0.01 450 0.58 ± 0.00 501 0.55 ± 0.03 601 0.55 ± 0.00 700 0.49 ± 0.02 7 20 1.27 ± 0.02 25 1.18 ± 0.03 40 1.02 ± 0.01 55 1.11 ± 0.01 60 1.58 ± 0.01 60 1.86 ± 0.01 70 1.78 ± 0.01 80 1.42 ± 0.02 95 1.34 ± 0.06 100 1.07 ± 0.01 110 1.47 ± 0.04 130 1.70 ± 0.01 145 1.77 ± 0.01 160 1.48 ± 0.01 160 0.92 ± 0.00 175 1.23 ± 0.06 200 0.93 ± 0.02 230 0.71 ± 0.00 250 0.70 ± 0.00 300 0.65 ± 0.01 350 0.70 ± 0.02 400 0.77 ± 0.02 450 0.54 ± 0.02 500 0.49 ± 0.00 600 0.45 ± 0.00 700 0.42 ± 0.00 800 0.33 ± 0.01 1000 0.36 ± 0.01 8 20 0.80 ± 0.00 50 0.95 ± 0.05 70 1.12 ± 0.00 80 1.82 ± 0.03 100 0.89 ± 0.01 120 1.16 ± 0.03 140 1.55 ± 0.02 175 0.72 ± 0.01 200 0.51 ± 0.01 250 0.62 ± 0.01 300 0.54 ± 0.01 64 350 0.53 ± 0.02 400 0.43 ± 0.01 600 0.47 ± 0.00 800 0.31 ± 0.00 1000 0.28 ± 0.01 9 20 1.38 ± 0.00 35 1.21 ± 0.03 50 1.40 ± 0.00 80 3.01 ± 0.01 90 2.92 ± 0.00 95 2.31 ± 0.03 110 1.87 ± 0.01 135 1.72 ± 0.04 200 0.82 ± 0.00 250 0.94 ± 0.00 300 0.99 ± 0.01 312 1.30 ± 0.02 325 1.17 ± 0.00 337 1.14 ± 0.02 350 1.01 ± 0.02 400 0.99 ± 0.02 450 0.88 ± 0.00 500 0.71 ± 0.01 600 0.74 ± 0.02 700 0.73 ± 0.01 800 0.56 ± 0.00 1000 0.37 ± 0.00 10 20 2.35 ± 0.01 40 2.36 ± 0.02 50 3.23 ± 0.06 70 2.33 ± 0.09 80 2.03 ± 0.01 100 1.34 ± 0.06 150 1.11 ± 0.01 200 1.01 ± 0.00 250 1.13 ± 0.01 275 1.08 ± 0.01 300 1.12 ± 0.01 325 1.06 ± 0.00 350 0.97 ± 0.00 400 0.96 ± 0.00 450 0.93 ± 0.01 500 0.89 ± 0.03 525 0.91 ± 0.00 550 0.93 ± 0.01 600 0.94 ± 0.00 65 700 0.74 ± 0.00 800 0.62 ± 0.00 1000 0.50 ± 0.00 11 20 2.67 ± 0.06 50 2.21 ± 0.00 60 2.13 ± 0.03 75 1.51 ± 0.01 100 1.41 ± 0.03 120 1.13 ± 0.09 200 0.83 ± 0.24 250 0.93 ± 0.01 300 0.92 ± 0.02 340 1.11 ± 0.02 360 0.96 ± 0.01 370 0.99 ± 0.04 380 0.99 ± 0.02 400 1.20 ± 0.16 450 0.88 ± 0.04 550 0.82 ± 0.03 700 0.87 ± 0.02 1000 0.49 ± 0.04 12 20 1.89 ± 0.00 50 1.69 ± 0.08 70 1.45 ± 0.03 80 1.42 ± 0.00 100 1.26 ± 0.06 150 0.63 ±0.01 200 0.63 ± 0.01 250 0.61 ± 0.02 275 0.89 ± 0.02 300 1.00 ± 0.00 320 1.08 ± 0.02 340 0.93 ± 0.00 400 0.97 ± 0.04 450 0.71 ± 0.00 500 0.37 ± 0.02 550 0.41 ± 0.01 600 0.40 ± 0.01 650 0.59 ± 0.04 700 0.40 ± 0.00 800 0.36 ± 0.01 900 0.54 ± 0.06 66 Fig.1-1. Map of the cruise track off coastal Peru in the Eastern Tropical South Pacific Ocean sampled in October-November 2005 aboard the R/V Knorr. 67 Fig.1-2. Depth profiles of oxygen for stations (A) 24, (B) 26, (C) 27, and (D) 29 in Transect 1 of KN-182-09 cruise. Station 24 Oxygen ( mol L -1 ) 0 50 100 150 200 250 300 Depth (m) 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 300 Depth (m) 0 200 400 600 800 1000 Station 26 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Depth (m) 0 200 400 600 800 1000 Station 27 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Depth (m) 0 50 100 150 200 Station 29 A. B. D. C. 68 Fig.1-3. Depth profiles of oxygen for stations (A) 19, (B) 21, & (C) 22 in Transect 2 of KN-182- 09 cruise. Oxygen ( mol L -1 ) 0 50 100 150 200 250 Depth (m) 0 200 400 600 800 1000 Station 19 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Depth (m) 0 100 200 300 400 500 Station 21 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Depth (m) 0 20 40 60 80 100 120 140 160 Station 22 C. B. A. 69 Fig.1-4. Depth profiles of oxygen for stations (A) 9, (B) 10, (C) 11, (D) 12 & (E) 14 in Transect 3 of KN-182-09 cruise. Oxygen ( mol L -1 ) 0 50 100 150 200 250 Depth (m) 0 200 400 600 800 1000 Station 9 Oxygen ( mol L -1 ) 0 50 100 150 200 Depth (m) 0 100 200 300 400 500 Station 10 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Depth (m) 0 200 400 600 800 1000 1200 Station 11 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Depth (m) 0 200 400 600 800 Station 12 Oxygen ( mol L -1 ) 0 50 100 150 200 Depth (m) 0 20 40 60 80 100 Station 14 E. D. C. A. B. 70 Fig.1-5. Depth profiles of temperature (red line) and salinity (blue line) for stations (A) 24, (B) 26, (C) 27, & (D) 29 in Transect 1 of KN-182-09 cruise. Temperature ( o C) 8 10 12 14 16 18 Depth (m) 0 100 200 300 400 500 Salinity 34.6 34.8 35.0 35.2 T S Station 24 Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity 34.5 34.6 34.7 34.8 34.9 35.0 Station 26 T S Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.5 34.6 34.7 34.8 34.9 35.0 T S Station 27 Station 29 Temperature ( o C) 12 13 14 15 16 17 Depth (m) 0 50 100 150 200 Salinity (ppt) 34.8 34.9 35.0 35.1 35.2 T S B. A. C. D. 71 Fig.1-6. Depth profiles of temperature (red line) and salinity (blue line) for stations (A) 19, (B) 21, and (C) 22 in Transect 2 of KN-182-09 cruise. Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.4 34.6 34.8 35.0 35.2 T S Station 19 Temperature ( o C) 9 10 11 12 13 14 15 16 Depth (m) 0 100 200 300 400 500 Salinity (ppt) 34.6 34.7 34.8 34.9 35.0 35.1 Station 21 T S Temperature ( o C) 11 12 13 14 15 16 Depth (m) 0 50 100 150 200 250 Salinity (ppt) 34.80 34.85 34.90 34.95 35.00 Station 22 T S A. B. C. 72 Fig.1-7. Depth profiles of temperature (red line) and salinity (blue line) for stations (A) 9, (B) 10, (C) 11, (D) 12 & (E) 14 in Transect 3 of KN-182-09 cruise. Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.4 34.6 34.8 35.0 35.2 Station 9 T S Temperature ( o C) 6 8 10 12 14 16 Depth (m) 0 100 200 300 400 500 Salinity (ppt) 34.6 34.7 34.8 34.9 T S Station 10 Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.4 34.6 34.8 35.0 35.2 Station 11 T S Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.4 34.6 34.8 35.0 35.2 Station 12 T S Temperature ( o C) 6 8 10 12 14 16 18 Depth (m) 0 100 200 300 400 500 Salinity (ppt) 34.6 34.7 34.8 34.9 35.0 35.1 Station 14 T S E. D. B. A. C. 73 Fig.1-8. Depth profiles of total dissolved Mn (closed circles) and nitrite (open circles) for stations (A) 24, (B) 26, (C) 27, and (D) 29 in Transect 1 of KN-182-09 cruise. Error bars for total dissolved [Mn] represent error propagation from calculation of standard deviation values (n=3). D. Station 29 Dissolved Mn (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 50 100 150 200 Nitrite (µmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Mn NO 2 - C. Station 27 Dissolved Mn (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 200 400 600 800 1000 Nitrite (µmol L -1 ) 0 1 2 3 4 5 Mn NO 2 - B. Station 26 Dissolved Mn (nmol L -1 ) 0 1 2 3 4 0 200 400 600 800 1000 Nitrite (µmol L -1 ) 0 2 4 6 8 Mn NO 2 - A. Station 24 Dissolved Mn (nmol L -1 ) 0 1 2 3 4 5 6 Depth (m) 0 200 400 600 800 1000 Nitrite (µmol L -1 ) 0 1 2 3 4 5 6 Mn NO 2 - 74 Fig.1-9. Depth profiles of total dissolved Mn (closed circles) and nitrite (open circles) for stations (A) 19, (B) 21, (C) 22, and (D) 23 in Transect 2 of KN-182-09 cruise. Error bars for total dissolved [Mn] represent error propagation from calculation of standard deviation values (n=3). C. Station 22 Dissolved Mn (nmol L -1 ) 0 1 2 3 4 Depth (m) 0 50 100 150 200 Nitrite (µmol L -1 ) 0 1 2 3 4 Mn NO 2 - B. Station 21 Dissolved Mn (nmol L -1 ) 0 1 2 3 4 5 0 100 200 300 400 Nitrite (µmol L -1 ) 0 1 2 3 4 5 Mn NO 2 - A. Station 19 Dissolved Mn (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 Depth (m) 0 200 400 600 800 1000 Nitrite (µmol L -1 ) 0 1 2 3 4 5 6 Mn NO 2 - D. Station 23 Dissolved Mn (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 20 40 60 80 100 Nitrite (µmol L -1 ) 0 1 2 3 4 5 Mn NO 2 - 75 Fig.1-10. Depth profiles of total dissolved Mn (closed circles) and nitrite (open circles) for stations (A) 9, (B) 10, (C) 11, (D) 12 & (E) 14 in Transect 3 of KN-182-09 cruise. Error bars for total dissolved [Mn] represent error propagation from calculation of standard deviation values (n=3). A Station 9 Dissolved Mn (nmol L -1 ) 0 1 2 3 4 5 Depth (m) 0 100 200 300 400 500 600 Nitrite (µmol L -1 ) 0 1 2 3 4 5 6 7 Mn NO 2 - B. Station 10 Dissolved Mn (nmol L -1 ) 0 1 2 3 4 5 0 100 200 300 400 Nitrite (µmol L -1 ) 0 1 2 3 4 5 6 7 Mn NO 2 - C. Station 11 Dissolved Mn (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Depth (m) 0 200 400 600 800 1000 Nitrite (µmol L -1 ) 0 2 4 6 8 10 Mn NO 2 - D. Station 12 Dissolved Mn (nmol L -1 ) 0 1 2 3 4 5 6 7 0 200 400 600 800 1000 Nitrite (µmol L -1 ) 0 1 2 3 4 5 6 Mn NO 2 - E. Station 14 Dissolved Mn (nmol L -1 ) 0 1 2 3 4 Depth (m) 0 200 400 600 800 1000 Nitrite (µmol L -1 ) 0 1 2 3 4 5 6 7 Mn NO 2 - 76 Fig.1-11. Map of the cruise track in the eastern tropical South Pacific Ocean that was sampled from 19 January to 3 March 2010 aboard the R/V Atlantis. 77 Fig.1-12. Upper 300 m oceanographic sections of temperature along the (A) 10 o S and (B) 20 o S zonal transect. The southern zonal section ranged from 80 o W to 100 o W along 20 o S (stations 2, 3, 4, 5 and 12). The northern zonal section ranged from 82 o 30’W to 100 o W along 10 o S (stations 7, 8, 9, 10 and 11). A. B. 78 Fig.1-13. Upper 1000 m oceanographic sections of salinity along the (A) 10 o S and (B) 20 o S zonal transect. A. B. 79 Fig.1-14. Upper 1000 m oceanographic sections of oxygen concentrations along the (A) 10 o S and (B) 20 o S zonal transect. A. B. 80 Fig.1-15. Upper 1000 m oceanographic sections of nitrite ((B) north; (D) south) and total dissolved Mn ((A) north; (C) south)) concentrations. B. C. A. D. 81 Chapter 2 Iron speciation in the Eastern Tropical South Pacific oxygen minimum zone off of Peru 82 2.1. Abstract Vertical profiles of total dissolved iron (DFe), Fe (II) and hydrographic parameters were obtained along three transects across the continental shelf off the Peruvian coast in October 2005. Fe (II) and DFe concentrations were determined using an automated flow injection analysis system (FIA, FeLume II Waterville Analytical) using luminol chemiluminescence (CL) and isotope dilution inductively coupled plasma mass spectrometry (ICP-MS), respectively. In nearshore stations off the broad continental shelf along the northern and central transects, exceedingly high DFe (~ 50 - 75 nmol L -1 ) were measured with most of the dissolved Fe present as Fe(II) below the oxycline. In southern Peru, the shelf is narrower, and dissolved Fe concentrations were 10-fold lower. Moreover, a smaller fraction of the dissolved Fe was present as Fe (II) in the south, even below the oxycline. At the western ends of the transects, Fe (II) maxima were coincident with deep (i.e. secondary) nitrite maxima, even when there were two maxima within the oxygen deficient zone. This suggests a relationship between nitrate reduction and Fe (II) accumulation in the water column. However, over the shelf, Fe (II) was also influenced by benthic processes. Large lateral gradients in dissolved Fe across the shelf-slope break reflect Fe removal by oxidative scavenging and it seems plausible that much of the Fe is “trapped” by redox cycling on the shelf. Nevertheless the maintenance of dissolved Fe (II) filaments within secondary nitrite maxima constitutes an important mechanism for Fe transport offshore. 83 2.2. Introduction Iron, the fourth most abundant element in the Earth’s crust, is an important biologically relevant trace element that plays a dominant role in several key enzymes associated with the nitrogen cycle. It plays a central role in photosynthesis and nitrogen assimilation, especially in marine photoautotrophs (Sunda and Huntsman 1997). Although Fe (III) is the thermodynamically favored form of Fe in oxygenated seawater, at pH 8.6, Fe (III) is strongly hydrolyzed as compared to Fe (II) and oxidation rates of Fe (II) are high. Hence, considerable interest is shown to identify non-equilibrium reactions that work on this large pool of Fe leading to accumulation of Fe(II), the reduced, more bioavailable form of Fe. Almost 40% of the world’s oceans (Moore et al. 2002) have been stated to be iron limited along with reports of possible iron limitation of phytoplankton in the Peru Upwelling /Humbolt Current system (Chavez et al. 2008; Hutchins et al. 2002; Pennington et al. 2006) ,thereby giving rise to HNLC conditions in those areas. New evidence shows that Fe limits nitrogen fixation in the oligotrophic waters offshore (Dekaezemacker and Bonnet 2011), while in the oxygen- depleted waters underneath, concentrations of Fe were high (Bruland et al. 2001). Previous studies in different oxygen minimum zones (OMZs) have noted the presence of a well-defined Fe(II) plume associated with the secondary nitrite maximum (SNM) at their low oxygen core (Kondo and Moffett 2013; Moffett et al. 2007). Recent studies indicate that sediments underlying the OMZ may be important source of Fe(II) to the suboxic zone especially in coastal upwelling systems with low atmospheric input like the Peru upwelling region (Noffke et al. 2012). As a result, high concentrations of Fe (II) and total dissolved Fe (DFe) extend well off the shelf in the upper water column. This phenomenon seems closely coupled with the SNM, since both 84 denitrifiers and anammox have high Fe requirements. Here, we argue that these elevated concentrations arise because a large fraction of the Fe is present as soluble Fe(II). Hong and Kester (1986) were the first investigators to examine the total concentration and redox speciation of iron off the Peruvian coast by colorimetry but their method was not sensitive enough to measure offshore transport due to low detection limits. They observed high concentrations of dissolved Fe (>50 nmol L -1 ) over the Peruvian shelf and stated that the DFe was trapped within 35 km of the coastline and seemed to be confined on the shelf. Total dissolved Fe concentrations have also been reported by Bruland et.al. (2005) along three transects across the Peruvian shelf, very similar to the transects sampled during this study. Their study further emphasized the role of Fe limitation and the importance of continental shelves as a source of Fe. High concentrations of DFe (>100 nmol L -1 ) were reported across the broad northern continental shelf while waters off the southern narrow continental shelf contained relatively low amounts of Fe (~ 5nmol L -1 ). Thus, the main impetus for this study was to decipher the role of reduced Fe in maintaining the high concentrations of DFe supply into the Peruvian upwelling zone and to examine the importance of variability in continental shelf width as a sizeable supply source of dissolved and reduced Fe. Latest reports on the vertical expansion and intensification of the equatorial Pacific OMZs (Karstensen et al. 2008; Stramma et al. 2008) make it increasingly important to understand the dynamic relationship between the highly productive, eastern boundary upwelling systems and the offshore transport of Fe within them. 2.2.1. Study area The Eastern Tropical South Pacific has one of the shallowest and most intense oxygen minimum zones characterized by the presence of a well-defined SNM. Recently, it has been found that the oxygen values (2 nmol L -1 ) at the core of the ETSP OMZ were lower than the 85 detection limit (1-10 nmol L -1 O 2 ) of STOX sensors (Revsbech et al. 2009; Thamdrup et al. 2012). The Eastern Tropical South Pacific forms the most intense “nitrate deficit” maximum zone (nitrate deficit max ranges between 22 and 27 µmol L -1 ), corresponding to the intense OMZ core (O 2min between 2 and 3 µmol L -1 ) (Paulmier and Ruiz-Pino 2009). The OMZ is thickest (>600m) between 5 and 13 o S and extends to about 1000km offshore (Fuenzalida et al. 2009) resulting from the combined effects of high productivity (and export) arising from wind-driven coastal upwelling and reduced ventilation (Fuenzalida et al. 2009; Karstensen et al. 2008). Sampling during this study took place along three transects across different shelf width regions of the Peruvian margin, beginning near shore and extending offshore into oceanic waters (Fig. 2-1). Transect 1 (~11.7 o S- 12.5 o S), comprising of Stations 24, 26, 27 and 29, was sampled across the broader, northern Peru shelf. Transect 2 (~13.28 o S -13.3 o S; stations 19, 22, and 23) was sampled off of the relatively broad, central Peruvian shelf while Transect 3 (~15.6 o S-17.6 o S; stations 9, 10, 11 and 12) across the narrow, southern Peruvian shelf. High benthic fluxes of Fe and PO 4 have been reported from the Peruvian upwelling zone thereby highlighting the importance of sediments underlying anoxic water bodies as nutrient sources to the ocean (Noffke et al. 2012). Scholz et al. (2011) have also observed high concentrations of dissolved Fe in the water column in the near shore stations of the transect in the ETSP off the Peruvian continental margin. We hypothesize that there would be a profound effect of reduced Peruvian sediments underlying the OMZ on the Fe distribution on this region and the presence of the OMZ would facilitate the advective transport of Fe into the adjoining ocean. Based on previous studies on the effect of shelf width on Fe distribution (Bruland et al. 2005; Chase et al. 2005), we also expect to observe variability in Fe distribution over the wide range in shelf width at our sampling region. 86 2.3. Methods 2.3.1. Sample collection Samples on the KN-182-09 cruise during October-November 2005 were collected aboard the R/V Knorr. Sampling was concentrated on three transects off the Peruvian shelf, at 11 o S, 13 o S and 16 o S (Fig. 2-1). Seawater samples for total dissolved Fe analysis were collected in 10L Niskin bottles with Teflon coated interior surfaces (Ocean Test Equipment) mounted on a trace metal clean rosette (Sea-Bird Electronics). Samples were acidified to pH ~1.7 and stored for analysis in the lab. All hydrographic data during this cruise were obtained using a CTD-O 2 probe (Seabird). 2.3.2. Cleaning protocol The 500 ml LDPE (VWR) sampling bottles were thoroughly cleaned in a sequential four-step process that consisted of soaking them in a 5% Citranox acid detergent bath (Alconox) for at least a day followed by another overnight soak in a 10% hydrochloric acid bath (VWR). They were then filled with 10% HCl and baked at 60°C for at least 2 days and finally, filled with 0.1% trace metal grade HCl (Optima, Fisher) and baked at 60°C again for another 2 days. The insides and outsides of the bottles were thoroughly rinsed at least five times with Milli-Q water (18.2 MΩ; Millipore) in between each step. Samples were prepared in 15 mL polypropylene centrifuge tubes (VWR) which were first cleaned in a two-step process by soaking them in 10% HCl at 60°C for 48 hours and then, rinsing each tube at least five times with Milli-Q water. After the rinses, the tubes were filled to a positive meniscus with 0.5% trace metal grade HCl, capped and then baked at 60°C overnight. After retrieving them from the oven, the tubes were left capped & stored until further use. Upon 87 analysis, the tubes were emptied and rinsed three times with Milli-Q water and at least once with the sample. In order to minimize contamination from the beads and before addition to the samples, the NTA resin was cleaned using the following procedure (Lee et al. 2011). 25 ml of the NTA resin solution was poured into a clean 50 mL polypropylene centrifuge tube (Corning) and then washed five times with Milli-Q water. In between washes, the tube was spun down in a 5810-R centrifuge (Eppendorf) maintained at 8°C for 10 min at 4000 rpm. After decanting the supernatant, Milli-Q water was added for the next wash. The resin was then washed five times with 1.5 mol L -1 trace metal grade HCl (Optima, Fisher) and several more times with Milli-Q water after that to bring the pH of the solution above 4, to indicate that all of the HCl had been removed from the solution. For the final cleaning step, the resin solution was washed five times with 0.5 mol L -1 trace metal grade HNO 3 (Optima, Fisher). The resin solution was placed on an analog shaker for several hours for the first wash and then left overnight on the shaker for the last wash. After the final wash, the resin solution was again washed at least five times with Milli-Q water until the pH had risen above 4 in order to remove all of the HNO 3 . The resin solution was diluted twofold with 25 mL Milli-Q water and stored in the refrigerator for future use. 25 µL of the working resin suspension contains ~100 – 400 beads, which is 1:50 dilution of the primary resin solution. 2.3.3. Nitrite measurement Nitrite was measured spectrophotometrically (Strickland and Parsons 1968) by allowing small Acropak-filtered sample aliquots (15 mL or less) to react with an acidified sulfanilamide solution to form a diazonium compound. The compound was then mixed with N-(1-Naphthyl)- ethylenediamine dihydrochloride to produce a colored azo dye, whose extinction was measured 88 on a Shimadzu UV-1700 UV-VIS spectrophotometer to determine the nitrite concentration of each sample. 2.3.4. Measurement of Fe(II) The dissolved Fe(II) concentrations were determined using an automated flow injection analysis system (FIA, FeLume II Waterville Analytical) employing a luminol chemiluminescence (CL) based detection system as described by (King et al. 1995). The FeLume was fitted with a standard quartz flow cell and a Hamamatsu HC135 photon counter configured with the following settings: pump speed: 15 rpm; photon counter integration time: 200 milliseconds; load time: 20 seconds; No. of data pts.: 100. Samples were analyzed immediately after sampling to minimize potential oxidation by pressurizing the Niskin bottles with N 2 and the time lag between retrieving the sampling Niskin bottles and Fe(II) analysis was about 20 minutes. Briefly, an alkaline luminol reagent (~pH – 10.3) reacts with an Fe(II)-containing solution, resulting in luminol oxidation with concurrent chemiluminescent emission (Croot and Laan 2002; Rose and Waite 2001). At pH 10.3, Fe(II) is oxidized by oxygen on a millisecond time scale forming reaction intermediates that catalyze the oxidation of luminol and producing 426 nm of light. The mixing and reaction takes place in a Plexiglas spiral flow cell positioned in front of a photomultiplier tube. The sample and luminol reagent is continually mixed in the flow cell by omitting the injection valve (Hopkinson and Barbeau 2007; Rose and Waite 2001; Roy et al. 2008). Lab View software (National Instruments) controls the loading and injection of the sample and luminol into the flow cell while the intensity of the luminescence is recorded as the sample passes through the reaction coil. The quantification of the signal is done by measuring the height of the signal peak. The need for 89 sample concentration is avoided as this method generates intense peaks that enable highly sensitive Fe (II) determinations at ambient seawater pH. Detection limits were determined for surface samples where ferrous iron was negligible based on a standard 3σ evaluation of the baseline signal (Kondo and Moffett 2013; Moffett et al. 2007). This led to a detection limit of 14pM. However, samples containing high concentrations of Fe(II) (> 5nM) showed more variability, presumably because oxidation is faster at higher Fe(II) concentrations (King et al. 1995). 2.3.5. Total dissolved Fe (DFe) measurement After sampling and before analysis, all samples were acidified to below pH 2 by the addition of concentrated trace metal grade HCl (Optima, Fisher) and stored for at least a month. All samples were analyzed in triplicate using the Finnegan Element 2 (Thermo Scientific) Inductively Coupled Plasma Mass Spectrophotometer (ICP-MS) on a medium resolution mode. The total dissolved Fe concentrations were determined using a single batch nitrilotriacetatic acid (NTA) resin extraction and isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) method adapted from Lee et al. (2011). Dissolved Fe was pre-concentrated in the samples by adding a chelating resin - NTA Superflow resin (Qiagen) in the preparatory stage. 15 ml centrifuge tubes were filled with ~7.5 mL of sample (with the exact volume determined gravimetrically) and spiked with enough 57 Fe-enriched spike (BDH Aristar Plus, VWR) to bring the final concentration to ~2 nmol L -1 . 0.1 mL of 1.5 mol L -1 trace metal grade hydrogen peroxide (H 2 O 2 ; Optima, Fisher) was then added to each sample and left to equilibrate for at least an hour at room temperature, to completely oxidize any Fe 2+ to Fe 3+ (Lee et al. 2011). Next, 200 µL (~800 beads) of the working resin suspension was added to each sample, and the tubes were placed on a shaker for two to three days. The samples were then centrifuged for 10 90 min at 4000 rpm, and the seawater was carefully siphoned off to leave only the resin beads at the bottom. The beads were washed twice with 3 mL Milli-Q water to remove salts and the tubes were once again centrifuged using the same settings. After the final wash, 1 mL of 5% trace metal grade HNO 3 (Optima, Fisher) was added to each tube and, after leaving them on the shaker again for one day, the samples were ready for analysis. Procedural seawater blanks were prepared in triplicates, in the same way as samples using ~0.2 mL low trace metal surface seawater from the 2004 SAFe cruise ([Fe] = 0.09 ± 0.007 nmol L -1 ). The average detection limit and internal blank value for this method (n=3) for Fe was 0.01 nmol L -1 and 0.06 nmol L -1 , respectively. The accuracy of the method was evaluated by measuring SAFe reference standards S1 and D1 (Johnson et al. 2007). The Fe values obtained by this method for S1 and D1 were 0.094 ± 0.01 nmol L -1 and 0.645 ± 0.02 nmol L -1 , respectively. The certified consensus values were 0.090 ± 0.007 nmol L -1 Fe (S1) and 0.67 ± 0.07 nmol L -1 Fe (D1). These values are within the range of the latest consensus numbers (http://www.geotraces.org/science/intercalibration). 2.4. Results 2.4.1. Hydrographic features Surface salinity, temperature & oxygen distribution are shown in Fig. 2-2. Surface temperature and salinity distributions (Fig. 2-2A & B) indicate strong upwelling of low temperature (~ 16 o C) and low salinity (~ 35 psu) waters approximately between 11-14 o S latitude. This low temperature-salinity feature is also coherent with a low oxygen (150-175 µmol L -1 ) region (Fig. 2-2C) indicating upwelling of low temperature and salinity, oxygen depleted- deeper waters. South of this upwelling core, an increase in temperature and salinity of surface waters is noted. Another low T-S feature (~15.8 o C; 34.9) was recorded south of 16 o S latitude, but this 91 feature was accompanied by high surface oxygen concentrations of up to 200 µmol L -1 . Salinity and temperature depth profiles for Transects 1, 2 and 3 are depicted in Fig. 2-3, 2-4 & 2-5 respectively. The depth of the mixed layer (15-55m) gradually becomes deeper as we move southward along the Peruvian shelf towards Transect 3 (Fig. 2-5). Below the mixed layer, the shallow oxycline (30-70m) deepened in the offshore stations. Oxygen levels at mid-depths fell to suboxic levels (< 10 µmol L -1 ) immediately below the oxycline and remained uniform up to an average depth of 400m. The vertical span of OMZ also became shorter and deeper as we moved southward away from the upwelling center, spanning from about ~60-600m in Transect 1, ~ 80- 500 in Transect 2 and ~ 100-400m in Transect 3. Transects 1 and 2 were off a broader shelf while the southernmost Transect 3 was off of a relatively narrower shelf (Fig. 2-1). All three transects were characterized by an intense oxygen deficient zone over the shelf, which deepens on moving offshore. The OMZ characterized by dissolved oxygen values < 5 µmol L -1 extended from 70-600 m in Transect 1, 100- 500 m in Transect 2 and from 150-450 m in Transect 3. However, the OMZ extended deeper from 200- 500 m along the 10 o S zonal transect during AT-15-61 cruise. Overall nitrite concentrations during the KN-182-09 were around 3.5 – 8 µM while along the 10 o S zonal transect, nitrite concentrations were as high as 2.5 µM. 2.4.2. Fe distribution along Transect 1 Transect 1 was sampled right off the broad northern Peruvian shelf between ~11.7 o S -12.5 o S (Fig. 2-1). The surface mixed layer at this transect was shallow ranging from ~ 60m at the offshore station to ~45 m at the station closest to the shore (Fig. 2-3). Surface DFe values range between 0.3 – 0.8 nmol L -1 at stations 24, 26, and 27 (Fig. 2-6A & B; Fig. 2-7A) while highest surface DFe concentrations (1.63 nmol L -1 ) were measured at the near shore station 29 (Fig. 2- 92 7B). Below the mixed layer, a marked decrease in DFe concentrations was noted offshore. The oxygen minimum layer ([O 2 ] < 5 µmol L -1 ) extended from ~ 60m to 600m at this transect accompanied with a double nitrite peak at all stations except for a broad peak at station 26 (Fig. 2-6D). Almost 50% of DFe was present in its reduced form (Fe II) except for Station 29 (Fig. 2- 7B). Deep water DFe was high (1.6 to 2 nmol L -1 ) at all deep stations. This transect, located at the core of the upwelling (Pennington et al. 2006) recorded the highest DFe and Fe(II) measured during the entire cruise. Station 24 is characterized by a sharp nitrite peak (5.5 µmol L -1 ) at 85 m and a broad, deep nitrite max (3µmol L -1 ) at 260 m (Fig. 2-6C). Fe(II) & DFe peaks corresponding to the shallow SNM are displaced deeper into the water column at about 160 m.50% of the DFe (2.5 nmol L -1 ) was present as Fe(II) (1.2 nmol L -1 ) at this depth. At around 600m ([O 2 ] ~ 11 µmol L -1 ), there is a corresponding increase in DFe (2.23 nmol L -1 ) and Fe(II) (1.9 nmol L -1 ) coincident with a high DMn plume (4.97 nmol L -1 ), thereby hinting towards a deeper source of reduced trace metals at this depth (J. Vedamati; in prep). Station 26 exhibits a broad SNM (100-260m) associated with high nitrite concentrations of up to 7µmol L -1 (Fig. 2-6D). This broad SNM was associated with sharp peaks in DFe (2.58 nmol L -1 ) and Fe (II) (1.25 nmol L -1 ) at 250m (Fig. 2-6B) with almost 50% of the DFe being present as Fe (II). Relatively high DFe, deep water values (400-1000 m) probably reflect efflux from continental margins with about 20-30% of this Fe being present as Fe(II). Station 27 (Fig. 2-7A & C) is marked by the presence of two sharp peaks each of nitrite, Fe(II) and DFe concentrations, at 120 and 300m depth. The shallow, high nitrite peak (4 µmol L - 1 ) is associated with a slightly broad Fe (II) (0.9 - 1 nmol L -1 ) and DFe (1.51 - 1.7 nmol L -1 ) peaks. The deeper nitrite peak (2.33 µmol L -1 ) is associated with a sharp, high Fe (II) (1.1 nmol 93 L -1 ) and DFe (2.02 nmol L -1 ) peak hinting towards rapid recycling of Fe. Elevated DFe (2.51 nmol L -1 ) concentration at 600m is also accompanied with a corresponding Fe (II) peak (0.52 nmol L -1 ). Deep water values for DFe were high (1.98 nmol L -1 ) at this station reflect its proximity to slope sediments. At the near shore, shallow station 29 (Fig. 2-7D), the oxygen minimum zone extended from ~ 60m to the deepest depth sampled at 180m. As oxygen disappears from the water column, both Fe(II) and DFe concentrations increase corresponding to 1-1.5µmol L -1 concentration of NO 2 - at the SNM. DFe values at this shallow station were slightly less than Fe(II), but within the standard error of the measurement. Near bottom concentrations (130 m) of DFe (74.7 nmol L -1 ) and Fe (II) (90 nmol L -1 ) are an order of magnitude higher than near surface DFe values (1.63 and 0.13 nmol L -1 ) respectively (Fig. 2-7B). 2.4.3. Fe distribution along Transect 2 Transect 2 was sampled entirely on the central Peruvian shelf between ~13.28 o S -13.3 o S (Fig. 2-1). Surface DFe concentrations ranged between 1.4 – 1.9 nmol L -1 (Fig. 2-8 & 2-9). The OMZ extended from 100-500 m at the deeper station 19 (Fig. 2-8B) while it was as shallow as 30m at the nearshore stations 22 and 23 (Fig. 2-9C & D). DFe was very high within the OMZ at these stations, ranging between 2.6 - 33 nmol L -1 . Fe(II) was very high, and the profile features were similar to DFe. We did not report DFe and Fe (II) data for Station 21, which had very poor precision for reasons we could not determine. Station 19, the deepest station of this transect, had a deep mixed layer at around 100m (Fig. 2-8B). At 50 m, there is the presence of a primary nitrite maximum (5.5 µmol L -1 ) corresponding to [O 2 ] ~ 50 µmol L -1 . The Fe peaks seem to be displaced higher as compared to the SNM at 120m (Fig. 2-8A). Two well-defined DFe and Fe (II) peaks are present at 100 and 300m which 94 coincide with the broad SNM. DFe values at the double peaks were 3.4 nmol L -1 (120m) and 2.7 nmol L -1 (300m), corresponding to Fe (II) values of 1.91 nmol L -1 and 1.38 nmol L -1 . 50% of the DFe was present as Fe (II). Owing to the proximity of this transect to the Peruvian shelf, all stations in this transect were characterized by high DFe (1.6 – 35 nmol L -1 ) at the deepest depths sampled. Deep Fe(II) values (1000m) were low at Station 19 (0.02 nmol L -1 ). Stations 22 and 23 were characterized with high Fe(II) concentrations (11.9 and 31 nmol L -1 ) at the bottom most depths sampled (180 and 80m)(Fig. 2-9A & B). Both DFe and Fe(II) concentrations increase towards the bottom in these over-the-shelf stations. Highest DFe and Fe(II) concentrations were lower than Transect 1 even though this transect was closer to the continental shelf and there was a gradual decrease in DFe and Fe(II) concentrations as we moved away from the shelf. The shallowest station (80m) of this transect, Station 23 (Fig. 2-9B), is characterized by high Fe(II) and DFe values throughout the water column. Even though this station is shallower than Station 29, Fe (II) concentrations were lower (~ 31 nmol L -1 ) than the northern transect 1. DFe values at this shallow station were within the rather large errors of Fe (II). Station 22 is characterized by a shallow OMZ, the presence of a nitrite max at 40 m and high Fe (II) (0.76 - 12 nmol L -1 ) and DFe (1.6 - 15 nmol L -1 ) values throughout the water column (Fig. 2-9A & C). Our results suggest that in areas of high dissolved Fe on Transects 2 and 3, the fraction present as Fe (II) approaches 100%. This is not surprising, since the solubility of Fe (III) in seawater is as much as two orders of magnitude lower than concentrations we report. Nevertheless our DFe and Fe (III) data are not always in exact agreement. Indeed, the Fe (II) data are frequently higher than the corresponding DFe collected from the same bottle, although within standard error for all but two depths. It is not unreasonable that these numbers are not 95 identical. They were determined using completely different methodologies over 6 years apart. Reported concentrations of DFe for consensus standards show similar ranges, even when based on analysis of the same acidified standards. However, the consistently higher values of Fe (II) suggest a systematic difference between protocols. We see no reason why the Fe (II) should be consistently higher. But sample preservation protocols may have influenced the DFe values. Samples were acidified to pH 1.7 in accordance with GEOTRACES protocols on board ship. However, they were not acidified until two weeks after the collection of the samples collected on transects 2 and 3. During this time, the high concentrations of Fe(II) would have oxidized and could have formed solid phases adsorbed to the bottle walls that would resist solubilization. A rule of thumb developed for lead analysis is to allow twice the time in the bottle with acid as the time between sampling and acidification for complete recovery (EA Boyle, personal communication), but that may not be sufficient for high Fe. The GEOTRACES “Cookbook” provides no guidance about the timing of acidification, and indicates post-cruise acidification is acceptable. However, GEOTRACES has not addressed the issue of high Fe analysis in any of its intercalibration exercises. This effect is probably small, but a difference of only 10-15% in our highest samples would account for this discrepancy. 2.4.4. Fe distribution along Transect 3 Transect 3 was sampled off the southern narrow Peruvian shelf between ~15.6 o S -17.6 o S (Fig. 2-1). Owing to the narrow southern continental shelf, DFe is lower throughout this transect than anywhere else along the shelf consistent with findings of (Bruland et al. 2005). Transect 3 exhibits a wider range in DFe (0.38 – 2.16 nmol L -1 ) and Fe (II) values (0.05 – 0.5 nmol L -1 ) (Fig. 2-10 and 2-11) than transect 1 and 2. The OMZ (O 2 < 5µmol L -1 ) ranges between ~ 80 - 96 400m in depth and is narrower in comparison to Transect 2 and 3. Fe (II) constitutes only about 30% of DFe at this transect. High DFe (>3 nmol L -1 ) occurs throughout the water column below 200m at Station 11 (Fig. 2-10A). A single broad nitrite (8µM) peak is present at 150m (Fig. 2-10C), which coincides with a smaller Fe (II) peak (1 nmol L -1 ) at 150 m and a deeper, larger peak (1.8 nmol L -1 ) at 250m. Approximately, 11% of the DFe is present as Fe(II). Similarly, Station 12 is characterized by the presence of a broad SNM present between 100- 200m (Fig. 2-10D). 3.7 nmol L -1 of DFe is associated with the SNM peak along with 0.5 nmol L - 1 Fe(II) which accounts for only ~ 13% of DFe (Fig. 2-10B). Below 300m, high DFe (1.6 - 2.3 nmol L -1 ) is present throughout the water column while the Fe(II) concentrations remained low (0.06 - 0.1 nmol L -1 ). There was the presence of a double nitrite peak at Station 9 - a shallow PNM at 60m (6.5µmol L -1 ) and the deeper broader SNM peak at 250m (3.5µmol L -1 ) (Fig. 2-11C). A similar structure in the DFe and Fe (II) distribution with depth is noted, although the peaks are displaced deeper than the nitrite maxima (Fig. 2-11A). The DFe (~ 6.5 nmol L -1 ) peak at 220m seems to coincide with the Fe(II) peak (~ 1.6 nmol L -1 ) at 250m. However, the DFe peak at 400m does not seem to be associated with a corresponding Fe(II) peak, thereby indicating a near shore shelf- slope sediment source of Fe(III). Station 10 (Fig. 11B & D) is characterized by a well-defined high surface DFe (1.95 nmol L - 1 ) along with a high Fe(II) concentration (0.56 nmol L -1 ) (Fig. 2-11B). A broad DFe peak (2.3 - 4.2 nmol L -1 ) is present at mid depths (100-250m). A broad nitrite peak is also present with maxima at 80 and 250m (Fig. 2-11D). The shallow and broad nitrite maximum at the oxycline does not coincide with an Fe(II) peak while the deeper nitrite maximum at 250 m is characterized 97 by a well-defined Fe(II) peak (300m, 2 nmol L -1 ). 20-30 % of the DFe is in the form of Fe(II). Upper 1000m oceanographic sections of DFe and Fe(II) is denoted in an ODV plot in Fig. 2-12. 2.5. Discussion 2.5.1. High Fe off the Peruvian continental shelf The impact of a broad continental shelf on a productive, upwelling regime has been demonstrated previously by several researchers (Bruland et al. 2001; Chase et al. 2005; Johnson et al. 2001; Johnson et al. 1999). The importance of Fe supply in the Peruvian upwelling regime has also been in focus, when it was found that extensive parts of the Peru/ Humbolt upwelling system were iron limited (Hutchins et al. 1998; Hutchins et al. 2002). Coastal waters off Peru were characterized by exceedingly high concentrations of Fe, particularly in waters overlying the broad coastal shelf in central and northern Peru and by large variability in DFe concentrations near the bottom depending on the size of the shelf (Bruland et al. 2005). Consequently, the Peru margin is most certainly an important source of Fe to the central South Pacific basin and such inputs are likely to be strongly affected by redox processes owing to the large OMZ off the coast. Similarly during this study, high DFe concentrations (~75 nmol L -1 ) were associated with the widest shelf in the north (Transect 1) while in the southern, narrow shelf (Transect 3), the concentrations in the near bottom suboxic waters were more than an order of magnitude lower. The near-shore stations of the two northernmost transects were characterized by very high DFe (~ 50 nmol L -1 ) while the southernmost transect, off a narrow shelf, was characterized by lower Fe(II) and DFe concentrations. Thus, Fe speciation measurements conducted along these three transects in October 2005, clearly indicate that Fe(II) is the dominant redox state within the OMZ and these high Fe(II) concentrations associated with the shallow transects hint towards the strong role of reduced Fe in maintaining a constant supply of bioavailable Fe to this upwelling regime. 98 In a study of pore water concentrations and solid phase data of redox-sensitive elements across a transect along 11 o S, (Scholz et al. 2011) also observed high pore water DFe concentrations (35 µmol L -1 ) and high flux values for Fe in the shallowest sediments (~ 50m water depth) over the Peruvian margin. Andesite, the Peruvian detrital background (Boning et al. 2004), is the principal source of Fe to the upwelling sediments in this region. Deeper regions of their transect (~300-600m) had slightly lower DFe concentrations and much less steep concentration gradients. The values reported previously (Noffke et al. 2012; Severmann et al. 2010) along with that during this study, clearly indicate that sediments are the principal focal point of reduction of Fe in the Peruvian OMZ. Thus, increased inputs of detrital continental material along with lateral transport of reduced Fe from mid depths accompanied by subsequent oxidation and deposition results in "trapping" of excess Fe in the shallow sediments. With the commencement of suboxic conditions, these sediments become a source of reduced Fe which gets recycled into the water column leading to higher Fe concentrations observed in near shore, shallow stations. A simplistic view of eastern boundary upwelling zones - that surface waters move offshore driven by Ekman transport, while subsurface waters move onshore to replace them, does not provide a mechanism for the offshore transport of Fe or other materials within the OMZ. However, (Czeschel et al. 2011) showed that subsurface circulation, while generally “sluggish”, was complex, with many adjacent westward and eastward filaments and two regions of frequent eddies at 9 o S and 16 o S that were associated with strong westward flow. Westward flowing filaments bear a strong signature from the shelf and slope regimes and are a plausible mechanism for westward, subsurface Fe transport. This phenomenon further explains the formation of the shallow Fe feature in stations off the continental shelves. 99 The narrow shelf in the south leads to smaller benthic inputs (Bruland et al. 2005; Chase et al. 2005), and accounts for the lower values in near shore stations of Transect 3. Iron declines rapidly at the shelf-slope break but a subsurface “tongue” extends offshore (especially in Transects 2 and 3) that is comprised of Fe(II). Indeed, while Fe(II) is distributed throughout the water column on the shelf, its westward extension is primarily along this tongue at the top of the OMZ, coinciding with the secondary nitrite maximum. Stations 24, 26 and 27 in Transect 1 are characterized by the presence of two Fe peaks – one in the 100-200 m depth range and the second below 250 m. (Noffke et al. 2012) studied the benthic fluxes of Fe in Peruvian OMZ and found that the fluxes of Fe(II) from sediments off Peru were massive, but 20 fold higher in the upper 250m than in the zone between 250m and 600m. That depth dependence could also contribute to the structure in Fe(II) and DFe distributions observed in stations with a double Fe peak. The upper Fe peak could thus, be attributed to fluxes from the sediment that cause elevated concentrations of Fe (II) and DFe between ~ 100-200m while the deeper Fe feature seems to be associated with the SNM and OMZ. This feature is prominent in stations (24, 26, and 27) in Transect 1 while it diminishes in Transect 2 (Station 19) and absent in any station in Transect 3. 5.2. Lower Fe throughout the water column in offshore stations in northern transects Shallow, near shore stations in all three transects recorded high Fe concentrations with higher values off the broader shelf in the north as compared to the shallower, southern stations off of a narrower shelf. However, as we move southward from Transect 1 to Transect 3, the Fe concentrations in the water column increased at the offshore stations. This difference in Fe concentrations could be attributed to differences in primary productivity in the region along with variations in aeolian deposits in the transects. Fernández et al. (2009) discussed the spatial 100 distribution of surface nitrate during KN-182-09. Along the southernmost transect, Station 14 (not reported in the paper due to lack of oxygen data) showed lower surface nitrate concentrations as compared to the coastal station 9 along Transect 2. On the contrary, they noted that along the northern transect, surface nitrate concentrations in the offshore station (St 24, 15 µmol L -1 ) were higher than values observed near the coast (St 28, 6.6 µmol L -1 ). Fernández et al. (2009) also reported that the levels of primary production observed during the KN182-09 cruise were in the range of other estimations for waters off Peru as well as other coastal upwelling systems. In their observations, higher values were obtained in the northern part of the study area (2.4 ± 2.4 g m -2 d -1 ) compared to southern Peru (0.9 ± 0.6 g m -2 d -1 ). In northern stations (i.e. Stations 28 and 20), the reported carbon fixation rates also reflected higher biological consumption compared to the southern transect. However, in all stations of the coast-to-offshore transect, primary production rates were higher near the coast, reflecting the dynamic nature of biogeochemical processes in this area. This difference in primary productivity and carbon uptake rates in the northern and southern transects could explain the range of Fe concentrations in all three transects. Higher productivity in the northern transect at the core of the upwelling region (11 o S), (Pennington et al. 2006), could result in the drawdown of Fe, and thereby lower values than in the southern transect. The bottom waters in the nearshore stations of Transect 1 contained extremely high concentrations of dissolved iron (~ 74 nmol L -1 ) while, the highest dissolved Fe concentration observed in surface waters of this region, was only 1.63 nmol L -1 . Presumably, these upwelled waters initially contained extremely high dissolved Fe that had been subsequently assimilated and depleted by both the high diatom biomass and luxury Fe-uptake during early stages of the bloom. Bruland et al. (2005) attributed the tendency of these coastal upwelling 101 systems to be driven towards Fe limitation to the development of a diatom bloom and subsequent sedimentation of iron-rich diatom biomass, that eventually exports Fe out of the surface photic zone. These processes can thus, lead to the rapid development of blue water HNLC conditions observed offshore. However, the weaker C uptake rates in southern Peru, thought to be associated with iron limitation (Bruland et al. 2005; Hutchins et al. 2002) could thus lead to inefficient drawdown of Fe in the near shore coastal waters, leading to higher concentrations in the offshore surface waters. 2.5.3. Sources of Fe(II) to the OMZ The subsurface maxima in Fe(II) in the Peruvian OMZ is an anomaly since it is at odds with the thermodynamic predictions of the redox cascade (Codispoti et al. 2005), and observations over redox gradients elsewhere. However, such anomalies are interesting because they provide new insights into the actual processes that define the evolution of chemical gradients in the ocean. Fe (II) is thermodynamically unstable at seawater pH in the presence of oxygen or nitrate. Since oxygen levels within the Peruvian OMZ are probably truly zero (Revsbech et al. 2009), Fe(II) is thought to be maintained by a dynamic kinetic balance between in situ reduction during metabolic remineralisation (Hopkinson and Barbeau 2007; Moffett et al. 2007), inputs from dust(Duce and Tindale 1991) , predominantly shallow reducing sediments (Hong and Kester 1986; Lohan and Bruland 2008; Noffke et al. 2012) and oxidation at the OMZ boundaries and perhaps within the OMZ by microbially catalyzed oxidation with nitrate. Raiswell and Canfield (2012) have argued that this reaction ultimately places limits on the accumulation of Fe(II) in the presence of nitrate. Croot et al. (2010) reported Fe(II) and iodide profiles off Peru at the 2010 Ocean Sciences Meeting. The iodide profiles were similar at the top of the OMZ, but it stayed high throughout the OMZ, rather than decreasing quickly with depth below the secondary nitrite 102 maxima, like Fe(II) does. This suggests that iodide and Fe(II) might have similar formation pathways, but different oxidation pathways. Interestingly, nitrate levels are generally high at depths where Fe(II) is found (greater than 10µmol L -1 ) and maintain an Eh regime too oxidizing for Fe(II). Based on thermodynamics, the energy sequence of terminal electron acceptors for respiration is O 2 > IO 3 - > NO 3 - > Mn(IV) > Fe(III) > SO 4 2- (Murray et al. 1995). Nitrate and nitrite do not oxidize Fe(II) spontaneously, but microbially catalyzed chemoautotrophic oxidation of Fe(II) by nitrate has been observed in sedimentary regimes (Straub et al. 1996). That is reasonable because chemoautotrophic oxidation of iodide by nitrate is not thermodynamically viable. There are four possible sources of the Fe(II) in the secondary nitrite maximum. The first possible source could be dissimilatory reduction by heterotrophic bacteria using Fe(III) as a terminal electron acceptor. Fe(III) reduction by sulfide produced by microbial sulfate reduction could also account for the Fe (II) peak while selective reduction of Fe(III) by microbes could also lead to Fe(II) accumulation, perhaps as part of an Fe acquisition strategy (Moffett et al. 2007). Long range transport of Fe (II) from reducing sediments is the only process that requires no in situ reduction mechanisms. Benthic sources seem like the only explanation for the massive Fe(II) concentrations that we observe over the shelf. Finally, westward flowing subsurface filaments have been reported to be active off Peru and can transport Fe(II) offshore (Czeschel et al. 2011). This suggests maintenance via some internal processes, perhaps in addition to benthic inputs. Offshore, atmospheric inputs may also be more important (Duce and Tindale 1991). Dust and aerosol transport have been studied in the VOCALS program and they are derived from arid regions in northern Chile and southern Peru (Hawkins et al. 2010). 103 2.6. References Boning, P. and others 2004. Geochemistry of Peruvian near-surface sediments. Geochim. Cosmochim. Acta 68: 4429-4451. Bruland, K. W., E. L. Rue, and G. J. Smith. 2001. Iron and macronutrients in California coastal upwelling regimes: Implications for diatom blooms. Limnol. Oceanogr. 46: 1661-1674. Bruland, K. W., E. L. Rue, G. J. Smith, and G. R. Ditullio. 2005. Iron, macronutrients and diatom blooms in the Peru upwelling regime: brown and blue waters of Peru. Mar. Chem. 93: 81-103. Chase, Z. and others 2005. Manganese and iron distributions off central California influenced by upwelling and shelf width. Mar. Chem. 95: 235-254. Chavez, F. P., A. Bertrand, R. Guevara-Carrasco, P. Soler, and J. Csirke. 2008. The northern Humboldt Current System: Brief history, present status and a view towards the future. Prog. Oceanogr. 79: 95-105. Codispoti, L., C. Flagg, V. Kelly, and J. H. Swift. 2005. Hydrographic conditions during the 2002 SBI process experiments. Deep Sea Research Part II: Topical Studies in Oceanography 52: 3199-3226. Croot, P. L., and P. Laan. 2002. Continuous shipboard determination of Fe(II) in polar waters using flow injection analysis with chemiluminescence detection. Anal. Chim. Acta 466: 261-273. Czeschel, R., L. Stramma, F. U. Schwarzkopf, B. S. Giese, A. Funk, and J. Karstensen. 2011. Middepth circulation of the eastern tropical South Pacific and its link to the oxygen minimum zone. J. Geophys. Res.-Oceans 116. 104 Dekaezemacker, J., and S. Bonnet. 2011. Sensitivity of N-2 fixation to combined nitrogen forms (NO3- and NH4+) in two strains of the marine diazotroph Crocosphaera watsonii (Cyanobacteria). Mar. Ecol.-Prog. Ser. 438: 33-46. Duce, R. A., and N. W. Tindale. 1991. Atmospheric transport of iron and its deposition in the ocean. Limnology and Oceanography;(United States) 36. Fernández, C., L. Farías, and M. E. Alcaman. 2009. Primary production and nitrogen regeneration processes in surface waters of the Peruvian upwelling system. Prog. Oceanogr. 83: 159-168. Fuenzalida, R., W. Schneider, J. Garces-Vargas, L. Bravo, and C. Lange. 2009. Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 56: 1027-1038. Hawkins, L. N., L. M. Russell, D. S. Covert, P. K. Quinn, and T. S. Bates. 2010. Carboxylic acids, sulfates, and organosulfates in processed continental organic aerosol over the southeast Pacific Ocean during VOCALS-REx 2008. J. Geophys. Res.-Atmos. 115. Hong, H. S., and D. R. Kester. 1986. REDOX STATE OF IRON IN THE OFFSHORE WATERS OF PERU. Limnol. Oceanogr. 31: 512-524. Hopkinson, B. M., and K. A. Barbeau. 2007. Organic and redox speciation of iron in the eastern tropical North Pacific suboxic zone. Mar. Chem. 106: 2-17. Hutchins, D. A., G. R. Ditullio, Y. Zhang, and K. W. Bruland. 1998. An iron limitation mosaic in the California upwelling regime. Limnol. Oceanogr. 43: 1037-1054. Hutchins, D. A. and others 2002. Phytoplankton iron limitation in the Humboldt Current and Peru Upwelling. Limnol. Oceanogr. 47: 997-1011. 105 Johnson, K. S. and others 2001. The annual cycle of iron and the biological response in central California coastal waters. Geophys. Res. Lett. 28: 1247-1250. Johnson, K. S., F. P. Chavez, and G. E. Friederich. 1999. Continental-shelf sediment as a primary source of iron for coastal phytoplankton. Nature 398: 697-700. Johnson, K. S. and others 2007. Developing standards for dissolved iron in seawater. Eos, Transactions American Geophysical Union 88: 131-132. Karstensen, J., L. Stramma, and M. Visbeck. 2008. Oxygen minimum zones in the eastern tropical Atlantic and Pacific oceans. Prog. Oceanogr. 77: 331-350. King, D. W., H. A. Lounsbury, and F. J. Millero. 1995. RATES AND MECHANISM OF FE(II) OXIDATION AT NANOMOLAR TOTAL IRON CONCENTRATIONS. Environ. Sci. Technol. 29: 818-824. Kondo, Y., and J. W. Moffett. 2013. Dissolved Fe(II) in the Arabian Sea oxygen minimum zone and western tropical Indian Ocean during the inter-monsoon period. Deep Sea Research Part I: Oceanographic Research Papers 73: 73-83. Lee, J. M., E. A. Boyle, Y. Echegoyen-Sanz, J. N. Fitzsimmons, R. F. Zhang, and R. A. Kayser. 2011. Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batch nitrilotriacetate resin extraction and isotope dilution inductively coupled plasma mass spectrometry. Anal. Chim. Acta 686: 93-101. Lohan, M. C., and K. W. Bruland. 2008. Elevated Fe(II) and dissolved Fe in hypoxic shelf waters off Oregon and Washington: An enhanced source of iron to coastal upwelling regimes. Environ. Sci. Technol. 42: 6462-6468. 106 Moffett, J. W., T. J. Goeffert, and S. W. A. Naqvi. 2007. Reduced iron associated with secondary nitrite maxima in the Arabian Sea. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 54: 1341- 1349. Moore, J. K., S. C. Doney, D. M. Glover, and I. Y. Fung. 2002. Iron cycling and nutrient- limitation patterns in surface waters of the World Ocean. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 49: 463-507. Murray, J. W., L. A. Codispoti, and G. E. Friederich. 1995. OXIDATION-REDUCTION ENVIRONMENTS - THE SUBOXIC ZONE IN THE BLACK-SEA, p. 157-176. In C. P. Huang, C. R. Omelia and J. J. Morgan [eds.], Aquatic Chemistry: Interfacial and Interspecies Processes. Advances in Chemistry Series. Noffke, A. and others 2012. Benthic iron and phosphorus fluxes across the Peruvian oxygen minimum zone. Limnol. Oceanogr. 57: 851-867. Paulmier, A., and D. Ruiz-Pino. 2009. Oxygen minimum zones (OMZs) in the modern ocean. Prog. Oceanogr. 80: 113-128. Pennington, J. T., K. L. Mahoney, V. S. Kuwahara, D. D. Kolber, R. Calienes, and F. P. Chavez. 2006. Primary production in the eastern tropical Pacific: A review. Prog. Oceanogr. 69: 285-317. Raiswell, R., and D. E. Canfield. 2012. The iron biogeochemical cycle past and present. Geochem. Perspect 1: 1-220. Revsbech, N. P., L. H. Larsen, J. Gundersen, T. Dalsgaard, O. Ulloa, and B. Thamdrup. 2009. Determination of ultra-low oxygen concentrations in oxygen minimum zones by the STOX sensor. Limnol. Oceanogr. Meth. 7: 371-381. 107 Rose, A. L., and T. D. Waite. 2001. Chemiluminescence of luminol in the presence of iron(II) and oxygen: Oxidation mechanism and implications for its analytical use. Anal. Chem. 73: 5909-5920. Roy, E. G., M. L. Wells, and D. W. King. 2008. Persistence of iron(II) in surface waters of the western subarctic Pacific. Limnol. Oceanogr. 53: 89-98. Scholz, F., C. Hensen, A. Noffke, A. Rohde, V. Liebetrau, and K. Wallmann. 2011. Early diagenesis of redox-sensitive trace metals in the Peru upwelling area - response to ENSO- related oxygen fluctuations in the water column. Geochim. Cosmochim. Acta 75: 7257- 7276. Severmann, S., J. Mcmanus, W. M. Berelson, and D. E. Hammond. 2010. The continental shelf benthic iron flux and its isotope composition. Geochim. Cosmochim. Acta 74: 3984- 4004. Stramma, L., G. C. Johnson, J. Sprintall, and V. Mohrholz. 2008. Expanding oxygen-minimum zones in the tropical oceans. Science 320: 655-658. Straub, K. L., M. Benz, B. Schink, and F. Widdel. 1996. Anaerobic, nitrate-dependent microbial oxidation of ferrous iron. Appl. Environ. Microbiol. 62: 1458-1460. Strickland, J. D., and T. R. Parsons. 1968. A practical handbook of seawater analysis. Fisheries Research Board of Canada Ottawa. Sunda, W. G., and S. A. Huntsman. 1997. Interrelated influence of iron, light and cell size on marine phytoplankton growth. Nature 390: 389-392. Thamdrup, B., T. Dalsgaard, and N. P. Revsbech. 2012. Widespread functional anoxia in the oxygen minimum zone of the Eastern South Pacific. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 65: 36-45. 108 Fig.2-1. Map of the cruise track off coastal Peru in the Eastern Tropical South Pacific Ocean that was sampled in October-November 2005 aboard the R/V Knorr. 109 Fig.2-2. Surface distributions of (A) salinity; (B) temperature and (C) oxygen at our sampling region off the coast of Peru along three transects sampled in October-November 2005. C. A. B. 110 Fig.2-3. Depth profiles of temperature (red line) and salinity (blue line) for stations (A) 24, (B) 26, (C) 27, & (D) 29 in Transect 1 of KN-182-09 cruise. Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 100 200 300 400 500 Salinity (ppt) 34.6 34.8 35.0 35.2 35.4 T S Station 24 Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.6 34.8 35.0 35.2 35.4 Station 26 T S Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.6 34.8 35.0 35.2 35.4 T S Station 27 Station 29 Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 50 100 150 200 Salinity (ppt) 34.6 34.8 35.0 35.2 35.4 T S A. B. C. D. 111 Fig.2-4. Depth profiles of temperature (red line) and salinity (blue line) for stations (A) 19, (B) 21, and (C) 22 in Transect 2 of KN-182-09 cruise. Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.6 34.8 35.0 35.2 35.4 T S Station 19 Temperature ( o C) 9 10 11 12 13 14 15 16 Depth (m) 0 100 200 300 400 500 Salinity (ppt) 34.6 34.8 35.0 35.2 35.4 Station 21 T S Temperature ( o C) 11 12 13 14 15 16 Depth (m) 0 50 100 150 200 250 Salinity (ppt) 34.8 34.9 35.0 35.1 35.2 Station 22 T S A. B. C. 112 Fig.2-5. Depth profiles of temperature (red line) and salinity (blue line) for stations (A) 9, (B) 10, (C) 11, (D) 12 & (E) 14 in Transect 3 of KN-182-09 cruise. Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.4 34.6 34.8 35.0 35.2 Station 9 T S Temperature ( o C) 6 8 10 12 14 16 Depth (m) 0 100 200 300 400 500 Salinity (ppt) 34.6 34.7 34.8 34.9 T S Station 10 Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.4 34.6 34.8 35.0 35.2 Station 11 T S Temperature ( o C) 2 4 6 8 10 12 14 16 18 Depth (m) 0 200 400 600 800 1000 Salinity (ppt) 34.4 34.6 34.8 35.0 35.2 Station 12 T S Temperature ( o C) 6 8 10 12 14 16 18 Depth (m) 0 100 200 300 400 500 Salinity (ppt) 34.6 34.7 34.8 34.9 35.0 35.1 Station 14 T S A. B. C. D. E. 113 Fig.2-6. Depth profiles of total dissolved Fe (closed circles), Fe (II) (open circles), nitrite (closed triangles) and total dissolved oxygen (open triangles) for station 24 (A & C), and station 26 (B & D) along Transect 1 of KN-182-09 cruise. Error bars for total dissolved [Fe] represent error propagation from calculation of standard deviation values (n=3). Station 26 Nitrite (µmol L -1 ) 0 2 4 6 8 0 200 400 600 800 1000 Dissolved Oxygen (µmol L -1 ) 0 50 100 150 200 250 300 NO 2 - O 2 Station 26 Concentration (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Depth (m) 0 200 400 600 800 1000 DFe Fe (II) Station 24 Nitrite (µmol L -1 ) 0 1 2 3 4 5 6 0 200 400 600 800 1000 Dissolved Oxygen (µmol L -1 ) 0 50 100 150 200 250 NO 2 - O 2 Station 24 Concentration (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Depth (m) 0 200 400 600 800 1000 DFe Fe (II) A. B. D. C. 114 Fig.2-7. Depth profiles of total dissolved Fe (closed circles), Fe (II) (open circles), nitrite (closed triangles) and total dissolved oxygen (open triangles) for station 27 (A & C), and station 29 (B & D) in Transect 1 of KN-182-09 cruise. Error bars for total dissolved [Fe] represent error propagation from calculation of standard deviation values (n=3). Station 29 Nitrite (µmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 Depth (m) 0 50 100 150 200 Dissolved Oxygen (µmol L -1 ) 0 50 100 150 200 250 NO 2 - O 2 Station 29 Concentration (nmol L -1 ) 0 20 40 60 80 100 Depth (m) 0 50 100 150 200 DFe Fe (II) Station 27 Nitrite (µmol L -1 ) 0 1 2 3 4 5 0 200 400 600 800 1000 Dissolved Oxygen (µmol L -1 ) 0 50 100 150 200 250 NO 2 - O 2 Station 27 Concentration (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Depth (m) 0 200 400 600 800 1000 DFe Fe (II) A. B. D. C. 115 Fig.2-8. Depth profiles of total dissolved Fe (closed circles), Fe (II) (open circles), nitrite (closed triangles) and total dissolved oxygen (open triangles) for station 19 (A & B) along Transect 2 of KN-182-09 cruise. Error bars for total dissolved [Fe] represent error propagation from calculation of standard deviation values (n=3). Station 19 Nitrite (µmol L -1 ) 0 1 2 3 4 5 6 0 200 400 600 800 1000 Dissolved Oxygen (µmol L -1 ) 0 50 100 150 200 250 NO 2 - O 2 Station 19 Concentration (nmol L -1 ) 0 1 2 3 4 Depth (m) 0 200 400 600 800 1000 DFe Fe (II) A. B. 116 Fig.2-9. Depth profiles of total dissolved Fe (closed circles), Fe(II) (open circles), nitrite (closed triangles) and total dissolved oxygen (open triangles) for station 22 (A & C), and station 23 (B & D) along Transect 2 of KN-182-09 cruise. Error bars for total dissolved [Fe] represent error propagation from calculation of standard deviation values (n=3). Nitrite ( mol L -1 ) 0 1 2 3 4 5 0 50 100 150 200 250 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 NO 2 - O 2 Nitrite ( mol L -1 ) 0 1 2 3 4 5 0 20 40 60 80 100 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 Concentration (nmol L -1 ) 0 2 4 6 8 10 12 14 16 Depth (m) 0 50 100 150 200 250 DFe Fe (II) Concentration (nmol L -1 ) 0 10 20 30 40 Depth (m) 0 20 40 60 80 100 DFe Fe (II) Station 23 NO 2 - O2 Station 23 Station 22 Station 22 A. B. D. C. 117 Fig.2-10. Depth profiles of total dissolved Fe (closed circles), Fe (II) (open circles), nitrite (closed triangles) and total dissolved oxygen (open triangles) for stations for station 11 (A & C), and station 12 (B & D) along in Transect 3 of KN-182-09 cruise. Error bars for total dissolved [Fe] represent error propagation from calculation of standard deviation values (n=3). Station 12 Nitrite ( mol L -1 ) 0 2 4 6 8 10 0 200 400 600 800 1000 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 300 NO 2 - O 2 Station 11 Nitrite ( mol L -1 ) 0 2 4 6 8 10 0 200 400 600 800 1000 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 300 NO 2 - O 2 Station 12 Concentration (nmol L -1 ) 0 1 2 3 4 Depth (m) 0 200 400 600 800 1000 DFe Fe(II) Station 11 Concentration (nmol L -1 ) 0 1 2 3 4 Depth (m) 0 200 400 600 800 1000 DFe Fe(II) A. B. C. D. 118 Fig.2-11. Depth profiles of total dissolved Fe (closed circles), Fe(II) (open circles), nitrite (closed triangles) and total dissolved oxygen (open triangles) for station for station 9 (A & C), and station 10 (B & D) along Transect 3 of KN-182-09 cruise. Error bars for total dissolved [Fe] represent error propagation from calculation of standard deviation values (n=3). Station 10 Nitrite ( mol L -1 ) 0 1 2 3 4 5 6 7 0 100 200 300 400 500 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 300 NO 2 - O 2 Station 10 Concentration (nmol L -1 ) 0 1 2 3 4 5 6 Depth (m) 0 100 200 300 400 500 DFe Fe (II) Concentration (nmol L -1 ) 0 2 4 6 8 Depth (m) 0 200 400 600 800 DFe Fe (II) Station 9 Station 9 Nitrite ( mol L -1 ) 0 1 2 3 4 5 6 7 0 200 400 600 800 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 NO 2 - O 2 A. B. D. C. 119 Fig.2-12. Upper 1000m oceanographic sections of (A) total dissolved Fe,and (B) Fe (II) for Transects 1, 2 and 3 during the KN-182-09 cruise. A. B. 120 Chapter 3 Iron speciation at the Costa Rica Upwelling Dome 121 3.1. Abstract Vertical profiles of total dissolved iron (DFe), Fe (II) and hydrographic parameters were obtained at five stations at the Costa Rica Upwelling dome in June-July 2010. Fe (II) and DFe concentrations were determined using an automated flow injection analysis system (FIA, FeLume II Waterville Analytical) using luminol chemiluminescence (CL) and isotope dilution inductively coupled plasma mass spectrometry (ICP-MS), respectively. Of all the secondary nitrite maxima (SNM) features observed at different OMZs, this is the deepest SNM - Fe(II) feature (400 – 500 m) studied till date. Unlike in any other OMZs, slightly elevated Fe(II) concentrations (~ 0.1 – 0.2 nmol L -1 ) were found below the oxycline at all stations. A broad local DFe maximum was coincident with the SNM at all stations and ~10 – 20 % of the DFe concentration at the peak was present as Fe (II). This broad and local DFe maxima found deep in the water column could be attributed to lateral, advective sources. During this study, the Fe(II) maxima observed within the OMZ, either coincides or is shallower than the SNM and could be significant. This deep Fe (II)-SNM peak seems to be a persistent feature in the region especially since it was present during sampling in June and November 2005 and also during our study in June-July 2010. 122 3.2. Introduction The Costa Rica Upwelling Dome is an open ocean upwelling system in the eastern equatorial Pacific which is also an HNLC (Hutchins et al. 2002), where growth of large autotrophs is strongly limited by low levels of dissolved Fe (Coale et al. 1996; Landry et al. 2000). Usually, upwelling regimes are dominated by fast-growing eukaryotic phytoplankton such as diatoms, rather than by cyanobacteria. However, in the Costa Rica dome, Synechococcus dominate surface waters and are present in concentrations much higher than observed in other open ocean environments (Campbell et al. 1994; Durand et al. 2001; Goericke and Welschmeyer 1993; Saito et al. 2005). This type of enhanced productivity is unusual for the cyanobacteria in marine waters since Synechococcus is mostly associated with microbial loop ecosystems characterized by tight recycling and regeneration in oligotrophic gyres. (Saito et al. 2005) have suggested that smaller aeolian iron inputs, too less to induce such a major shift in phytoplankton assemblage, may nonetheless stimulate new production by enhancing Synechococcus growth in such open ocean upwelling systems. The primary source of iron to the iron limited, open ocean regions of the eastern tropical Pacific is dust fallout transported at high altitude from Asia (Duce and Tindale 1991; Meskhidze et al. 2003).Recent study by (Kaupp et al. 2011) have determined that the upwelling of the upper part of the Equatorial Undercurrent (EUC) is the main source of Fe to the surface waters of this region, since eolian deposition is minimal. Now since iron plays a central role in photosynthesis and nitrogen assimilation, especially in marine photoautotrophs, it is more important in controlling rates of metabolism and growth than cell yields (Sunda and Huntsman 1997). Thus, the presence of iron is critical to the growth and metabolism of marine phytoplankton and even the presence of small amounts of Fe in the system creates a lot of interest. 123 Fe (III) is the thermodynamically favored form of Fe in oxygenated seawater. At pH 8.6, Fe (III) is strongly hydrolyzed as compared to Fe (II) and oxidation rates of Fe (II) are high. Considerable interest is exhibited to decipher non-equilibrium reactions that work on this large pool of Fe leading to accumulation of the dissolved, reduced, more bioavailable form of Fe – Fe (II). Previous studies in different OMZs have noted the presence of a distinct Fe(II) plume associated with the secondary nitrite maximum at the core of oxygen OMZs (Kondo and Moffett 2013; Moffett et al. 2007). However, this is the first ever attempt to study the speciation of Fe in an open ocean upwelling system which is away from any direct continental shelf influence and is dominated by an assemblage of Synechococcus, found in mostly oligotrophic regimes. Previously, we have studied the speciation of Fe in eastern boundary upwelling zones such as Peruvian upwelling zone and the Arabian Sea OMZ, which are characterized by the presence of fast growing eukaryotic phytoplankton blooms. These eastern boundary upwelling regimes are under a strong influence of the continental shelf and large inputs of Fe from the continental shelf have been measured (J. Vedamati; in prep) from the reducing sediments underlying the shelf. A distinct Fe (II) maximum coincident with the DFe maximum associated with the SNM is observed in all these regimes. In contrast to these eastern boundary upwelling regimes, Costa Rica Upwelling dome is an low productivity, open ocean upwelling regime (Pennington et al. 2006) without a dominant influence of adjacent continental shelves and characterized by the presence of an OMZ rather deep (400-500 m) in the water column. Thus, it will be interesting to characterize the differences in Fe distribution and speciation within this unique open ocean upwelling system and to see if a distinct Fe(II) maxima, like in other OMZs, is associated with the SNM. The primary goal of this cruise to the Costa Rica Upwelling Dome was to characterize biological process studies at the dome and we utilized it as a cruise of opportunity to study Fe 124 speciation at this unique upwelling regime. A Lagrangian mode of sampling was undertaken and a surface water mass was followed for five days, each known as a cycle. All samples for our study were collected over these five days while following the surface water mass. 3.2.1. Study Area The Costa Rica Dome is a permanent, tropical thermocline dome, about 300-500 km in diameter located at the terminus of the 10 o N west to east thermocline ridge (Fiedler 2002; Kessler 2006; Wyrtki 1964). This feature forms between the westward North Equatorial Current and the eastward North Equatorial Counter Current (NECC) and appears to be generated in winter by the wind stress curl (Papagayo wind jet and westerly remnants of the SE trades) while it is maintained during summer by the left-handed turning of the NECC as it reaches the Central American coast to become the Costa Rica Coastal Current. These cyclonic current conditions result in an offshore doming of the thermocline (Fiedler 2002; Kessler 2002; Kessler 2006). Thus, nutrient rich thermocline waters lie close to the surface of the dome and wind mixing and/or upwelling brings high nutrients to the surface resulting in high primary productivity at the dome (Fiedler and Talley 2006; King 1986). Increased salinity, high nutrient concentrations and low temperature waters signify upwelling at the Costa Rica Dome (Wyrtki 1964). It is known that high phytoplankton production at the surface coupled with the presence of a sharp pycnocline along with sluggish deep circulation results in the extreme oxygen deficiency of the eastern tropical Pacific oxygen minimum zone (Fiedler and Talley 2006). The depletion of oxygen at intermediate depths in the water column occurs due to the consumption of oxygen during remineralisation of organic matter combined with the sluggish water transport. After the depletion of oxygen in the water column, nitrate becomes the primary electron acceptor and thus, denitrification becomes the primary respiratory 125 pathway for facultative bacteria. The intermediate oxygen minimum layer is primarily characterized by the presence of a secondary nitrite maximum (SNM) formed due to the dissimilatory reduction of NO 3 - (Codispoti et al. 1986). 3.3. Methods 3.3.1. Sample collection Samples on the MV1008 cruise during June-July 2010 were collected aboard the R/V Melville. Sampling was concentrated on five cycles (lasting for five days each) within and outside the dome using a Lagrangian sampling mode following surface water mass (Fig. 3-1). Seawater samples for Fe (II) and total dissolved Fe analysis were collected in 5L Niskin bottles with Teflon coated interior surfaces (Ocean Test Equipment) mounted on a trace metal clean rosette (Sea-Bird Electronics). Samples were acidified to pH ~1.7 and stored for analysis in the lab. All hydrographic data during this cruise were obtained using a CTD-O 2 probe (Seabird). 3.2.2. Cleaning protocol The 500 ml LDPE (VWR) sampling bottles were thoroughly cleaned in a sequential four-step process that consisted of soaking them in a 5% Citranox acid detergent bath (Alconox) for at least a day followed by another overnight soak in a 10% hydrochloric acid bath (VWR). They were then filled with 10% HCl and baked at 60°C for at least 2 days and finally, filled with 0.1% trace metal grade HCl (Optima, Fisher) and baked at 60°C again for another 2 days. The insides and outsides of the bottles were thoroughly rinsed at least five times with Milli-Q water (18.2 MΩ; Millipore) in between each step. Samples were prepared in 15 mL polypropylene centrifuge tubes (VWR) which were first cleaned in a two-step process by soaking them in 10% HCl at 60°C for 48 hours and then, rinsing each tube at least five times with Milli-Q water. After the rinses, the tubes were filled to a 126 positive meniscus with 0.5% trace metal grade HCl, capped and then baked at 60°C overnight. After retrieving them from the oven, the tubes were left capped & stored until further use. Upon analysis, the tubes were emptied and rinsed three times with Milli-Q water and at least once with the sample. In order to minimize contamination from the beads and before addition to the samples, the NTA resin was cleaned using the following procedure (Lee et al. 2011). 25 ml of the NTA resin solution was poured into a clean 50 mL polypropylene centrifuge tube (Corning) and then washed five times with Milli-Q water. In between washes, the tube was spun down in a 5810-R centrifuge (Eppendorf) maintained at 8°C for 10 min at 4000 rpm. After decanting the supernatant, Milli-Q water was added for the next wash. The resin was then washed five times with 1.5 mol L -1 trace metal grade HCl (Optima, Fisher) and several more times with Milli-Q water after that to bring the pH of the solution above 4, to indicate that all of the HCl had been removed from the solution. For the final cleaning step, the resin solution was washed five times with 0.5 mol L -1 trace metal grade HNO 3 (Optima, Fisher). The resin solution was placed on an analog shaker for several hours for the first wash and then left overnight on the shaker for the last wash. After the final wash, the resin solution was again washed at least five times with Milli-Q water until the pH had risen above 4 in order to remove all of the HNO 3 . The resin solution was diluted twofold with 25 mL Milli-Q water and stored in the refrigerator for future use. 25 µL of the working resin suspension contains ~100 – 400 beads, which is 1:50 dilution of the primary resin solution. 3.3.3. Nitrite measurement Nitrite was measured spectrophotometrically (Strickland and Parsons 1968) by allowing small Acropak-filtered sample aliquots (15 mL or less) to react with an acidified sulfanilamide 127 solution to form a diazonium compound. The compound was then mixed with N-(1-Naphthyl)- ethylenediamine dihydrochloride to produce a colored azo dye, whose extinction was measured on a Shimadzu UV-1700 UV-VIS spectrophotometer to determine the nitrite concentration of each sample. 3.3.4. Measurement of Fe(II) The dissolved Fe(II) concentrations were determined using an automated flow injection analysis system (FIA, FeLume II Waterville Analytical) employing a luminol chemiluminescence (CL) based detection system as described by (King et al. 1995). The FeLume was fitted with a standard quartz flow cell and a Hamamatsu HC135 photon counter configured with the following settings: pump speed: 15 rpm; photon counter integration time: 200 milliseconds; load time: 20 seconds; No. of data pts.: 100. Samples were analyzed immediately after sampling to minimize potential oxidation by pressurizing the Niskin bottles with N 2 and the time lag between retrieving the sampling Niskin bottles and Fe(II) analysis was about 20 minutes. Briefly, an alkaline luminol reagent (~pH – 10.3) reacts with an Fe(II)-containing solution, resulting in luminol oxidation with concurrent chemiluminescent emission (Croot and Laan 2002; Rose and Waite 2001). At pH 10.3, Fe(II) is oxidized by oxygen on a millisecond time scale forming reaction intermediates that catalyze the oxidation of luminol and producing 426 nm of light. The mixing and reaction takes place in a Plexiglas spiral flow cell positioned in front of a photomultiplier tube. The sample and luminol reagent is continually mixed in the flow cell by omitting the injection valve as described in (Hopkinson and Barbeau 2007; Rose and Waite 2001; Roy et al. 2008). Lab View software (National Instruments) controls the loading and injection of the sample and luminol into the flow cell while the intensity of the luminescence is recorded as the sample passes through the reaction 128 coil. The quantification of the signal is done by measuring the height of the signal peak. The need for sample concentration is avoided as this method generates intense peaks that enable highly sensitive Fe (II) determinations at ambient seawater pH. 3.3.5. Total dissolved Fe (DFe) measurement After sampling and before analysis, all samples were acidified to below pH 2 by the addition of concentrated trace metal grade HCl (Optima, Fisher) and stored for at least a month. All samples were analyzed in triplicate using the Finnegan Element 2 (Thermo Scientific) Inductively Coupled Plasma Mass Spectrophotometer (ICP-MS) on a medium resolution mode. The total dissolved Fe concentrations were determined using a single batch nitrilotriacetatic acid (NTA) resin extraction and isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) method adapted from Lee et al. (2011). Dissolved Fe was pre-concentrated in the samples by adding a chelating resin - NTA Superflow resin (Qiagen) in the preparatory stage. 15 ml centrifuge tubes were filled with ~7.5 mL of sample (with the exact volume determined gravimetrically) and spiked with enough 57 Fe-enriched spike (BDH Aristar Plus, VWR) to bring the final concentration to ~2 nmol L -1 . 0.1 mL of 1.5 mol L -1 trace metal grade hydrogen peroxide (H 2 O 2 ; Optima, Fisher) was then added to each sample and left to equilibrate for at least an hour at room temperature, to completely oxidize any Fe 2+ to Fe 3+ (Lee et al. 2011). Next, 200 µL (~800 beads) of the working resin suspension was added to each sample, and the tubes were placed on a shaker for two to three days. The samples were then centrifuged for 10 min at 4000 rpm, and the seawater was carefully siphoned off to leave only the resin beads at the bottom. The beads were washed twice with 3 mL Milli-Q water to remove salts and the tubes were once again centrifuged using the same settings. After the final wash, 1 mL of 5% trace 129 metal grade HNO 3 (Optima, Fisher) was added to each tube and, after leaving them on the shaker again for one day, the samples were ready for analysis. Procedural seawater blanks were prepared in triplicates, in the same way as samples using ~0.2 mL low trace metal surface seawater from the 2004 SAFe cruise ([Fe] = 0.09 ± 0.007 nmol L -1 ). The average detection limit and internal blank value for this method (n=3) for Fe was 0.01 nmol L -1 and 0.06 nmol L -1 , respectively. The accuracy of the method was evaluated by measuring SAFe reference standards S1 and D1 (Johnson et al. 2007). The Fe values obtained by this method for S1 and D1 were 0.094 ± 0.005 nmol L -1 and 0.645 ± 0.020 nmol L -1 , respectively. The certified consensus values were 0.090 ± 0.007 nmol L -1 Fe (S1) and 0.67 ± 0.07 nmol L -1 Fe (D1). These values are within the range of the latest consensus numbers (http://www.geotraces.org/science/intercalibration). 3.4. Results Lagrangian water sampling during MV1008 was done at five cycles (Fig. 3-1), each cycle lasting for about 5 days. Each cycle was sampled for DFe, Fe(II) and nitrite along with hydrographic parameters such as oxygen along with temperature and salinity (Fig. 3-2). 3.4.1. Cycle 1 Fe(II) during Cycle 1(Fig. 3-3) ranged from 0.02 – 0.06 nmol L -1 throughout all depths, with the highest concentrations measured slightly above the broad SNM (ranging between 0.8-0.9 µmol L -1 ). The DFe concentrations throughout the water column spanned between 0.04 – 1.6 nmol L -1 , with slightly elevated surface values (0.17 nmol L -1 ). Lowest DFe (0.04 nmol L -1 ) concentrations were measured coinciding with the PNM at 40m, drawn down most likely due to phytoplankton uptake (Fig. 3-3B). Below 40m, the DFe concentrations gradually increase resulting in a maximum (1.26 nmol L -1 ) at 265 m coinciding with that of the Fe (II) peak (0.57 130 nmol L -1 ).At this depth, Fe(II) constitutes almost 85% of the DFe. The DFe concentrations remain elevated even at deeper depths without a corresponding Fe(II) peak, thereby hinting towards an external source of DFe to the region (Fig. 3-3A). 3.4.2. Cycle 2 Average Fe(II) concentrations measured during this cycle (Fig. 3-4) were around 0.1 nmol L - 1 , with a relatively smaller variability within the water column than in cycle 1. Fe(II) value remains elevated throughout the water column without the presence of a distinct Fe(II) peak. However, a greater variability in DFe concentrations was observed (0.09 -1.56 nmol L -1 ), with the lowest values at the surface. Both Fe(II) and DFe (0.03 and 0.1 nmol L -1 , respectively) exhibit a minima coinciding with the PNM at 45m (Fig. 3-4B). Highest DFe values seemed to be associated with the SNM around 450m. Almost 100% of the DFe at the surface consisted of Fe(II), while Fe (II) constituted only ~ 8 -15% of DFe at deeper depths. 3.4.3. Cycle 3 During Cycle 3 (Fig. 3-5) , Fe(II) concentrations ranged between 0.02 – 0.3 nmol L -1 while DFe concentrations as low as 0.04 nmol L -1 was observed at the surface, which then increased gradually throughout the water column. Coincident with the PNM at 20m, lowest concentrations of Fe(II) and DFe (0.07 & 0.06 nmol L -1 respectively) were observed. Fe (II) values at the surface 20m are slightly higher than DFe concentrations which could be attributed towards error in calibration. No sharp peak in DFe was observed to be associated with the SNM (Fig. 3-5), although overall high values (~1.1 nmol L -1 ) were measured. However, a distinct Fe(II) peak (0.3 nmol L -1 ) coincides with the SNM at 400m, that constituted ~30% of the DFe (1.04 nmol L -1 ) at that depth. 131 3.4.4. Cycle 4 Small variability in Fe (II) concentrations during Cycle 4 (Fig. 3-6) (~ 0.05 - 0.22 nmol L-1) was observed throughout the water column, except at subsurface which has the lowest values (0.06 nmol L -1 ). Overall, Fe (II) concentrations remain elevated throughout the water column with small local peaks at several depths. It is possible that due to sparse sampling between 300 – 400 m, we might have missed the Fe(II) feature especially since it’s sharp in Cycle 3. The upper water column (15-25 m) Fe (II) concentration was higher than DFe. DFe concentrations range between 0.01 – 1.36 nmol L -1 , characterized by low surface value (0.01 nmol L -1 ) and a gradual increase below 50m. Instead of a sharp DFe peak, a broad elevation in DFe concentrations was coincident with the SNM around 300m. At the SNM, Fe(II) constitutes about 14-15 % of the DFe. 3.4.5. Cycle 5 Fe(II) concentrations during Cycle 5 (Fig. 3-7 )ranged between 0.03-0.23 nmol L -1 with the highest concentrations associated with the SNM peak at ~400m. At 12m depth, Fe (II) concentrations (0.19 nmol L -1 ) were higher than the observed DFe (0.05 nmol L -1 ). DFe values range from 0.05 – 1.23 nmol L -1 throughout the water column, with DFe concentrations gradually increasing below 70m. Highest DFe concentrations (1.4 nmol L -1 ) were associated with the SNM and Fe (II) constituted about 16-20% of the DFe. Highest nitrite concentration (3.9 µmol L -1 ) was observed at 150 m during this cycle, which was however not associated with any corresponding Fe (II) or DFe peak. 3.4.6. Station 11 DFe at Station 11 (Fig. 3-8) sampled during June 2005 ranged between 0.07 – 1.27 nmol L -1 while Fe(II) ranged between 0.2 – 0.6 nmol L -1 . An Fe (II) maxima (0.6 nmol L -1 ) was coincident 132 with a maxima in DFe (1.27 nmol L -1 ) at 360 m. Fe(II) concentrations remain elevated (~0.2 nmol L -1 ) within 100 – 600 m depth, very similar to that observed in stations sampled during June 2010. 3.4.7. Station 39 DFe at Station 39 (Fig. 3-9) during PEZ 2005 ranged between 0.12 – 1.49 nmol L -1 while Fe(II) ranged between 0.05 – 0.38 nmol L -1 . A PNM associated with draw down in DFe and Fe(II) was present at 30m. A deep SNM was present at 450m coinciding with an elevation in DFe values along with a small peak in Fe (II).At this depth, almost 25% of the DFe was present as Fe(II). 3.5. Discussion 3.5.1. Surface Fe (II) maxima There was a distinct surface Fe (II) maxima present at all cycles. However, at Cycles 2, 3, 4, and 5, excess [Fe (II)] over [DFe] was observed at the surface. These results are similar to those obtained by (Kondo and Moffett 2013) who ascribed this apparent excess to light dependent processes that could have led to this artifact in Fe(II) measurement. Similar to the protocol used by them, prior to analysis, dissolved Fe samples were acidified to pH <1.8 in order to leach the colloidal and/or strongly complexed fractions of Fe (Johnson et al. 2007) in accordance with protocols established in several international intercalibration exercises (Cutter et al. 2010). Therefore, this excess Fe (II) over DFe suggests that another photochemically-produced species, is contributing to the chemiluminescence signal in the surface waters. 3.5.2. Fe (II) within the OMZ A distinct Fe (II) maximum coincident with the DFe peak at the OMZ core, similar to those observed at other OMZs (Kondo and Moffett 2013; Moffett et al. 2007), was observed. This 133 could be attributed to metabolic remineralisation coupled with in-situ reduction processes occurring within the water column. However, unlike in other OMZs, the Fe (II) peak was placed somewhat shallower than the SNM and might have significant reasons/ implications. Sharp Fe (II) maxima of ~ 0.2 - 0.6 nmol L -1 are observed at stations outside the dome (Cycles 1, 3 & 5). Maximum Fe (II) concentration (0.5 nmol L -1 ) was coincident with the SNM is at Cycle 1, which is closest to the shore. Away from the shore, it decreases to about 0.2-0.3 nmol L -1 at the SNM peak. DFe profiles do not show much variability among cycles. There’s a similar pattern of uptake at the surface and increase below the euphotic zone. Similar data was obtained at Station 11 (sampled in June 2005) and at Station 39 (sampled in November 2005) using similar Fe(II) measurement methods and thus, provides us with further confidence regarding measurement precision of Fe(II) and DFe in these samples. Within the upwelling dome (Cycles 2 and 4), no sharp Fe (II) peak was observed although Fe(II) values remain elevated throughout the water column (~ 0.1 – 0.2 nmol L -1 ). This was similar to that observed at Station 39 sampled within the dome in June 2005. This could be ascribed to the tight recycling and regeneration of Synechococcus bloom at the surface resulting in lower carbon export into deeper waters, typical of oligotrophic systems. Thus, rapid recycling of organic matter in the surface waters could lead to low remineralisation rates in the subsurface mid-depth waters. Furthermore, rapid scavenging on dissolved Fe in particles through the water column could lead to substantial depletion in dissolved Fe concentrations as it reaches the OMZ. 3.5.3. Fe (II): DFe ratios Metabolic remineralisation (Hopkinson and Barbeau 2007; Moffett et al. 2007) along with advective transport from adjacent shelf sediments (Hong and Kester 1986; Landing and Bruland 1987) have been attributed to the formation of the dissolved Fe peak observed in the OMZs. 134 Previous results from the Arabian Sea OMZ (Kondo and Moffett 2013) has also confirmed that a close relationship exists between Fe(II) and the secondary nitrite maxima, thereby suggesting that the rich microbial community within this feature is closely involved with Fe redox cycling. Thus, in this Costa Rica upwelling dome, an open ocean upwelling regime away from the influence of continental shelves, metabolic remineralisation associated with in-situ reducing processes seems to be the most plausible reason of elevated Fe (II) throughout the water column. Recent investigation of Fe sources to the open ocean equatorial Pacific regions indicate that upwelling of EUC is the primary source of Fe into the region (Kaupp et al. 2011) and low productivity is due to Fe limitation (Coale et al. 1996; Landry et al. 2000; Pennington et al. 2006). Thus, ratios of Fe (II): DFe at the Fe (II) maxima peak associated with the SNM could indicate strength of the microbial community within the OMZ. It is now known that denitrification in the tropical North and South Pacific OMZs appears to be limited by organic carbon (Ward et al. 2008). Thus, if the main source of Fe(II) is the regeneration of the exported organic matter by the denitrifying bacteria, less [Fe(II)] would be produced in the deep OMZ in the Synechococcus dominated Costa Rica Upwelling dome. It is now known that cells such as Synechococcus are favored under iron limitation,which are subject to intense grazing by microzooplankton thereby preventing blooms (Rocap et al. 2003). This picophytoplankton bloom forms a tight recycling system which prevents export of organic carbon into the deep ocean. These and other factors interact to maintain HNLC conditions in the Costa Rica Upwelling dome which prevents carbon export into deeper waters and subsequent limitation of denitrification rates. 135 3.6. References Campbell, L., H. A. Nolla, and D. Vaulot. 1994. THE IMPORTANCE OF PROCHLOROCOCCUS TO COMMUNITY STRUCTURE IN THE CENTRAL NORTH PACIFIC-OCEAN. Limnol. Oceanogr. 39: 954-961. Coale, K. H. and others 1996. A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature 383: 495-501. Codispoti, L. A. and others 1986. HIGH NITRITE LEVELS OFF NORTHERN PERU - A SIGNAL OF INSTABILITY IN THE MARINE DENITRIFICATION RATE. Science 233: 1200-1202. Croot, P. L., and P. Laan. 2002. Continuous shipboard determination of Fe(II) in polar waters using flow injection analysis with chemiluminescence detection. Anal. Chim. Acta 466: 261-273. Cutter, G. and others 2010. Sampling and sample-handling protocols for GEOTRACES Cruises. Tech. rep., GEOTRACES. Duce, R. A., and N. W. Tindale. 1991. Atmospheric transport of iron and its deposition in the ocean. Limnology and Oceanography;(United States) 36. Durand, M. D., R. J. Olson, and S. W. Chisholm. 2001. Phytoplankton population dynamics at the Bermuda Atlantic Time-series station in the Sargasso Sea. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 48: 1983-2003. Fiedler, P. C. 2002. The annual cycle and biological effects of the Costa Rica Dome. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 49: 321-338. Fiedler, P. C., and L. D. Talley. 2006. Hydrography of the eastern tropical Pacific: A review. Prog. Oceanogr. 69: 143-180. 136 Goericke, R., and N. A. Welschmeyer. 1993. THE MARINE PROCHLOROPHYTE PROCHLOROCOCCUS CONTRIBUTES SIGNIFICANTLY TO PHYTOPLANKTON BIOMASS AND PRIMARY PRODUCTION IN THE SARGASSO SEA. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 40: 2283-2294. Hong, H. S., and D. R. Kester. 1986. REDOX STATE OF IRON IN THE OFFSHORE WATERS OF PERU. Limnol. Oceanogr. 31: 512-524. Hopkinson, B. M., and K. A. Barbeau. 2007. Organic and redox speciation of iron in the eastern tropical North Pacific suboxic zone. Mar. Chem. 106: 2-17. Hutchins, D. A. and others 2002. Phytoplankton iron limitation in the Humboldt Current and Peru Upwelling. Limnol. Oceanogr. 47: 997-1011. Johnson, K. S. and others 2007. Developing standards for dissolved iron in seawater. Eos, Transactions American Geophysical Union 88: 131-132. Kaupp, L. J., C. I. Measures, K. E. Selph, and F. T. Mackenzie. 2011. The distribution of dissolved Fe and Al in the upper waters of the Eastern Equatorial Pacific. Deep Sea Research Part II: Topical Studies in Oceanography 58: 296-310. Kessler, W. S. 2002. Mean three-dimensional circulation in the northeast tropical Pacific. J. Phys. Oceanogr. 32: 2457-2471. ---. 2006. The circulation of the eastern tropical Pacific: A review. Prog. Oceanogr. 69: 181-217. King, D. W., H. A. Lounsbury, and F. J. Millero. 1995. RATES AND MECHANISM OF FE(II) OXIDATION AT NANOMOLAR TOTAL IRON CONCENTRATIONS. Environ. Sci. Technol. 29: 818-824. King, F. D. 1986. THE DEPENDENCE OF PRIMARY PRODUCTION IN THE MIXED LAYER OF THE EASTERN TROPICAL PACIFIC ON THE VERTICAL 137 TRANSPORT OF NITRATE. Deep-Sea Research Part a-Oceanographic Research Papers 33: 733-754. Kondo, Y., and J. W. Moffett. 2013. Dissolved Fe(II) in the Arabian Sea oxygen minimum zone and western tropical Indian Ocean during the inter-monsoon period. Deep Sea Research Part I: Oceanographic Research Papers 73: 73-83. Landing, W. M., and K. W. Bruland. 1987. THE CONTRASTING BIOGEOCHEMISTRY OF IRON AND MANGANESE IN THE PACIFIC-OCEAN. Geochim. Cosmochim. Acta 51: 29-43. Landry, M., J. Constantinou, M. Latasa, S. Brown, R. Bidigare, and M. Ondrusek. 2000. Biological response to iron fertilization in the eastern equatorial Pacific (IronEx II). III. Dynamics of phytoplankton growth and microzooplankton grazing. Mar. Ecol.-Prog. Ser. 201: 453. Lee, J. M., E. A. Boyle, Y. Echegoyen-Sanz, J. N. Fitzsimmons, R. F. Zhang, and R. A. Kayser. 2011. Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batch nitrilotriacetate resin extraction and isotope dilution inductively coupled plasma mass spectrometry. Anal. Chim. Acta 686: 93-101. Meskhidze, N., W. Chameides, A. Nenes, and G. Chen. 2003. Iron mobilization in mineral dust: Can anthropogenic SO2 emissions affect ocean productivity? Geophys. Res. Lett. 30: 2085. Moffett, J. W., T. J. Goeffert, and S. W. A. Naqvi. 2007. Reduced iron associated with secondary nitrite maxima in the Arabian Sea. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 54: 1341- 1349. 138 Pennington, J. T., K. L. Mahoney, V. S. Kuwahara, D. D. Kolber, R. Calienes, and F. P. Chavez. 2006. Primary production in the eastern tropical Pacific: A review. Prog. Oceanogr. 69: 285-317. Rocap, G. and others 2003. Genome divergence in two Prochlorococcus ecotypes reflects oceanic niche differentiation. Nature 424: 1042-1047. Rose, A. L., and T. D. Waite. 2001. Chemiluminescence of luminol in the presence of iron(II) and oxygen: Oxidation mechanism and implications for its analytical use. Anal. Chem. 73: 5909-5920. Roy, E. G., M. L. Wells, and D. W. King. 2008. Persistence of iron(II) in surface waters of the western subarctic Pacific. Limnol. Oceanogr. 53: 89-98. Saito, M. A., G. Rocap, and J. W. Moffett. 2005. Production of cobalt binding ligands in a Synechococcus feature at the Costa Rica upwelling dome. Limnol. Oceanogr. 50: 279- 290. Strickland, J. D., and T. R. Parsons. 1968. A practical handbook of seawater analysis. Fisheries Research Board of Canada Ottawa. Sunda, W. G., and S. A. Huntsman. 1997. Interrelated influence of iron, light and cell size on marine phytoplankton growth. Nature 390: 389-392. Ward, B. B., C. B. Tuit, A. Jayakumar, J. J. Rich, J. Moffett, and S. W. A. Naqvi. 2008. Organic carbon, and not copper, controls denitrification in oxygen minimum zones of the ocean. Deep Sea Research Part I: Oceanographic Research Papers 55: 1672-1683. Wyrtki, K. 1964. The thermal structure of the eastern Pacific Ocean. Deutsches Hydrographisches Institut. 139 Fig.3-1. Map of the cruise track in the Costa Rica Upwelling Dome that was sampled during June – July 2010 aboard the R/V Melville (MV1008). 140 Fig.3-2. Depth profiles of temperature (red line) and salinity (blue line) for Cycles (A) 1, (B) 2, (C) 3, (D) 4, & (E) 5 during the Costa Rica Upwelling Dome cruise in 2010. Cycle 1 Temperature ( o C) 5 10 15 20 25 30 Depth (m) 0 100 200 300 400 500 Salinity (psu) 33.0 33.5 34.0 34.5 35.0 T S Cycle 2 Temperature ( o C) 5 10 15 20 25 30 0 100 200 300 400 500 Salinity (psu) 33.0 33.5 34.0 34.5 35.0 35.5 36.0 T S Temperature ( o C) 5 10 15 20 25 30 0 100 200 300 400 500 Salinity (psu) 33.0 33.5 34.0 34.5 35.0 T S Cycle 3 Temperature ( o C) 5 10 15 20 25 30 Depth (m) 0 100 200 300 400 500 Salinity (psu) 33.0 33.5 34.0 34.5 35.0 T S Cycle 4 Temperature ( o C) 5 10 15 20 25 30 0 100 200 300 400 500 Salinity (psu) 33.0 33.5 34.0 34.5 35.0 T S Cycle 5 A. B. C. D. E. 141 Fig.3-3. Depth profiles of (A) total dissolved Fe (closed circles) & Fe (II) (open circles) and (B) nitrite (closed triangle) & oxygen (dotted line) Cycle 1 in the Costa Rica Upwelling dome sampled in June-July 2010. Error bars for total dissolved [Fe] represent error propagation from the calculation of standard deviation values (n=3). Fe (II) (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 Depth (m) 0 200 400 600 Dissolved Fe(nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Nitrite ( mol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Depth (m) 0 200 400 600 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 Fe (II) Dissolved Fe Cycle 1 NO 2 - O 2 Cycle 1 B. A. B. 142 Fig.3-4. Depth profiles of (A) total dissolved Fe (closed circles) & Fe (II) (open circles) and (B) nitrite (closed triangles) & oxygen (dotted line) Cycle 2 in the Costa Rica Upwelling dome sampled in June-July 2010. Error bars for total dissolved [Fe] represent error propagation from the calculation of standard deviation values (n=3). Nitrite ( mol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Depth (m) 0 200 400 600 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 Dissolved Fe(nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Depth (m) 0 200 400 600 Fe(II) (nmol L -1 ) 0.0 0.1 0.2 0.3 0.4 Dissolved Fe Fe (II) Cycle 2 Cycle 2 NO 2 - O 2 A. B. 143 Fig.3-5. Depth profiles of (A) total dissolved Fe (closed circles) & Fe (II) (open circles) and (B) nitrite (closed triangles) & oxygen (dotted line) Cycle 3 in the Costa Rica Upwelling dome sampled in June-July 2010. Error bars for total dissolved [Fe] represent error propagation from the calculation of standard deviation values (n=3). Dissolved Fe (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 200 400 600 Fe (II) (nmol L -1 ) 0.0 0.1 0.2 0.3 0.4 Nitrite ( mol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Depth (m) 0 200 400 600 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 Cycle 3 Dissolved Fe Fe(II) Cycle 3 NO 2 - O 2 A. B. 144 Fig.3-6. Depth profiles of (A) total dissolved Fe (closed circles) & Fe (II) (open circles) and (B) nitrite (closed triangles) & oxygen (dotted line) Cycle 4 in the Costa Rica Upwelling dome sampled in June-July 2010. Error bars for total dissolved [Fe] represent error propagation from the calculation of standard deviation values (n=3). Dissolved Fe (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 200 400 600 Fe (II) (nmol L -1 ) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Nitrite ( mol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Depth (m) 0 200 400 600 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 Dissolved Fe Fe(II) Cycle 4 Cycle 4 NO 2 - O 2 A. B. 145 Fig.3-7. Depth profiles of (A) total dissolved Fe (closed circles) & Fe (II) (open circles) and (B) nitrite (closed triangles) & oxygen (dotted line) Cycle 5 in the Costa Rica Upwelling dome sampled in June-July 2010. Error bars for total dissolved [Fe] represent error propagation from the calculation of standard deviation values (n=3). Dissolved Fe (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 200 400 600 Fe (II) (nmol L -1 ) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Nitrite ( mol L -1 ) 0 1 2 3 4 5 Depth (m) 0 200 400 600 Dissolved Oxygen ( mol L -1 ) 0 50 100 150 200 250 Dissolved Fe Fe (II) Cycle 5 Cycle 5 NO 2 - O 2 A. B. 146 Fig.3-8. Depth profiles of total dissolved Fe (closed circles), Fe (II) (open circles) and nitrite (closed triangle) for Station 11 in the Costa Rica Upwelling dome sampled in June 2005. Error bars for total dissolved [Fe] represent error propagation from the calculation of standard deviation values (n=3). Concentration (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Depth (m) 0 200 400 600 800 Total dissolved Fe Fe (II) Station 11 147 Fig.3-9. Depth profiles of total dissolved Fe (closed circles), Fe (II) (open circles) and nitrite (closed triangles) for Station 39 in the Costa Rica Upwelling dome sampled in November 2005. Error bars for total dissolved [Fe] represent error propagation from the calculation of standard deviation values (n=3). Station 39 Concentration (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Depth (m) 0 200 400 600 800 Nitrite ( mol L -1 ) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Total dissolved Fe Fe (II) NO 2 - 148 Chapter 4 Comparative behavior and distribution of copper in the Arabian Sea oxygen minimum zone and in the Eastern Tropical South Pacific oxygen minimum zone off of Peru. 149 4.1. Abstract Total dissolved copper (DCu) distributions were studied in an offshore transect across the Arabian Sea OMZ (September 2007) and along three near shore transects off the Peruvian coast (October 2005) to compare and assess the extent of the influence of oxygen minimum conditions combined with eastern boundary inputs from near shore reducing sediments. DCu concentrations were determined using isotope dilution inductively coupled plasma mass spectrometry (ICP- MS). Uniform DCu concentrations throughout the water column were measured in stations closer to the Peruvian coast, probably reflecting offshore transport from the shelf. Higher surface DCu concentrations over the southern narrow shelf (~ 1.2 nmol L -1 ) as compared to that over the northern broad Peruvian shelf ( ~ 0.97 nmol L -1 ) were attributed to the presence of copper smelters along the Peruvian and Chilean coasts that could impact particle concentrations in the aerosols, especially on the southern region of the Peruvian coast. Offshore and beyond the influence of the OMZ, the lowest DCu was within the primary nitrite maxima (PNM), where ammonia oxidation and nitrate reduction rates are important. In the transect through the Arabian Sea OMZ, a distinct draw down in DCu concentrations ( ~ 0.8 nmol L -1 ) was observed at mid- depths coincident with the secondary nitrite maximum (SNM) while no such feature was present in stations outside the denitrification zone. A similar feature was also observed in offshore stations along our three transects off the Peruvian OMZ. This finding is of particular interest because of the importance of Cu in several metalloenzymes involved in the N cycle especially nitrous oxide reduction during denitrification. The low concentrations of dissolved Cu coincident with the SNM here could, thus impose significant constraints on the rates of these OMZ processes, especially in regions which are known to have a disproportionately large impact on the biogeochemical cycling of nitrogen in the oceans. 150 4.2. Introduction Oxygen minimum zones (OMZs) are redox active, mid-depth oceanic regions formed due to a unique interplay of biogeochemical cycling and physical ocean ventilation, are characterized by high surface productivity along with high rates of subsurface remineralisation. Large horizontal and vertical gradients in the concentrations of redox reactive metals like Fe and Mn in these subsurface low oxygen regions (J. Vedamati; in prep) have also been reported recently. Furthermore, the presence of the secondary nitrite maximum (SNM) denotes intense denitrification by denitrifying microbial assemblages and thus, represents a very bioactive region where any change in the chemical behavior of bioactive trace metals could have huge impacts on the N-cycle. Few data about the behavior and distribution Cu in these redox active, upwelling regions (Boyle et al. 1977; Saager et al. 1992) along with J. Jacquot ;in press) are available, resulting in very limited understanding of the impact of OMZs on Cu distribution. Like other biologically relevant trace elements such as Fe and Mn, the distribution of Cu in the oceanic nutricline has been found to be regulated by phytoplankton uptake and its subsequent regeneration at depth (Sunda and Huntsman 1995). Several studies in both modern and ancient settings have also stated that Cu is enriched in reducing, organic-rich sediments and is brought to the marine sediments with settling biodetritus from surface waters, either adsorbed onto biodetrital surfaces or incorporated into soft tissue of plankton (Algeo and Maynard 2004; Brumsack 2006; Calvert and Price 1983). Work by Boning et al. (2004) also suggests that even though Cu is slightly elevated in the Peruvian sediments, it is most enriched in the sediments from within the OMZ and least on the upper edge. Furthermore, it has long been known that copper speciation is strongly influenced by the presence of small concentrations of sulfides and thiols, which can be found in oxygen-depleted 151 waters (Al-Farawati and Van Den Berg 1999; Laglera and Van Den Berg 2003; Luther et al. 1991). Saito et al. (2003) have also argued that Cu levels in hypoxic and anoxic regimes such as OMZs would be low due to the formation of sulfide precipitates. Recent work by Canfield et al. (2010) off the coast of northern Chile, also suggests that sulfide may be more abundant in OMZ waters than previously thought because of a heretofore unknown cryptic sulfur cycle, which might lead one to think that dissolved Cu concentrations should be correspondingly low. Theberge et al. (1997) also measured about 1 – 2 nmol L -1 of metal–sulfide complexes in the Arabian Sea OMZ, thereby implying that most of the copper may be tightly bound in the form of inert sulfide complexes. Thus, we hypothesize that dissolved Cu concentrations would show considerable drawdown in the OMZ owing to the presence of sulfides and that reducing, organic sediments on the Peruvian shelf would affect the overall distribution of copper at near shore stations. Copper (Cu) performs a vital role in the nitrogen cycle, acting as a cofactor for the enzymes associated with nitrous oxide reduction (nitrous oxide reductase; NoSZ), nitrite reduction (nitrite reductase; NiRK), and ammonia oxidation (ammonium monooxidase; AMO)(Francis et al. 2007; Philippot 2002). Recent studies indicate that there could be some linkage between copper availability and the nitrogen cycle, as there is a substantial Cu requirement for microbes catalyzing these processes (J. Jacquot; in press). Thus, the effect of the OMZ conditions and the extent of influence of the Cu enriched sediments as a source to the oceans was studied during two separate cruises in transects through the Arabian Sea OMZ and in near shore transects off the Peruvian coast. 152 4.2.1. Study area The Arabian Sea contains one of the ocean’s three major oxygen minimum zones (OMZs) where oxygen levels become low enough (colorimetric O 2 < 1 µmol L -1 ) for denitrification to occur, but not fully anoxic to support sulfate reduction. A well-defined portion of the OMZ, characterized by elevated nitrite (at the secondary nitrite maximum) and covering a maximal depth range of 150–700 m, exhibits production of N 2 through denitrification and probably also through anaerobic ammonium oxidation that accounts for at least one-third of the global pelagic N 2 production rate (Bange et al. 2005; Codispoti et al. 2001). Our east-west transect through the Arabian Sea (Fig. 4-1) during the SW monsoons (Aug-Sep 2007) comprised of eight stations within the core of the denitrification zone (Naqvi 1991) as well as six stations closer to the upwelling center along the Omani coast. Fueled by high primary productivity, Eastern Tropical South Pacific has one of the shallowest and most intense oxygen minimum zones. The Peruvian oxygen minimum zone is thickest (>600m) between 5 and 13 o S and extends to about 1000km offshore (Fuenzalida et al. 2009). The core of the OMZ is characterized by the presence of a well- defined secondary nitrite minimum and the oxygen values (2 nmol L -1 ) are lower than the detection limit (1-10 nmol L -1 O 2 ) of STOX sensors (Thamdrup et al. 2012). The Eastern Tropical South Pacific (off Chile and Peru) forms the most intense “nitrate deficit” maximum zone (nitrate deficit max ranges between 22 and 27 µmol L -1 ), also corresponding to the most intense OMZ core (O 2min between 2 and 3 µmol L -1 ) (Paulmier and Ruiz-Pino 2009). The OMZ results from the combined effects of high productivity (and export) arising from wind-driven coastal upwelling and reduced ventilation (e.g., (Fuenzalida et al. 2009; Karstensen et al. 2008). Sampling during this study took place along three transects (Fig. 4-2) across different shelf width regions of the Peruvian margin, beginning near shore and extending offshore into oceanic 153 waters. Transect 1 (~11.7 o S- 12.5 o S), comprising of Stations 24, 26, 27 and 29, was sampled across the broader, northern Peru shelf. Transect 2 (~13.28 o S -13.3 o S; stations 19, 21, 22, and 23) was sampled off of the relatively broad, central Peruvian shelf while Transect 3 (~15.6 o S- 17.6 o S ; stations 9, 10, 11, 12 and 14) across the narrow, southern Peruvian shelf. This is the first such comprehensive study of copper distribution in an east west transect in the Arabian Sea OMZ after (Saager et al. 1992). Our study spans from stations closer to the Indian coast, into the core of the denitrification zone towards the upwelling dominated Omani coast, thereby providing us with a unique opportunity of examining the effect of the denitrification zone in the OMZ on copper distribution. We also attempt to compare and distinguish copper distribution in another upwelling dominate OMZ off the Peruvian coast and the effect of reducing, organic rich sediments on its distribution. 4.3. Methods 4.3.1. Sample collection Samples on the Arabian Sea transect were collected aboard R/V Roger Revelle (RR0708) during the SW monsoon between August- September 2007 (Fig. 4-1) from 20 m down to 1000 m using 5 L Teflon coated external spring Niskin-type bottles (Ocean Test Equipment) mounted on a trace metal clean rosette (Sea-Bird Electronics). The rosette was lowered over the side of the ship on a Kevlar line, and the bottles were preprogrammed to trip on the downcast at pre- specified depths. Samples on the KN-182-09 cruise during October-November 2005 were collected aboard the R/V Knorr. Sampling was concentrated on three transects off the Peruvian shelf, at 11 o S, 13 o S and 16 o S. Seawater samples for total dissolved Cu analysis were collected in 10L Niskin bottles with Teflon coated interior surfaces (Ocean Test Equipment) mounted on a trace metal clean 154 rosette (Sea-Bird Electronics). Samples were acidified to pH ~1.7 and stored for analysis in the lab. All hydrographic data during this cruise were obtained using a CTD-O2 probe (Seabird). 4.3.2. Nitrite measurements Nitrite was measured spectrophotometrically after (Strickland and Parsons 1968). Small Acropak-filtered sample aliquots (15 mL or less) were each made to react with an acidified sulfanilamide solution to form a diazonium compound. The compound was then mixed with N- (1-Naphthyl)-ethylenediamine dihydrochloride to produce a colored azo dye, whose extinction was measured on a Shimadzu UV-1700 UV-VIS spectrophotometer to determine the nitrite concentration of each sample. 4.3.3. Cleaning protocol The 500 ml LDPE (VWR) sampling bottles were thoroughly cleaned in a sequential four-step process that consisted of soaking them in a 5% Citranox acid detergent bath (Alconox) for at least a day followed by another overnight soak in a 10% hydrochloric acid bath (VWR). They were then filled with 10% HCl and baked at 60°C for at least 2 days and finally, filled with 0.1% trace metal grade HCl (Optima, Fisher) and baked at 60°C again for another 2 days. The insides and outsides of the bottles were thoroughly rinsed at least five times with Milli-Q water (18.2 MΩ; Millipore) in between each step. Samples were prepared in 15 mL polypropylene centrifuge tubes (VWR) which were first cleaned in a two-step process by soaking them in 10% HCl at 60°C for 48 hours and then, rinsing each tube at least five times with Milli-Q water. After the rinses, the tubes were filled to a positive meniscus with 0.5% trace metal grade HCl, capped and then baked at 60°C overnight. After retrieving them from the oven, the tubes were left capped & stored until further use. Upon 155 analysis, the tubes were emptied and rinsed three times with Milli-Q water and at least once with the sample. In order to minimize contamination from the beads and before addition to the samples, the NTA resin was cleaned using the following procedure (Lee et al. 2011). 25 ml of the NTA resin solution was poured into a clean 50 mL polypropylene centrifuge tube (Corning) and then washed five times with Milli-Q water. In between washes, the tube was spun down in a 5810-R centrifuge (Eppendorf) maintained at 8°C for 10 min at 4000 rpm. After decanting the supernatant, Milli-Q water was added for the next wash. The resin was then washed five times with 1.5 mol L -1 trace metal grade HCl (Optima, Fisher) and several more times with Milli-Q water after that to bring the pH of the solution above 4, to indicate that all of the HCl had been removed from the solution. For the final cleaning step, the resin solution was washed five times with 0.5 mol L -1 trace metal grade HNO 3 (Optima, Fisher). The resin solution was placed on an analog shaker (Thomas Scientific) for several hours for the first wash and then left overnight on the shaker for the last wash. After the final wash, the resin solution was again washed at least five times with Milli-Q water until the pH had risen above 4 in order to remove all of the HNO 3 . The resin solution was diluted twofold with 25 mL Milli-Q water and stored in the refrigerator for future use. 25 µL of the working resin suspension contains ~100 – 400 beads, which is 1:50 dilution of the primary resin solution. After sampling and before analysis, all samples were acidified to below pH 2 by the addition of concentrated trace metal grade HCl (Optima, Fisher) and stored for at least a month. All samples were analyzed in triplicate using the Finnegan Element 2 (Thermo Scientific) Inductively Coupled Plasma Mass Spectrophotometer (ICP-MS) on a medium resolution mode. 156 4.3.4. Total dissolved copper measurements The total dissolved Cu concentrations were determined simultaneously using a single batch nitrilotriacetatic acid (NTA) resin extraction and isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) method adapted from Lee et al. (2011). Dissolved Cu was pre- concentrated in the samples by adding a chelating resin - NTA Superflow resin (Qiagen) in the preparatory stage. 15 ml centrifuge tubes were filled with ~7.5 mL of sample (with the exact volume determined gravimetrically) and spiked with enough 65 Cu -enriched spike (BDH Aristar Plus, VWR) to bring the final concentration to ~2 nmol L -1 . Then, about 1.5 mL of 0.1 mol L -1 trace metal grade ammonium acetate buffer was added to increase the pH to at least 4 (Lee et al. 2011). The buffer was prepared by mixing 0.1 mol L -1 trace metal grade ammonium hydroxide (NH 4 OH; Optima, Fisher) and 0.1 mol L -1 trace metal grade acetic acid (CH 3 COOH; Optima, Fisher). Next, 200 µL (~800 beads) of the working resin suspension was added to each sample, and the tubes were placed on a shaker for two to three days. The samples were then centrifuged for 10 min at 4000 rpm, and the seawater was carefully siphoned off to leave only the resin beads at the bottom. The beads were washed twice with 3 mL Milli-Q water to remove salts and the tubes were once again centrifuged using the same settings. After the final wash, 1 mL of 5% trace metal grade HNO 3 (Optima, Fisher) was added to each tube and, after leaving them on the shaker again for one day, the samples were ready for analysis. Procedural seawater blanks were prepared the same way in triplicate, as samples using ~0.1 mL low trace metal surface seawater from the 2004 SAFe cruise ([Cu] = 0.51 ± 0.05 nmol L -1 ). The average detection limit and internal blank value (n=3) for Cu was 0.03 nmol L -1 and 0.07 nmol L -1 , respectively. The accuracy of the method was evaluated by measuring SAFe reference 157 standards S1 and D1 (Johnson et al. 2007). For Cu (n=3), values obtained by this method were 0.52 ± 0.001 nmol L -1 (S1) and 2.26 ± 0.017 nmol L -1 (D1) while the consensus values for S1 and D1 were 0.51 ± 0.05 nmol L -1 and 2.27 ± 0.11 nmol L -1 ,respectively. These values are within the range of the latest consensus numbers (http://www.geotraces.org/science/intercalibration). 4.4. Results 4.4.1. Total dissolved Cu distribution in the Arabian Sea Hydrographic data Based on the approximate geographical extent of the Arabian Sea’s ‘‘permanent’’ denitrification zone as demarcated by the 0.5 µmol L -1 NO 2 - contour (Naqvi 1991), we have classified the stations in our transect (Fig. 4-1) into those within the denitrification zone (Stations 3, 4, 5, 6, 7, 8, 22, 23) and those outside of it (Stations 11, 15, 16, 18, 20, 21). There is also a well-defined primary nitrite maximum (PNM) throughout the region. A sharp SNM (Fig. 4-3A., 0.5 – 4.5 µmol L -1 ) coincident with the low oxygen concentrations (Fig. 4-3B.) was observed between 150 to 500 m depth at all stations inside the denitrification zone. Slightly lower NO 3 - concentrations (20 µmol L -1 ) were also observed coincident with the SNM (Fig. 4-3C). As previously reported, although the OMZ and SNM were found in the eastern Arabian Sea, highest Chl A concentrations (Fig. 4-3D) were measured at stations outside of the denitrification zone (Station 20 & 21) in the western part of the transect. Our findings are in accordance to previous reports highlighting the spatial segregation of the denitrification zone and the high productivity region in the Arabian Sea (Buesseler et al. 1998; Naqvi et al. 2010). The eastern part of our transect within the denitrification zone is characterized by strong water column stratification, most likely due to the presence of a prominent high salinity (Fig. 4- 4A) , high temperature (Fig. 4-4B) water mass feature (~36.5 psu; ~27.5 o C) extending up to 158 150-175 m deep and as far as Station 8 and 23. Surface salinity and temperature (Fig. 4-4C & D) also show the high temperature and salinity features towards the Indian coast while stations closer to the Omani coast were characterized by lower salinity and temperature values (~34.5 psu ; 25.5 o C). Upper 1200 m oceanographic section of total dissolved Cu distributions during this transect through the Arabian Sea is represented in Fig. 4-5. Stations within the OMZ These stations are broadly similar and vastly different from other stations outside of the OMZ. Most stations within the OMZ (except Station 22) were characterized by a high salinity and temperature surface layer (Fig. 4-4 A & B), which was also, associated with high DCu concentrations (Fig. 4-5). High mixed layer DCu concentration of about 1.93 nmol L -1 is observed at stations 3, 4 and 5 (Fig. 4-6 A, B & C) which drops to ~ 0.8-0.9 nmol L -1 at the westernmost stations in the OMZ (Stations 6, 7, 8 and 23). Lowest surface DCu values were observed at the most offshore OMZ station, S22 (Fig.4-7C) with values as low as 0.69 nmol L -1 . The lowest concentrations of DCu in the top 100 m at each station except at station 5 (Fig. 4- 6C) were at the base of the euphotic zone, either within or just below the PNM. For these stations, the lowest DCu concentrations there, was significantly lower than the depths immediately above and below it, suggesting that processes at the base of the euphotic zone are related to these low values. At station 5 (Fig. 4-6C), there was no DCu minimum observed at the base of the euphotic zone, which could have possibly been missed out due to relatively sparse sampling in the upper 100m . Immediately below the DCu minimum, a maximum in DCu concentrations (1.2 – 1.5 nmol L - 1 ) at the upper oxycline is noted at all stations, except at station 3. The peak remains shallow (around 100m) at stations 4 and 5 and gradually becomes deeper (~125 - 150 m) as we progress 159 from Station 6 towards Station 23. This upper oxycline DCu maximum was the deepest (175 m) and with lowest DCu concentrations at the farthermost station 22 (Fig. 4-7C). The absence of this peak at Station 3 could either be due to low Chl A (< 0.1 mg m -3 ) or masking of the peak due to overall high DCu values (> 1.4 nmol L -1 ) in the top 100 m. Below the oxycline, a sharp drawdown of DCu coincident with the SNM is observed at all stations within the denitrification zone, with concentrations ranging between 0.7-0.8 nmol L -1 . The coupling of Cu drawdown and SNM becomes weaker as we move westward towards station 22, with the highest drawdown (~ 0.8 nmol L -1 ) observed at nearshore stations 3, 4, 5 and 6 (Fig. 4-6) and ~0.2 – 0.5 nmol L -1 DCu drawdown at Stations 8, 22 and 23(Fig. 4-7B,C & D). Below 600m, DCu concentrations gradually increase with increasing oxygen concentrations, reaching values up to 1.2 – 1.3 nmol L -1 . Stations outside the OMZ All stations outside the OMZ (Fig. 4-8 & 4-9; Stations 11, 15, 16, 18, 20, & 21) in the western Arabian Sea, were characterized by the presence of higher Chl A concentrations (up to ~0.6 mg m -3 ) and by the absence of an SNM. Each of these stations has unique features and was subject to varying hydrographic features due to influence of different water masses. Stations 20 and 21 (Fig. 4-8A & B) - Although almost constant DCu concentrations up to 400 m depth was measured at Stations 20 (~ 0.8 nmol L -1 ) and 21 (~ 0.9 nmol L -1 ), generic features such as slight draw down of DCu at the DCM and regeneration at depth were also seen. A weak SNM (0.18 µmol L -1 ) with no distinct drawdown of DCu is observed at S21 while no SNM was observed at station 20. A moderately high salinity (~ 36 psu) and temperature (~26.5 o C) surface water mass occupies the top 50m at Station 20 which is also associated with the highest Chl A concentrations ( ~ 0.6 mg m -3 ) for the entire transect. 160 Stations 11 and 18 (Fig. 4-8C & D) - A high salinity (~36 psu) and high temperature (~ 27 o C) water mass feature extending up to 350 m, was present these stations and the low oxygen (< 3 µmol L -1 ) layers spanned from 175 – 700 m. No DCu minimum coincident with the PNM is observed at these stations, which could most likely be due to the masking of the feature by overall high DCu concentrations in the water column. Station 18 is distinct due to the presence of a small SNM (0.3 µmol L -1 ) while Station 11, north of Station 18, lacks such a feature. Owing to low oxygen concentrations and presence of a small SNM, DCu distribution at this station looks similar to that in an OMZ. Stations 11 and 18 are characterized by high upper water column (350m) Cu concentrations ranging from 0.8 nmol L -1 at the surface to 1.5 nmol L -1 at 100 m to constant values of about 1.2- 1.3 nmol L -1 . A DCu max is present at the upper oxycline (1.2 nmol L -1 ), which gets drawn down between 200-600 m to about 1 nmol L -1 and further increases below 600m to high deep water values of 1.5 nmol L -1 . Stations 15 and 16 (Fig. 4-9A & B) - Stations 15 and 16 depict typical open ocean DCu profiles with slight decrease in DCu (~ 0.7 nmol L -1 ) concentrations (60-90 m), which correspond to decrease in Chl A concentrations. Low surface Cu concentration of ~0.8 nmol L -1 was observed at both stations. A DCu regeneration max (~ 0.93 nmol L -1 ) is present at 150m at Station 15, which then decreases to ~0.85 nmol L -1 below the remineralisation zone. DCu values gradually increase below 150 m at Station 16. 4.4.2. Total dissolved Cu distribution in the Peruvian OMZ Surface transects for salinity, temperature and oxygen are shown in Fig. 4-10A, B & C. Surface temperature and salinity distributions indicate strong upwelling of low temperature (~ 16 o C) and low salinity (~ 35 psu) waters approximately between 11-14 o S latitude. This low 161 temperature-salinity feature is also coherent with a low oxygen (150-175 µmol L -1 ) region indicating upwelling of low temperature, low salinity, and oxygen depleted- deeper waters. South of this upwelling core, an increase in temperature and salinity of surface waters was observed. Another low T-S feature (~15.8 o C; 34.9) was recorded south of 16 o S latitude, but this feature was accompanied by high surface oxygen concentrations of up to 200 µmol L -1 . The depth of the mixed layer (15-55m) gradually becomes deeper as we move southward along the Peruvian shelf towards Transect 3. Below the mixed layer, the shallow oxycline (30-70m) deepened in the offshore stations (Fig. 4-2). Oxygen levels at mid-depths fell to suboxic levels (< 10 µmol L -1 ) immediately below the oxycline and remained uniform up to an average depth of 400m. The vertical span of OMZ also became shorter and deeper as we moved southward away from the upwelling center, spanning from about ~60-600m in Transect 1, ~ 80-500 in Transect 1 and ~ 100-400m in Transect 3. DCu along Transect 1 Owing to its proximity to the continental shelf, surface DCu values of all stations in Transect 1 (Fig. 4-11) ranged between 0.97 – 1.24 nmol L -1 , with values gradually decreasing as we move offshore towards Station 24. At the shallowest, near shore Station 29 (Fig. 4-11D), uniformly high DCu concentrations (1.2 – 1.35 nmol L -1 ) were measured throughout all depths (30-180 m). Higher Cu closer to the coast indicates lateral transport from shelf waters and shelf and slope sediments. At the deeper, slightly offshore stations (Fig. 4-11A, B & C; 24, 26, 27), a subsurface Cu maxima coinciding with the upper oxycline was detected. This peak could be a result of remineralisation of sinking particulate and organic matter. At the core of the OMZ, a draw down in DCu concentrations (~ 0.08 nmol L -1 ) coincident with the SNM was noted at Stations 24, 26, and 27. DCu concentrations in the suboxic zone were 162 ~1 nmol L -1 , which gradually increase below 300 m to ~ 1.74 nmol L -1 . High DCu concentration (1.72 nmol L -1 ) at 700 m at Station 24 (Fig. 4-11A) is also coincident with a high DFe feature at the same depth. This feature could be attributed to an external advective source of input to this station since oxygen at that depth seems to be increasing with depth. DCu along Transect 2 Surface DCu concentrations at all stations in this near shore transect (Fig. 4-12) range between 1.1 – 1.6 nmol L -1 . At near shore stations 21, 22, and 23, a subsurface Cu minimum (coincident with the upper oxycline) below the surface DCu maxima was detected. This could be indicative of draw down by biological uptake and is distinctly evident since the values get drawn down from high surface values. At offshore station 19 (Fig. 4-12A), a subsurface DCu peak (1.54 nmol L -1 ) coinciding with the upper oxycline is present. A drawdown in DCu concentrations (1. 53 nmol L -1 at 50m to 1.29 nmol L -1 at 100m) associated with the deeper SNM was also observed between 100 – 300 m depth. DCu concentrations increase below 300m to reach values of ~ 2 nmol L -1 at 1000m in oxygenated deeper waters. DCu along Transect 3 Surface DCu concentrations at this transect (Fig. 4-13) range between 1.2 – 2.2 nmol L -1 . A subsurface DCu minima (0.99 nmol L -1 ; 80 m), along with two deep DCu maxima (1.3 and 1.57 nmol L -1 at 150 and 400 m respectively) were present at near shore station 9 (Fig. 4-13A). Stations 10 and 12 (Fig. 4-13B & D) exhibit similar features in DCu distribution - subsurface maxima at the upper oxycline, gradual drawdown within the OMZ and increase with oxygen concentrations depth below the OMZ. No subsurface peak was observed at Station 11(Fig. 4- 13C), which could also be attributed to low sampling frequency at this station or masking of the 163 maxima due to high surface values. A gradual increase in DCu with increasing oxygen concentrations is recorded below 300m at all stations. 4.5. Discussion 4.5.1. Surface DCu distribution Overall surface DCu concentrations in near shore transects off the Peruvian shelf were higher (~ 0.9 – 2.2 nmol L -1 ) than those measured in our offshore transect in the Arabian Sea (~0.7 – 1.9 nmol L -1 ). Although surface DCu (Fig. 4-5) concentrations remain higher (0.9 – 1.93 nmol L -1 ) at stations closer to the Indian subcontinent, these high surface DCu values measured at stations 3, 4, 5 and 6 (Fig. 4-6), may be attributed to the prominent high salinity and high temperature water mass at the surface which could have been enriched with DCu due to high fluvial and eolian inputs from the Indian subcontinent. However, higher range of surface DCu concentrations were measured in stations off southern transects 2 and 3 (Fig. 4-12 & 4-13; ~ 1.1 – 2.2 nmol L -1 ) than in stations off the northern transect 1 (Fig. 4-11; 0.97 – 1.2 nmol L -1 ). In a recent study of continental organic aerosols in the southeast Pacific marine boundary layer (Hawkins et al. 2010), it was found that sulfate composed a large fraction of observed submicron mass. Although no direct measurements of Cu content in aerosols were made, remote sensing measurements of cloud drop radius analyzed by (Bretherton et al. 2004) and (Huneeus et al. 2006) have indicated that the large pollution sources, especially copper smelters, along the Peruvian and Chilean coasts, could impact particle concentrations in the aerosols, especially on the southern region of the Peruvian coast. High DCu values in stations right off the Peruvian shelf could also be attributed to DCu influx from biogenically enriched sediments. It has long been suggested from studies in modern and ancient settings, that Cu is enriched in reducing, organic-rich sediments and is brought to the 164 marine sediments with settling biodetritus from surface waters, either adsorbed onto biodetrital surfaces or incorporated into soft tissue of plankton (Algeo and Maynard 2004; Brumsack 2006; Calvert and Price 1983) . Work by Boning et al. (2004) also suggests that even though Cu is slightly elevated in the Peruvian sediments, it is most enriched in the sediments from within the OMZ and least on the upper edge. This is clearly reflected in Transect 2 (Fig. 4-12), where low Cu concentrations (0.95 – 1.1 nmol L -1 ) were observed at all depths measured at the shallowest near shore station (Station 23), located at the upper edge of the OMZ. DCu concentrations increase towards Station 19, located right off the sediments from within the OMZ. DCu decreased as we moved offshore towards the western Arabian Sea and in remote, open ocean waters, such surface water enrichment was not observed. The absence of a pronounced surface maximum (at all stations except 3, 4, 5 and 6) is also indicative of regions of high biological productivity associated with upwelling. 4.5.2. PNM and DCu minima In a recent Cu speciation study in the ETSP off Peru (J. Jacquot; in press), it was found that the lowest dissolved and free [Cu 2+ ] were within the primary nitrite maxima (PNM), where ammonia oxidation and nitrate reduction rates are important. This finding is of interest because the two competing explanations for the PNM—iron (Fe) limitation of diatoms and high rates of ammonia oxidation relative to nitrite oxidation—have high Cu requirements. Thus, the low concentrations of free Cu 2+ measured in their study could impose significant constraints on the rates of these processes. Furthermore, the formation of the PNM has been linked to both bacterial nitrification and nitrite (NO 2 - ) excretion by phytoplankton at the base of the euphotic zone (Lomas and Lipschultz 2006) . Lam et al. (2009) have also determined that ammonia oxidation and nitrate reduction accounted for 6 – 33% and 67 – 94% of total nitrite production in the upper 165 OMZ, respectively. Hence, in this study, this pronounced minimum in the upper thermocline , either coincident or just below the PNM, is probably maintained by a combination of biological uptake by ammonia oxidizers and nitrate reducers in the upper water column along with general scavenging with subsequent incorporation into and removal by zooplanktonic fecal material. 4.5.3. Subsurface DCu maxima The presence of a distinct subsurface maxima coinciding with the upper oxycline at all stations in the Arabian Sea, hints towards its formation by remineralisation of sinking organic matter from productive surface waters at that depth (90-150 m). However, analogous to the Arabian Sea, we saw evidence of its formation in only some northern offshore stations (Stations 19, 24, 26 & 27) in transects 1 and 2 off the Peruvian shelf. Elsewhere, if these features existed, they were masked by high surface source. Results from a study by (Sunda and Huntsman 1995) support the hypothesis that Cu uptake by phytoplankton and subsequent regeneration at depth control Cu distribution in oceanic nutricline. It seems that DCu concentrations closest to the continents are further affected by the eolian and fluvial inputs while DCu concentrations in remote oceanic nutriclines (such as our stations in the Arabian Sea) are regulated by phytoplankton uptake and regeneration processes. 4.5.4. DCu drawdown within the OMZ Although, clear drawdown of DCu in the OMZ was noticed in offshore stations in stations from both the Arabian Sea and the Peruvian margin, no such clear correlation between DCu concentrations and SNM was observed in near shore stations right off the shelf (Stations 21, 22, 23, & 29). The most prominent and interesting feature in the DCu distribution in deep, offshore stations is the strong drawdown of Cu within the OMZ coincident with the SNM (esp. Fig. 4-5). This 166 feature seems in agreement with findings by (Saito et al. 2003) that Cu levels in hypoxic and anoxic regimes such as OMZs would be low due to the formation of sulfide precipitates. Recent work by Canfield et al. (2010) also suggests that sulfide may be more abundant in OMZ waters than previously thought because of a heretofore unknown cryptic sulfur cycle, which further supports our findings of correspondingly low DCu in the OMZ. Yet, in another study by (Theberge et al. 1997), although no free hydrogen sulfide was detected in the Arabian Sea OMZ, sulfide and metal complexes were observed throughout the water column along with a slight increase of sulfide concentrations within the OMZ. They further stated that multinuclear Cu complexes were relatively kinetically inert and less labile as compared to other metal sulfide complexes such as Fe, Mn etc. 167 4.6. References Al-Farawati, R., and C. M. G. Van Den Berg. 1999. Metal–sulfide complexation in seawater. Mar. Chem. 63: 331-352. Algeo, T. J., and J. B. Maynard. 2004. Trace-element behavior and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems. Chemical Geology 206: 289-318. Bange, H. W., S. W. A. Naqvi, and L. A. Codispoti. 2005. The nitrogen cycle in the Arabian Sea. Prog. Oceanogr. 65: 145-158. Boning, P. and others 2004. Geochemistry of Peruvian near-surface sediments. Geochim. Cosmochim. Acta 68: 4429-4451. Boyle, E. A., F. R. Sclater, and J. M. Edmond. 1977. DISTRIBUTION OF DISSOLVED COPPER IN PACIFIC. Earth Planet. Sci. Lett. 37: 38-54. Bretherton, C. S. and others 2004. The EPIC 2001 stratocumulus study. Bulletin of the American Meteorological Society 85: 967-+. Brumsack, H.-J. 2006. The trace metal content of recent organic carbon-rich sediments: Implications for Cretaceous black shale formation. Palaeogeography, Palaeoclimatology, Palaeoecology 232: 344-361. Buesseler, K. and others 1998. Upper ocean export of particulate organic carbon in the Arabian Sea derived from thorium-234. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 45: 2461- 2487. Calvert, S. E., and N. B. Price. 1983. Geochemistry of Namibian shelf sediments, p. 337-375. Coastal Upwelling Its Sediment Record. Springer. Codispoti, L. A. and others 2001. The oceanic fixed nitrogen and nitrous oxide budgets: Moving targets as we enter the anthropocene? Sci. Mar. 65: 85-105. 168 Francis, C. A., J. M. Beman, and M. M. M. Kuypers. 2007. New processes and players in the nitrogen cycle: the microbial ecology of anaerobic and archaeal ammonia oxidation. Isme Journal 1: 19-27. Fuenzalida, R., W. Schneider, J. Garces-Vargas, L. Bravo, and C. Lange. 2009. Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 56: 1027-1038. Hawkins, L. N., L. M. Russell, D. S. Covert, P. K. Quinn, and T. S. Bates. 2010. Carboxylic acids, sulfates, and organosulfates in processed continental organic aerosol over the southeast Pacific Ocean during VOCALS-REx 2008. J. Geophys. Res.-Atmos. 115. Huneeus, N., L. Gallardo, and J. A. Rutllant. 2006. Offshore transport episodes of anthropogenic sulfur in northern Chile: Potential impact on the stratocumulus cloud deck. Geophys. Res. Lett. 33. Johnson, K. S. and others 2007. Developing standards for dissolved iron in seawater. Eos, Transactions American Geophysical Union 88: 131-132. Karstensen, J., L. Stramma, and M. Visbeck. 2008. Oxygen minimum zones in the eastern tropical Atlantic and Pacific oceans. Prog. Oceanogr. 77: 331-350. Laglera, L. M., and C. M. G. Van Den Berg. 2003. Copper complexation by thiol compounds in estuarine waters. Mar. Chem. 82: 71-89. Lee, J. M., E. A. Boyle, Y. Echegoyen-Sanz, J. N. Fitzsimmons, R. F. Zhang, and R. A. Kayser. 2011. Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batch nitrilotriacetate resin extraction and isotope dilution inductively coupled plasma mass spectrometry. Anal. Chim. Acta 686: 93-101. 169 Luther, G. W., T. M. Church, and D. Powell. 1991. SULFUR SPECIATION AND SULFIDE OXIDATION IN THE WATER COLUMN OF THE BLACK-SEA. Deep-Sea Research Part a-Oceanographic Research Papers 38: S1121-S1137. Naqvi, S. W. A. and others 2010. The Arabian Sea as a high-nutrient, low-chlorophyll region during the late Southwest Monsoon. Biogeosciences 7: 2091-2100. Naqvi, W. A. 1991. GEOGRAPHICAL EXTENT OF DENITRIFICATION IN THE ARABIAN SEA IN RELATION TO SOME PHYSICAL PROCESSES. Oceanologica Acta 14: 281- 290. Paulmier, A., and D. Ruiz-Pino. 2009. Oxygen minimum zones (OMZs) in the modern ocean. Prog. Oceanogr. 80: 113-128. Philippot, L. 2002. Denitrifying genes in bacterial and Archaeal genomes. Biochimica Et Biophysica Acta-Gene Structure and Expression 1577: 355-376. Saager, P. M., H. J. W. Debaar, and R. J. Howland. 1992. CD, ZN, NI AND CU IN THE INDIAN-OCEAN. Deep-Sea Research Part a-Oceanographic Research Papers 39: 9-35. Saito, M. A., D. M. Sigman, and F. M. M. Morel. 2003. The bioinorganic chemistry of the ancient ocean: the co-evolution of cyanobacterial metal requirements and biogeochemical cycles at the Archean-Proterozoic boundary? Inorganica Chimica Acta 356: 308-318. Strickland, J. D., and T. R. Parsons. 1968. A practical handbook of seawater analysis. Fisheries Research Board of Canada Ottawa. Sunda, W. G., and S. A. Huntsman. 1995. REGULATION OF COPPER CONCENTRATION IN THE OCEANIC NUTRICLINE BY PHYTOPLANKTON UPTAKE AND REGENERATION CYCLES. Limnol. Oceanogr. 40: 132-137. 170 Thamdrup, B., T. Dalsgaard, and N. P. Revsbech. 2012. Widespread functional anoxia in the oxygen minimum zone of the Eastern South Pacific. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 65: 36-45. Theberge, S. M., G. W. Luther, and A. M. Farrenkopf. 1997. On the existence of free and metal complexed sulfide in the Arabian Sea and its oxygen minimum zone. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 44: 1381-1390. 171 Fig.4-1. Map of the cruise track in the Arabian Sea that was sampled during August – September 2007 aboard the R/V Roger Revelle (RR0708). 3 6 4 5 7 8 23 22 20 11 18 21 15 16 172 Fig.4-2. Map of the cruise track off coastal Peru in the Eastern tropical South Pacific that was sampled in October-November 2005 aboard the R/V Knorr (KN-182-09). 173 Fig.4-3. Upper 1200 m oceanographic sections of (A) nitrite; (B) oxygen, (C) nitrate; and upper 150 m oceanographic section of (D) Chl a concentrations in the Arabian Sea. B. D. C. A. 174 Fig.4-4. Upper 1200 m oceanographic sections of (A) salinity; and (B) temperature; and surface distribution of (C) salinity; and (D) temperature in the Arabian Sea. A. B. D. C. 175 Fig.4-5. Upper 1200 m oceanographic section of total dissolved Cu in the Arabian Sea. 176 Fig.4-6. Depth profiles of total dissolved Cu (closed circles), oxygen (open circles) and nitrite (closed triangle) for stations (A) 3, (B) 4, (C) 5, and (D) 6 in the Arabian Sea. Error bars for total dissolved [Cu] represent error propagation from the calculation of standard deviation values (n=3). Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0 1 2 3 4 5 6 Cu O 2 NO 2 - B. Station 4 Total dissolved Cu (nmol L -1 ) 0 1 2 3 4 Depth (m) 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0 1 2 3 4 Cu O 2 NO 2 - A. Station 3 Total dissolved Cu(nM) 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0 1 2 3 4 5 6 Cu O 2 NO 2 - D. Station 6 C. Station 5 Total dissolved Cu(nM) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0 1 2 3 4 Cu O 2 NO 2 - 177 Fig.4-7 Depth profiles of total dissolved Cu (closed circles), oxygen (open circles) and nitrite (closed triangle) for stations (A) 7, (B) 8, (C) 22, and (D) 23 in the Arabian Sea. Error bars for total dissolved [Cu] represent error propagation from the calculation of standard deviation values (n=3). B. Station 8 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0 1 2 3 4 5 Cu O 2 NO 2 - A. Station 7 Total dissolved Cu (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Depth (m) 0 200 400 600 800 1000 1200 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0 1 2 3 4 5 6 Cu O 2 NO 2 - D. Station 23 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0 1 2 3 4 5 Cu O 2 NO 2 - C. Station 22 Total dissolved Cu (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Depth (m) 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0 1 2 3 4 Cu O 2 NO 2 - 178 Fig.4-8. Depth profiles of total dissolved Cu (closed circles), oxygen (open circles) and nitrite (closed triangle) for stations (A) 20, (B) 21, (C) 11, and (D) 18 in the Arabian Sea. Error bars for total dissolved [Cu] represent error propagation from the calculation of standard deviation values (n=3). B. Station 21 Total dissolved Cu (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 300 Nitrite ( mol L -1 ) 0.0 0.5 1.0 1.5 2.0 Cu O 2 NO 2 - A. Station 20 Total dissolved Cu (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Depth (m) 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Cu O 2 NO 2 - D. Station 18 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 1200 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 Cu O 2 NO 2 - C. Station 11 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 200 400 600 800 1000 1200 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Cu O 2 NO 2 - 179 Fig.4-9. Depth profiles of total dissolved Cu (closed circles), oxygen (open circles) and nitrite (closed triangle) for stations (A) 15 and (B) 16 in the Arabian Sea. Error bars for total dissolved [Cu] represent error propagation from the calculation of standard deviation values (n=3). B. Station 16 Total dissolved Cu (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Cu O 2 NO 2 - A. Station 15 Total dissolved Cu (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 Depth (m) 0 200 400 600 800 1000 Oxygen ( mol L -1 ) 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0.0 0.1 0.2 0.3 0.4 0.5 Cu O 2 NO 2 - 180 Fig.4-10. Surface oceanographic distributions of (A) salinity; (B) temperature; and (C) oxygen during the KN-182-09 cruise off the Peruvian coast. B. A. C. 181 Fig.4-11. Depth profiles of total dissolved Cu (closed circles), oxygen (open circles) and nitrite (closed triangle) for stations (A) 24, (B) 26, (C) 27, and (D) 29 along Transect 1 during KN-182- 09 cruise. Error bars for total dissolved [Cu] represent error propagation from the calculation of standard deviation values (n=3). Oxygen ( mol/L) B. Station 26 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 0 50 100 150 200 250 300 Nitrite ( mol L -1 ) 0 2 4 6 8 10 Cu O 2 NO 2 - Oxygen ( mol L -1 ) A. Station 24 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 200 400 600 800 1000 0 50 100 150 200 250 300 Nitrite ( mol L -1 ) 0 1 2 3 4 5 6 Cu O 2 NO 2 - 0.0 0.5 1.0 1.5 2.0 0 50 100 150 200 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0.0 0.5 1.0 1.5 2.0 Cu O 2 NO 2 - D. Station 29 Total dissolved Cu (nmol L -1 ) C. Station 27 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 200 400 600 800 1000 0 50 100 150 200 250 Nitrite ( mol L -1 ) 0 1 2 3 4 5 Cu O 2 NO 2 - Oxygen ( mol/L) Oxygen ( mol L -1 ) 182 Fig.4-12. Depth profiles of total dissolved Cu (closed circles), oxygen (open circles) and nitrite (closed triangle) for stations (A) 19, (B) 21, (C) 22, and (D) 23 along Transect 2 during KN-182- 09 cruise. Error bars for total dissolved [Cu] represent error propagation from the calculation of standard deviation values (n=3). B. Station 21 Total dissolved Cu (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 200 400 600 800 1000 0 50 100 150 200 250 0 1 2 3 4 5 Cu O 2 NO 2 - Nitrite ( mol L -1 ) A. Station 19 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 Depth (m) 0 200 400 600 800 1000 0 50 100 150 200 250 0 1 2 3 4 5 6 Cu O 2 NO 2 - Nitrite ( mol L -1 ) Oxygen ( mol L -1 ) Oxygen ( mol/L) D. Station 23 Total dissolved Cu (nmol L -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 50 100 150 200 250 0 50 100 150 200 0 1 2 3 4 5 Cu O 2 NO 2 - C. Station 22 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 50 100 150 200 250 0 50 100 150 200 250 0 1 2 3 4 5 Oxygen ( mol/L) Oxygen ( mol/L) Nitrite ( mol L -1 ) Nitrite ( mol L -1 ) Cu O 2 NO 2 - 183 Fig.4-13. Depth profiles of total dissolved Cu (closed circles), oxygen (open circles) and nitrite (closed triangle) for stations (A) 9, (B) 10, (C) 11, and (D) 12 along Transect 3 during KN-182- 09 cruise. Error bars for total dissolved [Cu] represent error propagation from the calculation of standard deviation values (n=3). B. Station 10 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 0 50 100 150 200 0 1 2 3 4 5 6 7 Cu O 2 NO2- A. Station 9 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 Depth (m) 0 200 400 600 800 1000 0 50 100 150 200 250 0 1 2 3 4 5 6 7 Oxygen ( mol/L) Oxygen ( mol/L) Nitrite ( mol L -1 ) Nitrite ( mol L -1 ) Cu O 2 NO 2 - C. Station 12 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 0 50 100 150 200 250 0 1 2 3 4 5 6 C. Station 11 Total dissolved Cu (nmol L -1 ) 0.0 0.5 1.0 1.5 2.0 2.5 Depth (m) 0 200 400 600 800 1000 0 50 100 150 200 250 0 2 4 6 8 10 Cu O 2 NO 2 - Oxygen ( mol/L) Oxygen ( mol/L) Nitrite ( mol L -1 ) Nitrite ( mol L -1 ) Cu O 2 NO2- 184 Chapter 5 Distributions of total dissolved iron, manganese, zinc and copper along Line-P in the North eastern Pacific Ocean. 185 5.1. Abstract Total dissolved concentrations of bioactive trace metals - Fe, Cu, Mn and Zn were measured at seven stations (P1 – P8) across a dynamic physical/chemical front formed at the intersection of iron-limited, nitrate replete open-ocean and iron-replete, nitrate limited coastal water masses, along Line P in the North East Pacific. Total dissolved Fe, Cu and Zn (DFe, DCu & DZn) concentrations were determined using isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) while total dissolved Mn (DMn) concentrations were determined using simple dilution and matrix-matched external standardization ICP-MS. One of the most striking differences in DFe & DMn distribution was the presence of high Mn: Fe ratios off the continental shelf along Line P as compared to those obtained in transects off the Peruvian coast. Complex redox cycling of Fe and Mn in the reducing sediments along the continental margin underlying the Peruvian OMZ results in the “Fe trapping” while Mn diffuses off shore into the water column, thereby resulting in lower DMn values along the Peruvian continental shelf. No distinct subsurface DFe plume was present throughout the transect. DCu depth profiles along Line P exhibit general features of a nutrient like element and agree with previous data from the central North Pacific. However, in the absence of a SNM along Line P, no draw down of DCu was observed at mid-depths similar to DCu distributions within the Arabian Sea OMZ. Oceanographically consistent DZn profiles with silica-like distribution was obtained during this transect. Zn:Si and Zn:P relationships indicate strong coupling of zinc and nutrient remineralisation within the upper water column. Low surface DZn values in the range of 0.04 – 0.76 nmol L -1 and deeper DZn values of ~ 8.7 nmol L -1 measured were consistent with previously observed values in the North Pacific. 186 5.2. Introduction In recent times, the study of the NE sub-arctic Pacific has been taking place along Line P, which is a 1425 km-long transect between the coast of British Columbia and Ocean Station Papa (OSP; 50 o N and 145 o W) (Pena and Bograd 2007). Ocean Station Papa (OSP), the offshore end station of Line P, is one of the longest-running ocean time series in the world which was initiated in 1956, thereby providing a unique dataset that has immensely improved our understanding of North eastern Pacific Ocean processes. Ribalet et al. (2010) were the first to describe the presence of a 100 km wide transition zone between the iron-limited, nitrate replete open-ocean and iron-replete, nitrate limited coastal water masses. They reported the presence of a complex succession of blooms of five distinct phytoplankton classes in the transition zone with each group apparently constrained to a particular region within the transition zone. This hotspot of phytoplankton diversity and productivity was hypothesized to be most likely fuelled by the iron propelled off shore from the continental shelf which coincided with a gradient in nitrate supply. Waters over the continental shelf are rich in iron and silicic acid and support a high biomass composed of large centric diatoms (Taylor and Haigh 1996) while productivity is seasonally limited by nitrogen availability (Harris et al. 2009). It’s now known that, anticyclonic mesoscale eddy formation can export nutrient-rich waters from the productive continental shelf in a single eddy, and recirculate waters away from southwestern Alaska, eventually carrying nutrient-rich water from the coast to the vicinity of Ocean Station Papa within a few months (Crawford et al. 2007; Hongo et al. 2006; Wu et al. 2009) . These mesoscale eddies play an important role in advecting Fe-rich coastal waters and injecting iron and nutrients to the otherwise low iron regime 187 ,thereby fueling productivity and enriching at least 1000m in the Gulf of Alaska as a result (Whitney et al. 2005). We hypothesize that this physical/chemical front creates a distinctive biome with a disproportionate impact on the biogeochemistry of the region, an attribute that may be a fundamental feature of province boundaries. Bioactive trace metals such as Fe, Cu, Mn and Zn may play an essential role in regulating levels of primary productivity, trophic structure and species composition in such a dynamic scenario. Our research group has studied the behavior of redox-active trace metal Fe in several oxygen minimum zones (OMZs), characterized with active denitrification marked by the presence of nitrite and Fe(II). Thus, this unique environmental marine ecotone, in the absence of water column denitrification, provides us with an in-situ control for OMZ conditions, where redox-sensitive metals such as Fe and Mn are known to exhibit unique features within the OMZ core. The distributions of these bioactive trace metals- Fe, Cu, Mn and Zn at three different biogeochemical settings namely a coastal, transitional and an off shore zone, along a transect across the North East Pacific is presented here and interpreted with an oceanographic context. This pilot cruise for GeoMICS (Geological and Microbial Interactions across Chemical Surveys) was a collaborative effort aimed at mapping the distribution of these trace metals in a detailed surface to seafloor zonal survey across this zone enables us to decipher the complex interactions between biota and metals that lead to the formation of this distinct hotspot of phytoplankton diversity. Extensive measurements of particulate metals concentrations have also been carried out by Twining (in prep.) and will be referred to throughout this manuscript (Fig.5- 7). Subsequent papers will focus on parameters reported here along with the microbial aspects of the survey. 188 5.3. Methods 5.3.1. Hydrographic setting Samples were collected during the GEOMICS cruise, (Seattle, Washington to Seattle, Washington), from 16 May to 22 May 2012 onboard research vessel R.V. Thompson. The cruise track consisted of 7 stations, P1, P2, P3, P4, P6 and P8 along the well-studied Line P (Fig. 5-1). All hydrographic data during this cruise were obtained using hydrographic electrodes housed-in titanium pressure cases to completely eliminate the need for zinc sacrificial anodes and therefore, potential sample contamination (Seabird). 5.3.2. Sampling Seawater samples during GEOMICS cruise in May 2012 was collected aboard R/V Thompson using 12 L Teflon coated GO FLO bottles (General Oceanics, Model 108012T) mounted on a one, piece welded aluminum with a polyurethane electrostatic coating and a titanium lifting bail trace metal clean rosette (Sea-Bird Electronics) (Cutter and Bruland 2012). A conducting Vectran cable with an extruded polyester outer jacket was used for sampling and the GO-FLO bottles on the rosette were preprogrammed to trip on the up cast at pre-specified depths. The GO-FLO bottles were pressurized with filtered compressed nitrogen gas and the seawater samples were collected using Teflon tubing, through acid-cleaned 0.2 m Acropak capsules (Pall Corporation) into 500 mL acid cleaned, sample rinsed, low-density polyethylene bottles (LDPE, Nalgene). Samples for total dissolved metal analysis were then acidified to pH ~ 1.4 by adding trace metal grade HCl (Fisher Optima). All critical ship board manipulations such as sample acidification and loading of the sampling filters were carried out inside a positive pressure ISO-sized 20 feet aluminum container built to US UNOLS standards (Cutter and Bruland 2012). 189 5.3.3. Cleaning protocol The 500 ml LDPE (VWR) sampling bottles were thoroughly cleaned in a sequential four-step process that consisted of soaking them in a 5% Citranox acid detergent bath (Alconox) for at least a day followed by another overnight soak in a 10% hydrochloric acid bath (VWR). They were then filled with 10% HCl and baked at 60°C for at least 2 days and finally, filled with 0.1% trace metal grade HCl (Optima, Fisher) and baked at 60°C again for another 2 days. The insides and outsides of the bottles were thoroughly rinsed at least five times with Milli-Q water (18.2 MΩ; Millipore) in between each step. Samples were prepared in 15 mL polypropylene centrifuge tubes (VWR) which were first cleaned in a two-step process by soaking them in 10% HCl at 60°C for 48 hours and then, rinsing each tube at least five times with Milli-Q water. After the rinses, the tubes were filled to a positive meniscus with 0.5% trace metal grade HCl, capped and then baked at 60°C overnight. After retrieving them from the oven, the tubes were left capped & stored until further use. Upon analysis, the tubes were emptied and rinsed three times with Milli-Q water and at least once with the sample. In order to minimize contamination from the beads and before addition to the samples, the NTA resin was cleaned using the following procedure (Lee et al. 2011). 25 ml of the NTA resin solution was poured into a clean 50 mL polypropylene centrifuge tube (Corning) and then washed five times with Milli-Q water. In between washes, the tube was spun down in a 5810-R centrifuge (Eppendorf) maintained at 8°C for 10 min at 4000 rpm. After decanting the supernatant, Milli-Q water was added for the next wash. The resin was then washed five times with 1.5 mol L -1 trace metal grade HCl (Optima, Fisher) and several more times with Milli-Q water after that to bring the pH of the solution above 4, to indicate that all of the HCl had been 190 removed from the solution. For the final cleaning step, the resin solution was washed five times with 0.5 mol L -1 trace metal grade HNO 3 (Optima, Fisher). The resin solution was placed on an analog shaker (Thomas Scientific) for several hours for the first wash and then left overnight on the shaker for the last wash. After the final wash, the resin solution was again washed at least five times with Milli-Q water until the pH had risen above 4 in order to remove all of the HNO 3 . The resin solution was diluted twofold with 25 mL Milli-Q water and stored in the refrigerator for future use. 25 µL of the working resin suspension contains ~100 – 400 beads, which is 1:50 dilution of the primary resin solution. After sampling and before analysis, all samples were acidified to below pH 2 by the addition of concentrated trace metal grade HCl (Optima, Fisher) and stored for at least a month. All samples were analyzed in triplicate using the Finnegan Element 2 (Thermo Scientific) Inductively Coupled Plasma Mass Spectrophotometer (ICP-MS) on a medium resolution mode. 5.3.4. Total dissolved Fe (DFe) and Cu (DCu) analysis The total dissolved Fe and Cu concentration were determined simultaneously using a single batch nitrilotriacetatic acid (NTA) resin extraction and isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) method adapted from Lee et al. (2011). Dissolved Fe and Cu was pre-concentrated in the samples by adding a chelating resin - NTA Superflow resin (Qiagen) in the preparatory stage. 15 ml centrifuge tubes were filled with ~7.5 mL of sample (with the exact volume determined gravimetrically) and spiked with enough 57 Fe-enriched spike (BDH Aristar Plus, VWR) and 65 Cu to bring the final concentration to ~2 nmol L -1 . To measure Cu simultaneously, about 1.5 mL of 0.1 mol L -1 trace metal grade ammonium acetate buffer was added to increase the pH to at least 4 (Lee et al. 2011). The buffer was prepared by mixing 0.1 mol L -1 trace metal 191 grade ammonium hydroxide (NH 4 OH; Optima, Fisher) and 0.1 mol L -1 trace metal grade acetic acid (CH 3 COOH; Optima, Fisher). To measure Fe, 0.1 mL of 1.5 mol L -1 trace metal grade hydrogen peroxide (H 2 O 2 ; Optima, Fisher) was also added to each sample and left to equilibrate for at least an hour at room temperature, to completely oxidize any Fe 2+ to Fe 3+ (Lee et al. 2011). Next, 200 µL (~800 beads) of the working resin suspension was added to each sample, and the tubes were placed on a shaker for two to three days. The samples were then centrifuged for 10 min at 4000 rpm, and the seawater was carefully siphoned off to leave only the resin beads at the bottom. The beads were washed twice with 3 mL Milli-Q water to remove salts and the tubes were once again centrifuged using the same settings. After the final wash, 1 mL of 5% trace metal grade HNO 3 (Optima, Fisher) was added to each tube and, after leaving them on the shaker again for one day, the samples were ready for analysis. Procedural seawater blanks were prepared in triplicates in the same way as samples using ~0.1 mL low trace metal surface seawater from the 2004 SAFe cruise ([Fe] = 0.09 ± 0.007 nmol L -1 ;[Cu] = 0.51± 0.05 nmol L -1 ). The average detection limit and internal blank value for this method (n=3) for Fe was 0.01 nmol L -1 and 0.06 nmol L -1 , respectively. The average detection limit and internal blank value (n=3) for Cu was 0.03 nmol L -1 and 0.07 nmol L -1 , respectively. The accuracy of the method was evaluated by measuring SAFe reference standards S1 and D1 (Johnson et al. 2007). The Fe values obtained by this method for S1 and D1 were 0.094 ± 0.005 nmol L -1 and 0.645 ± 0.020 nmol L -1 , respectively. The certified consensus values were 0.090 ± 0.007 nmol L -1 Fe (S1) and 0.67 ± 0.07 nmol L -1 Fe (D1). For Cu (n=3), values obtained by this method were 0.52 ± 0.001 nmol L -1 (S1) and 2.26 ± 0.017 nmol L -1 (D1) while the consensus values for S1 and D1 were 0.51 ± 0.05 nmol L -1 and 2.27 ± 0.11 nmol L -1 ,respectively. These 192 values are within the range of the latest consensus numbers (http://www.geotraces.org/science/intercalibration). 5.3.5. Total dissolved Mn (DMn) analysis Total dissolved Mn concentrations were determined by seawater dilution using inductively coupled plasma mass spectrometry (ICP-MS) method adapted from (Field et al. 1999). The standardization of samples was done by a matrix-matched external calibration curve with variations in sensitivity corrected by normalizing to an added internal standard, In. 0.5 mL of sample was pipetted into the 15 ml pre-cleaned vials and 4.5 ml of 5% trace metal grade HNO 3 (Optima) was added to dilute each sample by 10 fold. The samples were then spiked with 1ppb of In, shaken and left to equilibrate for an hour. Each sample was analyzed in triplicates and the total Mn counts (cps) were determined on the instrument in medium resolution mode. Seawater Mn standards for external standardization were prepared the same way using low Mn deep seawater from the SAFe D2 reference samples ([Mn] = 0.35 ± 0.06 nmol L -1 (n=3)). The accuracy of the method was evaluated by measuring 2008 SAFe reference standards S1 and D2 (Johnson et al. 2007). The Mn values obtained by this method for S1 and D2 were 0.83 ± 0.007 nmol L -1 and 0.42 ± 0.004 nmol L -1 , respectively and were in agreement with certified consensus values of 0.79 ± 0.06 nmol L -1 Mn (S1) and 0.35 ± 0.06 nmol L -1 Mn (D2). 5.2.6. Total dissolved Zn (DZn) analysis Total dissolved Zinc concentrations were quantified using isotope dilution and magnesium hydroxide pre-concentration followed by analysis using inductively coupled plasma mass spectrometry (ICP-MS) after (Jakuba et al. 2008; Saito and Schneider 2006; Wu and Boyle 1998). 13.5 ml of the acidified sample was poured into a pre-cleaned 15 ml centrifuge tube and then spiked with a 67 Zn spike (BDH Aristar Plus, VWR) and allowed to equilibrate for a 193 minimum of 1 hour. After an hour, 125 µL of 11 mol L -1 ammonium hydroxide (Optima) was added to each tube. After 90 sec, the tube was inverted and after an additional 90 sec, tubes were centrifuged for 3 min at 4000 rpm using the 5810-R centrifuge (Eppendorf). The resulting supernatant was decanted carefully and then the tubes were respun for 3 min to form a firm pellet and the remaining supernatant was shaken out. Pellets were stored dry until the day of analysis and no longer than a few days. On the day of ICP-MS analysis, the pellets were dissolved using 1.5 mL of 5% nitric acid (Optima). ICP-MS measurements were made using a Finnigan ELEMENT2 in medium resolution mode, which was sufficient to resolve 64 Zn from the potential interference peak due to the MgAr + ion. The reported values have had the procedural blank subtracted. Procedural seawater blanks were prepared the same way as samples using 1 mL low trace metal surface seawater from the 2004 SAFe cruise. The average blank value was 0.08 nmol L -1 and the average detection limit (measured as thrice the standard deviation of the blank) was 0.06 nmol L -1 . The accuracy of the method was evaluated by measuring GEOTRACES reference standards GS and GD (Johnson et al. 2007). The Zn values obtained by this method for GS and GD were 0.04 ± 0.007 nmol L -1 and 1.67 ± 0.002 nmol L -1 , respectively and were in agreement with certified consensus Zn values of 0.038 ± 0.011 nmol L -1 (GS) and 1.64 ± 0.22 nmol L -1 (GD). 5.4. Results 5.4.1. Biogeochemical properties Our transect across the transition zone (Fig. 5-1) covers three distinctive biogeochemical settings namely, warm, low salinity nutrient deplete shelf waters (P1), a complex transition zone (P2, P3, P4, P5, P6) and high nutrient oceanic zone (P8). This transect along Line P was aimed at capturing the transition between coastal and oceanic regimes and is very well characterized by 194 the changes in temperature and salinity as we move offshore (Fig. 5-3A & B). Station 1 is representative of coastal near shore processes with low salinity (31-33.7 psu) and low temperatures (7-10 o C) throughout the water column. A strong salinity gradient (Fig. 5-4A) was observed as we moved from P1 to P2-P3 (Fig. 5-2). Salinity remained stable at stations P2, P3, P4 and P5 while a shallow lens of low salinity water mass was present at stations P6 and P8. According to the Acoustic Doppler Current Profiler (ADCP) data (Fig. 5-2), this shallow lens with low salinity appears to be a feature of the warm eddy front that extended up to about 300m at P6. Stations P6 and P8 also have deeper "nutriclines" consistent with the temperature and nutrient data. The transect along Line P was primarily characterized by strong currents at P2 and P4 up to a depth of 150 m and a warm eddy front at P6 which extended up to 300m deep. The ADCP data at P2 (35m) hints at the presence of a northward flowing current, which could result in the disturbance of the bottom layer at the shelf, thereby providing a source for dissolved metals. These strong currents at P2, P4 and P6 were associated with changes in surface temperature and salinity. Temperature and salinity remained stable during sampling except at station P4 where variability in temperature and salinity was observed. Temperature and salinity profiles indicate the mixed layer depth to be around 50m (Fig. 5-3). Oceanographic sections of both oxygen (Fig. 5-4A) and nitrite (Fig. 5-4B) support the absence of an OMZ at this site. Oxygen concentrations did not decrease below 50 µmol L -1 at mid-depths and there was no secondary nitrite maxima present. Oceanographic sections for nitrate, phosphate and silicate distributions have been showed in Fig. 5-5. 5.4.2. Biology An overview of the biology of along the transect revealed that the coastal stations were dominated by picoeukaryotes while Synechococcus was abundant in open ocean waters. 195 Relatively small changes in cell abundances were observed except between P5 and P6. Large picoeukaryotes were observed at P6 and P4, which could be due to enhanced total dissolved concentrations brought in by the eddies observed. The deep chlorophyll max (DCM) was stationed around 30 m for the entire transect while P3 had the highest Chl A concentrations. For P4 and P5, DCM was deeper at around 40-50m. Nitrate concentrations increase as we move off shore from P1 to P8. Thalosiossira oceanica FLDA1 expression was highest in the off shore stations P5, 6, 8 while it was low from P1 through P3. In spite of increased levels of Fe at station P1, no luxury uptake of iron was observed. This supporting data for GEOMICS was obtained from http://armbrustlab.ocean.washington.edu/labdb/field/cruises/104/reports. 5.4.3. Total dissolved Fe (DFe) distribution along the transect The average dissolved iron (DFe) concentrations in the upper 100m of the study area ranged between 0.16–7.46 nmol L -1 (Table 1). The DFe depth profiles were characterized by a surface mixed layer enrichment, a sharp decrease to a subsurface minimum and consequent increase in concentrations at deeper depths. In surface waters, the highest DFe concentration was observed at P1 (1.27 nmol L -1 ), which decreased progressively with distance from the shore with a slight exception at P6 which had a higher DFe concentration of 0.64 nmol L -1 (Fig. 5-6A). These surface DFe maxima are not unusual and have been attributed to the effect of external sources of Fe, such as fluvial (Bruland et al. 2001) and aeolian inputs (Bruland et al. 1994; Chase et al. 2005; Johnson et al. 2003). These surface DFe maxima also correspond to high DMn and low DZn surface values which further reiterates the fact the high DFe concentrations are not a sampling artifact, instead could be attributed to aeolian input and/or advective slope and shelf sediment inputs. The lowest surface DFe concentration measured for the entire transect was at P8 (0.26 nmol L -1 ). 196 Typical low values were associated with the DCM (0.2 - 1.2 nmol L -1 ) at all off shore deep stations. Except shallow stations P1 and P2, the minima in DFe at all stations coincided with the DCM. This minimum in DFe, observed between 30-70m, was associated with values as low as 0.16 nmol L -1 (P8). The subsurface DFe minimum could be attributed to downward mixing and biological uptake or scavenging within the euphotic zone (Wu et al. 2001). The subsequent increase in DFe concentration with depth indicates that the iron associated with sinking particulates was being remineralized. Shallow stations P1 and P2 were characterized by enhanced DFe concentrations throughout all depths (0.4 - 7.8 nmol L -1 ) which indicate a benthic source at these near shore, shallow stations. Several studies have noted that benthic fluxes and the re-suspension of Fe-rich bottom sediments may be responsible for the pronounced DFe enrichments in continental slope and shelf waters (Bruland et al. 2005; Martin et al. 1989). Consistent with those studies, relatively high levels of DFe was measured in the sample collected close to the seafloor at P2 (7.46 nmol L -1 ). The DFe concentrations measured in bottom waters over the shelf (P1 and P2) at our study site were lower than those reported in subsurface shelf waters off Peru (> 50 nmol L -1 ; Bruland et al., 2005; > 70 nmol L -1 ; J. Vedamati; in prep), but similar to those reported for central Gulf of California, Mexico and Monterey Bay continental margin (7.7 nmol L -1 and 4.47 nmol L -1 ; (Martin et al. 1989; Segovia-Zavala et al. 2010). Below the mixed layer, the DFe decreased from inshore to offshore stations, except at P6 where DFe concentrations were elevated. At P6, high DFe concentrations were measured throughout the water column especially around 300 m indicating the role of the warm eddy present at the site (Fig. 5-2). Increased DFe concentrations at station P6 may be attributed to 197 inputs from the warm eddy from the north as noted in the ADCP data (Fig. 5-2). A plume of high DFe (7.46 nmol L -1 ) at 100m appears to extend from P2 into the water column. The DFe concentrations in this high Fe plume decreases gradually as we move off shore from P2 to P8. This high DFe plume is consistent with particulate Fe and particulate Al plumes (Fig. 5-7) (Twining, in prep), thereby hinting towards a continental shelf or slope sediment source..Coale et al. (1996) observed a similar maximum in the section of particulate aluminum along the FeLine transect and suggested that the particulate matter may be lithogenic in origin. ADCP data for this transect also indicates the presence of strong currents at P2 which extend to about 150m depth and could well be the reason for re-suspension and eventual advection of shelf/slope sediments leading to the formation of the high DFe plume. At 100m, DFe at P8 (1.04 nmol L -1 ) was ~ 7 fold lower than that at station P2 (7.46 nmol L -1 ). However, if compared to surface water, the decrease was not as abrupt and below 200 m, concentrations >1 nmol L -1 were still measured at both P6 and P8. Another very evident high DFe plume is also observed between 500-1000m at station P3 which gradually decreases as we move off shore. Elevated particulate Fe (Fig. 5-7A) and Al concentrations at this depth hint towards the resuspension of shelf slope sediments, which in turn could contribute towards the elevated DFe concentrations. DFe concentrations within this deep plume range between 1.42 - 5.42 nmol L -1 . Coincident with the DFe plume are the particulate Fe and Al plumes, which indicate strongly towards a shelf source for subsurface increased DFe concentrations. The presence of benthic nephloid layer could most likely explain the presence of elevated particulate Fe, Cu, Mn, Zn, Al, Ti along with amplified bacterial numbers at that depth. This is consistent with elevated concentrations of dissolved Fe, Cu, Mn, and Zn values at that 198 depth range reported here. Deep water DFe values below 1500 m at Stations P6 and P8 were around 1.5 nmol L -1 . 5.4.4. Total dissolved copper (DCu) distribution along the transect Depth profiles for Cu display the general features reported earlier in the central North Pacific: enhanced Cu concentrations in surface waters, decrease in concentrations in the upper thermocline and a gradual increase with depth (Boyle et al. 1977). These depth profiles exhibit general features of a nutrient like element and agree with previous data from the central North Pacific (Boyle et al. 1977). Total dissolved Cu concentrations (1.25 - 2.76 nmol L -1 ) were typically more elevated in the near shore shallow stations P1 and P2 (Fig. 5-6B; Table 2). These concentrations seem to agree well with Cu concentrations measured previously by Boyle (1977). Higher Cu closer to the coast indicates lateral transport from shelf waters and shelf and slope sediments. Lowest DCu concentrations in the surface waters were measured at P3 and P4 (1.02 & 1.13 nmol L -1 , respectively), which suggests extensive scavenging near the surface, likely the result of biological uptake. Interestingly, these stations P3 and P4 were also characterized by high Chl A concentrations indicating enhanced productivity leading to increased uptake. ADCP data (Fig. 5- 2) indicates a strong temperature and salinity gradient across these stations and provides information regarding a strong current present at P2 up to 150m depth. High particulate Cu concentrations (Fig. 5-7B) were evident at the surface of stations P3, P4 and P5 along with a distinct plume in particulate Cu at 600 - 900 m depth at P3. 199 All off stations (P1-P8) are characterized by a sharp decrease from high surface DCu values to a subsurface minimum between 25-50m. This pronounced minimum in the upper thermocline is coincident either within or just below the PNM. For these stations, the lowest DCu concentrations there, was significantly lower than the depths immediately above and below it, suggesting that processes at the base of the euphotic zone are related to these low values. Below the thermocline, there is a gradual increase in DCu concentrations. No drawdown in DCu concentrations coincident with lower oxygen concentrations at mid depths was observed. DCu values remained high (1.3-1.9 nmol L -1 ) between 100 – 600m depth. This is in sharp contrast to an observed distinct DCu drawdown within the core of the Arabian Sea and Peruvian OMZs (J. Vedamati, in prep.). A continental slope source of deep dissolved Cu at 800m at P3 appears to be the source of dissolved Cu to mid-depths (700-1500m). Below 1500m, DCu concentrations gradually increase to relatively constant values of ~ 3 nmol L -1 . 5.4.5. Total dissolved Mn (DMn) distribution along the transect Vertical depth profiles of scavenged type element - Mn exhibits similar features as observed elsewhere- elevated surface Mn concentrations, subsurface minima, slight increase below the pycnocline and constant values in deep water (Landing and Bruland 1980; Lewis and Luther 2000). Manganese concentrations are characterized by a distinct surface maxima at all stations of this transect. The surface maxima appears to be maintained by photochemical reduction processes and photoinhibition of microbial Mn oxidation (Sunda 1989; Sunda and Huntsman 1988). Surface Mn concentrations range between 3.6 - 9 nmol L -1 and decrease as we move westward away from the shelf (Table 5-3; Fig. 5-6C). Particulate Mn concentrations over the shelf are elevated with respect to the off shore stations but the concentrations (~ 3 nmol L -1 ) remain lower than the dissolved fraction (Fig. 5-7C). This could be due to the photoinhibition of 200 biologically mediated Mn oxidation (Sunda and Huntsman 1988). The highest DMn concentration (9.02 nmol L -1 ) was measured at the shallow, near shore station P1. Similar elevated DMn concentrations in samples closer to the continental shelf have been observed previously in the NE Pacific (Landing and Bruland 1980). These high DMn concentrations at the surface could not only be due to vertical diffusion from sediments alone since the concentrations at the surface are higher than those closer to the bottom. These elevated Mn concentrations can be attributed to enhanced supply of Mn oxides from the sediments followed by their photoreduction in the surface waters by humic substances (Sunda and Huntsman 1988; Sunda et al. 1983). Lowest near surface Mn concentration was observed at P6 (3.66 nmol L -1 ) while the lowest mixed layer DMn concentration was measured at farthest off shore station P8 (1.5 nmol L -1 ). High DMn concentrations (~2-3 nmol L -1 ) are observed throughout the water column at stations P3, P4 and P5 due to their proximity to the coast. At each station in this transect, there was a subsurface Mn maximum coinciding with the upper oxygen deficient zone centered around 125-150m, which gradually became deeper in the offshore stations. This increase in DMn concentrations appears to correspond to a decrease in particulate Mn hinting towards an increase in reduction of Mn oxides by humic substances during remineralisation (Sunda and Huntsman 1988). Rapid decrease of DMn concentrations below the subsurface maxima appears to be due to increased Mn scavenging onto particles. No mid depth DMn maxima, coincident with the oxygen minimum (O 2 ~ 50 µmol L -1 ) in the water column was observed. At 700m, a plume of DMn coincident with the particulate Mn maximum (Fig. 5-7C) exists which decreases as we move westward towards P8. This elevated DMn concentrations at mid depths could result due to low in-situ Mn oxide formation leading to elevated DMn concentrations. A mid-depth particulate Mn desert is distinctly observed below 201 250m which also corresponds to the region of enhanced DMn concentrations. This particulate Mn minimum also appears to contribute significantly to the elevated DMn concentrations by reducing manganese loss via particle scavenging and settling (Sunda and Huntsman 1988). In turn, low in-situ Mn oxide production could have led to lower particulate Mn concentrations. However, Station P6 is distinct with higher DMn concentrations between 1.28 - 1.75 nmol L -1 below 500m depth. The warm eddy front at P6 seems to have led to the shoaling of the high DMn concentration plume boundary upwards into the water column. This results in higher DMn concentrations below 500m. Even though not as distinct as the Fe plume at 500m, the Mn plume seems to have been somewhat masked by the higher DMn concentrations throughout the water column, even though we observe a slight decrease as we move off shore. Below 1500m, DMn concentrations stabilize at constant values 0.9-1.2 nmol L -1 due to high rate of scavenging. Corresponding to the decrease in DMn concentrations and an increase in oxygen concentrations, particulate Mn shows an increase below 1500m. 5.4.6. Total dissolved Zn (DZn) distributions along the transect Vertical depth profiles for total dissolved Zn (DZn) in the NE Pacific exhibit the general features of a nutrient-like element such as depletion of Zn in surface waters and a gradual increase with depth (Bruland et al. 1978; Bruland et al. 1994; Jakuba et al. 2012; Lohan et al. 2002; Martin et al. 1989). Oceanographically consistent profiles (Fig. 5-6D) with a silica-like distribution were obtained during this transect (Bruland 1980). At all stations, DZn depth profiles were characterized by surface depletion and deep enrichment. Highest surface DZn concentrations were measured at the near shore station P1 (0.76 nmol L -1 ) while it decreased as we moved off shore and ranged between (0.04 - 0.22 nmol L -1 ) at the others (Table 4). Higher DZn concentrations at near shore stations indicate advective 202 inputs via coastal and shelf sediments enhanced by small scale mixing processes. These values are consistent with previous zinc measurements at Line P (Lohan et al. 2002). Station P6 is characterized by the presence of elevated particulate Zn concentrations (Fig. 5-7D; 0.3-0.5 nmol L -1 ) which do not result in enhanced DZn concentrations. This could imply that the warm eddy at P6 could be the possible source of particulate Zn at the station. Total DZn concentrations range between 0.04 - 1.96 nmol L -1 in the top 100m of the water column. Indeed, it is noteworthy that the concentrations reported at these stations in the upper water column are some of the lowest oceanic values reported anywhere. At each station, except station P2, the lowest Zn concentrations were observed at the depth coincident with the DCM hinting towards a horizontal correspondence between Chl A and DZn concentrations. Lowest total dissolved Zn concentration for the entire transect was observed at station P3 at 28m (0.02 nmol L -1 ), which is coincidentally the station that exhibits the highest Chl A max at 40 m (9 µg L -1 ). Typical of a nutrient like element, total dissolved Zn concentrations increase below the mixed layer. Consistent with DFe concentrations, DZn concentrations at station P2 exhibit a sharp increase at 100m hinting towards a small scale mixing process resulting in a benthic source of DZn at this station. Wind induced upwelling along the eastern boundary off the coast of California has been reported to result in enhanced surface water zinc concentrations (Bruland 1980) and thus, small scale mixing processes at 100m at P2 could result in enhancement of DZn concentrations. Particulate zinc concentrations for the transect (Fig. 5-7D) show distinct regions of particulate enrichment at stations P2 and P6 and a deep water source of particulate Zn at station 203 P4. This is consistent with other particulate trace metal enrichment regions like Fe and hint towards an advective transport of particulate metals into the ocean due to small scale mixing processes. Between 500-1000m, DZn concentrations increased as we move off shore from P3 to P8 and there appears to be a plume moving towards the shore at that depth. This is in contrast to DFe concentrations at that depth range which decrease as we move away from the particulate Fe source (Fig. 5-7A). The highest DZn values throughout the water column was measured offshore at P8 (8.74 nmol L -1 at 1500m) while the lowest values were at P3 (0.02 nmol L -1 at 28m). Consistent with previously observed values (Bruland et al. 1994; Lohan et al. 2002), deep water DZn concentrations were ~ 8.7 nmol L -1 . Regeneration with depth results in a profile similar to silica. These DZn concentrations were the highest observed in the water column (~ 8.7 nmol L -1 ) and were consistent with values observed by several other researchers in the deep waters of the North-east Pacific (Bruland 1980; Bruland et al. 1978; Martin et al. 1989). Zn: Si ratio: Zinc and silica profiles show a significantly close correlation throughout the water column at all stations of this transect (Fig. 5-8). The overall linear relationship between zinc and silica is [Zn] = 0.054 [Si] - 0.144 (R 2 = 0.944) where Zn is in units of nmol/L and Si is in units of µmol/L. This expression further reiterates that zinc undergoes a deep regeneration cycle which corresponds to the oceanic silica cycle. Zinc in surface waters is drawn down and regenerated at depth. Separate linear regressions of data from the central North Pacific yield almost identical slopes (~ 0.054 nmol: µmol) (Bruland 1980). The Zn: Si relationship is generally linear but data from this transect as well as from (Jakuba et al. 2012) and (Martin et al. 1989) trend above the linear relationship in the mid values (50-100 204 µmol L -1 Si; 3-7 nmol L -1 Zn) . Lower Zn and Si values trend slightly below the linear relationship and are similar to previously observed relationships by (Martin et al. 1989) and (Jakuba et al. 2012). A surface depletion of silica is observed at all stations (~10 µM) with further depletion at stations P3 and P4 (~ 2 - 4 µM). This trend corresponds very well with values obtained along Line P at station P4 although the data reported here are further lower (Lohan et al. 2002). Zn : P ratio: The zinc and phosphate relationship (Fig. 5-9) gives an idea of the drawdown of nutrients in the water column and is generally referred to as the "disappearance ratio". Dissolved zinc and phosphate are not strongly correlated throughout the water column especially in deeper waters where the phosphate remains constant. In surface water, low Zn:P ratios are indicative of drawdown of zinc by phytoplankton and lead us to believe that zinc could be the limiting factor before phosphate gets taken up. However, the Zn: P correlation (R 2 = 0.89) is stronger at mid- values where there is a direct relationship between zinc and phosphate drawdown. This was similar to the close correlation (R 2 > 0.9) found between Zn and P in the biologically active upper ocean by (Croot et al. 2011) in transects along the Zero meridian and across the Drake Passage. This indicates a close coupling of zinc and nutrient remineralisation in the upper water column. 5.5. Discussion We have investigated the behavior and distribution of Fe, Cu and Mn in upwelling dominated and redox active, oxygen minimum zones and to compare, these redox active trace metals were also studied in a non-OMZ oceanographic setting in the NE Pacific along the well-studied Line P. Both of these regimes differ widely in their water column circulation patterns, detrital inputs, 205 primary productivity and oxygen depletion at mid-depths. Some of the major differences in their behavior are summarized as below: 5.5.1. Mn:Fe ratios Mn:Fe ratios in reducing regimes throw light on important differences in the reductive release, deposition, transport and redox cycling of these elements. Although the redox behavior of both Fe and Mn has been studied previously, this is the first attempt at comparing it to a non- redox environment. Fe and Mn are both redox active metals and used as indicators of redox activity. However, the presence of a similar plume coinciding with the SNM at mid-depths of the OMZ leads us to believe that they behave similarly under redox conditions. This also leads us to believe that high concentrations of both Fe and Mn would be expected on a shallow transect off the highly reducing sediments underlying the suboxic zone off of Peru. Moreover, it is well established that the Mn and Fe flux across the benthic boundary is driven by reductive dissolution of reactive Mn and Fe oxyhydroxides in the surface sediments (Burdige 1993; Froelich et al. 1979; Pakhomova et al. 2007). So, high concentrations of both Fe and Mn was expected in samples off the shallow transect on the continental shelf off of Peru. As predicted, exceedingly high dissolved Fe concentrations (~50 - 75 nmol L -1 ) were measured off the Peruvian shelf while DFe concentrations an order of magnitude lower were observed in our transect along Line P. Recent studies have shown that Fe cycling in sediments and bottom waters under such reducing, suboxic conditions off the Peruvian margin is much tightly regulated in comparison to Mn (Scholz et al. 2011). In contrast, slow oxidation kinetics and shallower reduction zone for Mn leads to diffusion of Mn into the water column resulting in lower Mn: Al ratios at all regions on the continental shelf. Thus, the effective "trapping" of Fe in the sediments underlying the OMZ, results in distinct differences in the distribution patterns of 206 redox sensitive elements, Fe and Mn, which were previously thought to behave similarly under low oxygen conditions. In contrast, such mechanism might not be in play in sediments underlying the continental margin off NE Pacific. Under such non-suboxic conditions, both Fe and Mn are released in reasonable amounts leading to higher ratios of Mn:Fe as compared to the reducing, suboxic regime along the Peruvian coast. Off the Peruvian coast, Fe behaved as expected, however low Mn values at the same sites seemed perplexing. Thus, this complex interplay of reductive release, lateral transport, recycling and deposition creates a very dynamic system for Mn and Fe. 5.5.2. Fe subsurface plumes Significant concentrations of DFe and Fe(II) have been measured previously in oxygen deficient waters in the OMZs (Hopkinson and Barbeau 2007; Moffett et al. 2007). Similar subsurface DFe and Fe (II) plumes were also observed in the Peruvian upwelling zone along three transects off the continental coast (J. Vedamati; in prep.). In contrast, no such distinct subsurface DFe plume coinciding with low oxygen layers was observed in our transect in the NE Pacific. This key contrast in the behavior and distribution of dissolved Fe in these contrasting regimes indicates a strong role of reducing, suboxic environment within the OMZs in facilitating transport of Fe into the deeper ocean. This finding could have important implications since Fe is a key element required in the denitrification processes and in several other biological cycles. It has also been predicted that fluctuations in seasonal or perennial shelf hypoxia may intensify due to climate change and anthropogenically induced eutrophication (Diaz 2001; Diaz and Rosenberg 2008; Stramma et al. 2008) thereby resulting in a change in the distribution of Fe in the subsurface oceanic regimes. 207 5.5.3. Copper distribution in a non-suboxic setting Depth profiles for Cu obtained along Line P and in off shore stations outside the Arabian Sea OMZ (Stations 11, 15, 16, 18, 20, & 21) display the general features reported earlier in the central North Pacific: enhanced Cu concentrations in surface waters, decrease in concentrations in the upper thermocline and a gradual increase with depth (Boyle et al. 1977). These depth profiles exhibit general features of a nutrient like element and agree with previous data from the central North Pacific (Boyle et al. 1977). The most striking feature in the distribution of Cu in the NE Pacific is the lack of a distinct draw down in DCu concentrations at mid-depths coinciding with low oxygen (but not yet suboxic) of the water column. In our study of Cu in the OMZs (both Peru and in the Arabian Sea), a strong depletion of total dissolved Cu concentrations coincident with the SNM present within the oxygen minimum core was observed. No such phenomenon was observed in DCu concentrations in stations outside the denitrification zone of the OMZs. The presence of a cryptic sulfur cycle and the formation of sulfide precipitates within the OMZ were accounted for the low DCu concentrations at mid-depths (Canfield et al. 2010; Theberge et al. 1997). Due to its proximity to the coast, DCu values at stations right off the shelf at both equatorial and the NE Pacific seemed to be affected by lateral transport from shelf waters and shelf and slope sediments. However, DCu concentrations (1.25 - 2.76 nmol L -1 ) were higher at P1 and P2 in the NE Pacific than at stations along transects across the Peruvian continental shelf (0.97 – 2.2 nmol L -1 ). The presence of sulfides and eventual precipitation of copper sulfide precipitates could most likely explain the lower concentrations along transect 1 and 2 off Peru. Higher DCu concentrations along transect 3 could be attributed to the influence of copper smelters along the Chile and Peru coasts (Hawkins et al. 2010). 208 5.5.4. Evidence for coupled geochemistry? Based on comparison of Fe, Cu and Mn profiles within an OMZ with those outside an OMZ, seems to hint at the fact that none of these elements seemed to have a coupled geochemistry. All these elements seem to behave independently of each other and mostly are influenced by local physical dynamics, sediment composition and most importantly by the presence or absence of an OMZ. The interaction of the benthic layer on the continental shelf and the suboxic water column seems to play an important role in regulating the cycling of these redox-sensitive and bioactive trace elements. Within the OMZ, each of these element profiles exhibits unique characteristics in its distribution that is somewhat related to the low oxygen concentrations at mid depths. 209 5.6. References Boyle, E. A., F. R. Sclater, and J. M. Edmond. 1977. DISTRIBUTION OF DISSOLVED COPPER IN PACIFIC. Earth Planet. Sci. Lett. 37: 38-54. Bruland, K. W. 1980. OCEANOGRAPHIC DISTRIBUTIONS OF CADMIUM, ZINC, NICKEL, AND COPPER IN THE NORTH PACIFIC. Earth Planet. Sci. Lett. 47: 176- 198. Bruland, K. W., G. A. Knauer, and J. H. Martin. 1978. ZINC IN NORTHEAST PACIFIC WATER. Nature 271: 741-743. Bruland, K. W., K. J. Orians, and J. P. Cowen. 1994. REACTIVE TRACE-METALS IN THE STRATIFIED CENTRAL NORTH PACIFIC. Geochim. Cosmochim. Acta 58: 3171- 3182. Bruland, K. W., E. L. Rue, and G. J. Smith. 2001. Iron and macronutrients in California coastal upwelling regimes: Implications for diatom blooms. Limnol. Oceanogr. 46: 1661-1674. Bruland, K. W., E. L. Rue, G. J. Smith, and G. R. Ditullio. 2005. Iron, macronutrients and diatom blooms in the Peru upwelling regime: brown and blue waters of Peru. Mar. Chem. 93: 81-103. Burdige, D. J. 1993. The biogeochemistry of manganese and iron reduction in marine sediments. Earth-Science Reviews 35: 249-284. Canfield, D. E. and others 2010. A Cryptic Sulfur Cycle in Oxygen-Minimum-Zone Waters off the Chilean Coast. Science 330: 1375-1378. Chase, Z. and others 2005. Manganese and iron distributions off central California influenced by upwelling and shelf width. Mar. Chem. 95: 235-254. 210 Coale, K. H., S. E. Fitzwater, R. M. Gordon, K. S. Johnson, and R. T. Barber. 1996. Control of community growth and export production by upwelled iron in the equatorial Pacific Ocean. Crawford, W. R., P. J. Brickley, and A. C. Thomas. 2007. Mesoscale eddies dominate surface phytoplankton in northern Gulf of Alaska. Prog. Oceanogr. 75: 287-303. Croot, P. L., O. Baars, and P. Streu. 2011. The distribution of dissolved zinc in the Atlantic sector of the Southern Ocean. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 58: 2707- 2719. Cutter, G. A., and K. W. Bruland. 2012. Rapid and noncontaminating sampling system for trace elements in global ocean surveys. Limnol. Oceanogr. Meth. 10: 425-436. Diaz, R. J. 2001. Overview of hypoxia around the world. Journal of environmental quality 30: 275-281. Diaz, R. J., and R. Rosenberg. 2008. Spreading dead zones and consequences for marine ecosystems. Science 321: 926-929. Field, M. P., J. T. Cullen, and R. M. Sherrell. 1999. Direct determination of 10 trace metals in 50 mu L samples of coastal seawater using desolvating micronebulization sector field ICP- MS. J. Anal. At. Spectrom. 14: 1425-1431. Froelich, P. and others 1979. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochim. Cosmochim. Acta 43: 1075- 1090. Harris, S. L., D. E. Varela, F. W. Whitney, and P. J. Harrison. 2009. Nutrient and phytoplankton dynamics off the west coast of Vancouver Island during the 1997/98 ENSO event. Deep Sea Research Part II: Topical Studies in Oceanography 56: 2487-2502. 211 Hawkins, L. N., L. M. Russell, D. S. Covert, P. K. Quinn, and T. S. Bates. 2010. Carboxylic acids, sulfates, and organosulfates in processed continental organic aerosol over the southeast Pacific Ocean during VOCALS-REx 2008. J. Geophys. Res.-Atmos. 115. Hongo, Y., H. Obata, D. S. Alibo, and Y. Nozaki. 2006. Spatial variations of rare earth elements in North Pacific surface water. J. Oceanogr. 62: 441-455. Hopkinson, B. M., and K. A. Barbeau. 2007. Organic and redox speciation of iron in the eastern tropical North Pacific suboxic zone. Mar. Chem. 106: 2-17. Jakuba, R. W., J. W. Moffett, and S. T. Dyhrman. 2008. Evidence for the linked biogeochemical cycling of zinc, cobalt, and phosphorus in the western North Atlantic Ocean. Glob. Biogeochem. Cycle 22. Jakuba, R. W., M. A. Saito, J. W. Moffett, and Y. Xu. 2012. Dissolved zinc in the subarctic North Pacific and Bering Sea: Its distribution, speciation, and importance to primary producers. Glob. Biogeochem. Cycle 26. Johnson, K. S. and others 2007. Developing standards for dissolved iron in seawater. Eos, Transactions American Geophysical Union 88: 131-132. Johnson, K. S. and others 2003. Surface ocean-lower atmosphere interactions in the Northeast Pacific Ocean Gyre: Aerosols, iron, and the ecosystem response. Glob. Biogeochem. Cycle 17. Landing, W. M., and K. W. Bruland. 1980. MANGANESE IN THE NORTH PACIFIC. Earth Planet. Sci. Lett. 49: 45-56. Lee, J. M., E. A. Boyle, Y. Echegoyen-Sanz, J. N. Fitzsimmons, R. F. Zhang, and R. A. Kayser. 2011. Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batch 212 nitrilotriacetate resin extraction and isotope dilution inductively coupled plasma mass spectrometry. Anal. Chim. Acta 686: 93-101. Lewis, B. L., and G. W. Luther. 2000. Processes controlling the distribution and cycling of manganese in the oxygen minimum zone of the Arabian Sea. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 47: 1541-1561. Lohan, M. C., P. J. Statham, and D. W. Crawford. 2002. Total dissolved zinc in the upper water column of the subarctic North East Pacific. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 49: 5793-5808. Martin, J. H., R. M. Gordon, S. Fitzwater, and W. W. Broenkow. 1989. VERTEX - PHYTOPLANKTON IRON STUDIES IN THE GULF OF ALASKA. Deep-Sea Research Part a-Oceanographic Research Papers 36: 649-&. Moffett, J. W., T. J. Goeffert, and S. W. A. Naqvi. 2007. Reduced iron associated with secondary nitrite maxima in the Arabian Sea. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 54: 1341- 1349. Pakhomova, S. V., P. O. Hall, M. Y. Kononets, A. G. Rozanov, A. Tengberg, and A. V. Vershinin. 2007. Fluxes of iron and manganese across the sediment–water interface under various redox conditions. Mar. Chem. 107: 319-331. Pena, M. A., and S. J. Bograd. 2007. Time series of the northeast Pacific. Prog. Oceanogr. 75: 115-119. Ribalet, F. and others 2010. Unveiling a phytoplankton hotspot at a narrow boundary between coastal and offshore waters. Proc. Natl. Acad. Sci. U. S. A. 107: 16571-16576. Saito, M. A., and D. L. Schneider. 2006. Examination of precipitation chemistry and improvements in precision using the Mg (OH)< sub> 2</sub> preconcentration 213 inductively coupled plasma mass spectrometry (ICP-MS) method for high-throughput analysis of open-ocean Fe and Mn in seawater. Anal. Chim. Acta 565: 222-233. Scholz, F., C. Hensen, A. Noffke, A. Rohde, V. Liebetrau, and K. Wallmann. 2011. Early diagenesis of redox-sensitive trace metals in the Peru upwelling area - response to ENSO- related oxygen fluctuations in the water column. Geochim. Cosmochim. Acta 75: 7257- 7276. Segovia-Zavala, J. A., M. L. Lares, F. Delgadillo-Hinojosa, A. Tovar-Sanchez, and S. A. Sanudo-Wilhelmy. 2010. Dissolved iron distributions in the central region of the Gulf of California, Mexico. Deep-Sea Res. Part I-Oceanogr. Res. Pap. 57: 53-64. Stramma, L., G. C. Johnson, J. Sprintall, and V. Mohrholz. 2008. Expanding oxygen-minimum zones in the tropical oceans. Science 320: 655-658. Sunda, W. G. 1989. EFFECT OF SUNLIGHT ON THE REDOX CYCLING OF MANGANESE IN NEAR-SURFACE SEAWATER. Abstr. Pap. Am. Chem. Soc. 198: 100-GEOC. Sunda, W. G., and S. A. Huntsman. 1988. EFFECT OF SUNLIGHT ON REDOX CYCLES OF MANGANESE IN THE SOUTHWESTERN SARGASSO SEA. Deep-Sea Research Part a-Oceanographic Research Papers 35: 1297-1317. Sunda, W. G., S. A. Huntsman, and G. R. Harvey. 1983. PHOTO-REDUCTION OF MANGANESE OXIDES IN SEAWATER AND ITS GEOCHEMICAL AND BIOLOGICAL IMPLICATIONS. Nature 301: 234-236. Taylor, F., and R. Haigh. 1996. Spatial and temporal distributions of microplankton during the summers of 1992 1993 in Barkley Sound, British Columbia, with emphasis on harmful species. Can. J. Fish. Aquat. Sci. 53: 2310-2322. 214 Theberge, S. M., G. W. Luther, and A. M. Farrenkopf. 1997. On the existence of free and metal complexed sulfide in the Arabian Sea and its oxygen minimum zone. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 44: 1381-1390. Whitney, F. A., W. R. Crawford, and P. Harrison. 2005. Physical processes that enhance nutrient transport and primary productivity in the coastal and open ocean of the subarctic NE Pacific. Deep-Sea Res. Part II-Top. Stud. Oceanogr. 52: 681-706. Wu, J. F., A. Aguilar-Islas, R. Rember, T. Weingartner, S. Danielson, and T. Whitledge. 2009. Size-fractionated iron distribution on the northern Gulf of Alaska. Geophys. Res. Lett. 36. Wu, J. F., E. Boyle, W. Sunda, and L. S. Wen. 2001. Soluble and colloidal iron in the olgotrophic North Atlantic and North Pacific. Science 293: 847-849. Wu, J. F., and E. A. Boyle. 1998. Determination of iron in seawater by high-resolution isotope dilution inductively coupled plasma mass spectrometry after Mg(OH)(2) coprecipitation. Anal. Chim. Acta 367: 183-191. 215 Table 5-1: Total dissolved Fe concentrations in nmol L -1 Station Depth (m) [Fe] (nmol L -1 ) P1 10 1.28 ± 0.08 15 1.43 ± 0.07 30 1.19 ± 0.04 40 0.87 ± 0.05 70 2.82 ± 0.01 90 0.76 ± 0.02 P2 17 0.37 ± 0.02 22 0.55 ± 0.09 30 0.33 ± 0.02 40 0.39 ± 0.02 75 2.79 ± 0.05 100 7.47 ± 0.04 P3 20 0.49 ± 0.01 28 0.36 ± 0.00 45 0.94 ± 0.17 75 1.65 ± 0.03 100 2.87 ± 0.05 125 2.45 ± 0.03 200 3.28 ± 0.05 300 3.64 ± 0.06 500 4.28 ± 0.03 750 5.23 ± 0.17 780 5.54 ± 0.29 P4 20 0.64 ± 0.06 40 0.41± 0.08 55 0.73 ± 0.07 75 1.27 ± 0.09 100 2.55 ± 0.04 150 1.99 ± 0.04 200 2.92 ± 0.01 300 3.25 ± 0.03 500 5.42 ± 0.01 750 2.66 ± 0.06 1000 2.85 ± 0.08 P5 30 0.44 ± 0.02 40 0.40 ± 0.00 50 0.17 ± 0.01 75 0.54 ± 0.01 100 2.17 ± 0.01 216 Station Depth (m) [Fe] (nmol L -1 ) 125 2.54 ± 0.01 150 2.05 ± 0.02 200 2.31 ± 0.04 300 2.38 ± 0.06 P5 500 3.25 ± 0.03 750 2.04 ± 0.00 1000 2.59 ± 0.01 1500 2.90 ± 0.00 P6 20 0.46 ± 0.04 30 0.43 ± 0.01 45 1.09 ± 0.03 75 2.58 ± 0.03 100 2.79 ± 0.01 125 2.44 ± 0.03 150 2.47 ± 0.01 200 2.36 ± 0.18 300 3.06 ± 0.04 500 2.75 ± 0.02 750 1.76 ± 0.01 1000 2.52 ± 0.02 1500 2.65 ± 0.01 2000 1.49 ± 0.03 P8 20 0.27 ± 0.01 33 0.17 ± 0.01 50 0.16 ± 0.03 70 0.30 ± 0.06 100 1.04 ± 0.00 125 1.34 ± 0.03 150 1.90 ± 0.00 200 2.13 ± 0.01 500 1.43 ± 0.03 750 1.63 ± 0.01 1000 2.43 ± 0.02 1500 2.13 ± 0.01 2000 1.70 ± 0.02 217 Table 5-2: Total dissolved Cu concentrations in nmol L -1 Station Depth (m) [Cu] (nmol L -1 ) P1 10 2.76 ± 0.08 15 2.49 ± 0.07 30 1.73 ± 0.04 40 1.50 ± 0.05 70 1.49 ± 0.01 90 1.34 ± 0.02 P2 17 1.74 ± 0.05 22 1.26 ± 0.03 30 1.25 ± 0.01 40 1.32 ± 0.03 75 1.88 ± 0.08 100 1.73 ± 0.05 P3 20 1.02 ± 0.03 28 0.83 ± 0.03 45 0.89 ± 0.02 75 1.00 ± 0.01 100 1.16 ± 0.03 125 1.24 ± 0.03 200 1.32 ± 0.03 300 1.41 ± 0.03 500 1.63 ± 0.00 750 1.50 ± 0.05 780 1.60 ± 0.00 P4 20 1.13 ± 0.00 40 0.70 ± 0.01 55 0.80 ± 0.01 75 0.92 ± 0.00 100 1.00 ± 0.01 150 1.16 ± 0.03 200 1.49 ± 0.01 300 1.53 ± 0.03 500 1.61 ± 0.01 750 1.93 ± 0.03 1000 1.87 ± 0.03 P5 30 1.38 ± 0.01 40 1.36 ± 0.02 50 1.03 ± 0.00 75 1.22 ± 0.01 100 1.33 ± 0.01 218 Station Depth (m) [Cu] (nmol L -1 ) 125 1.54 ± 0.02 150 1.41 ± 0.01 200 1.48 ± 0.04 300 1.63 ± 0.00 500 1.62 ± 0.03 750 1.80 ± 0.02 1000 2.06 ± 0.01 1500 2.45 ± 0.00 20 1.35 ± 0.02 30 1.34 ± 0.02 45 1.18 ± 0.01 75 1.35 ± 0.00 100 1.37 ± 0.01 125 1.42 ± 0.03 150 1.49 ± 0.01 200 1.46 ± 0.04 300 1.68 ± 0.01 500 1.71 ± 0.02 750 1.83 ± 0.01 1000 1.84 ± 0.02 1500 2.55 ± 0.05 2000 3.26 ± 0.10 P8 20 1.39 ± 0.04 33 1.05 ± 0.01 50 1.09 ± 0.01 70 1.14 ± 0.00 100 1.45 ± 0.01 125 1.41 ± 0.01 150 1.45 ± 0.04 200 1.57 ± 0.01 500 1.87 ± 0.07 750 1.79 ± 0.01 1000 2.14 ± 0.01 1500 2.56 ± 0.02 2000 3.04 ± 0.02 219 Table 5-3: Total dissolved Mn concentrations in nmol L -1 Station Depth (m) [Mn] (nmol L -1 ) P1 10 9.03 ± 0.15 15 8.92 ± 0.02 30 5.32 ± 0.02 40 4.06 ± 0.04 70 2.77 ± 0.04 90 3.94 ± 0.07 P2 17 5.62 ± 0.09 22 1.75 ± 0.00 30 1.64 ± 0.03 40 1.39 ± 0.02 75 3.39 ± 0.11 100 2.15 ± 0.11 P3 20 4.76 ± 0.02 28 2.88 ± 0.13 45 2.53 ± 0.04 75 2.36 ± 0.02 100 2.35 ± 0.01 125 2.77 ± 0.01 200 2.02 ± 0.01 300 1.66 ± 0.01 500 1.93 ± 0.03 750 2.17 ± 0.02 780 2.20 ± 0.01 P4 20 4.31 ± 0.14 40 2.22 ± 0.03 55 2.10 ± 0.09 75 1.92 ± 0.04 100 1.74 ± 0.02 150 2.31 ± 0.03 200 2.28 ± 0.02 300 1.82 ± 0.04 500 2.18 ± 0.04 750 2.05 ± 0.01 1000 2.15 ± 0.01 P5 30 4.84 ± 0.00 40 4.43 ± 0.07 50 1.91 ± 0.02 75 1.85 ± 0.03 100 2.29 ± 0.04 220 Station Depth (m) [Mn] (nmol L -1 ) 125 2.45 ± 0.01 150 1.86 ± 0.00 200 1.91 ± 0.00 300 1.52 ± 0.03 500 1.91 ± 0.05 750 1.76 ± 0.02 1000 2.13 ± 0.05 1500 2.17 ± 0.01 P6 20 3.66 ± 0.01 30 3.05 ± 0.01 45 2.10 ± 0.06 75 2.45 ± 0.02 100 1.97 ± 0.03 125 2.36 ± 0.01 150 1.94 ± 0.02 200 1.56 ± 0.00 300 1.44 ± 0.03 500 1.68 ± 0.02 750 1.25 ± 0.06 1000 1.76 ± 0.01 1500 1.63 ± 0.02 2000 0.87 ± 0.02 P8 20 4.13 ± 0.02 33 1.89 ± 0.02 50 1.73 ± 0.03 P8 70 1.54 ± 0.05 100 1.64 ± 0.03 125 1.44 ± 0.02 150 1.79 ± 0.02 200 1.78 ± 0.01 500 1.28 ± 0.02 750 1.36 ± 0.00 1000 1.47 ± 0.01 1500 1.45 ± 0.02 2000 1.25 ± 0.03 221 Table 5-4: Total dissolved Zn concentrations in nmol L -1 Station Depth (m) [Zn] (nmol L -1 ) P1 10 0.76 ± 0.00 15 0.43 ± 0.01 30 0.65 ± 0.00 70 0.65 ± 0.00 90 0.21 ± 0.00 P2 17 0.06 ± 0.00 22 0.16 ± 0.01 30 0.04 ± 0.01 40 0.17 ± 0.02 75 1.09 ± 0.00 100 1.80 ± 0.00 P3 20 0.15 ± 0.00 28 0.03 ± 0.00 45 0.20 ± 0.01 75 0.79 ± 0.01 100 1.65 ± 0.00 125 2.06 ± 0.02 200 3.09 ± 0.00 300 3.92 ± 0.00 500 5.09 ± 0.00 750 5.31 ± 0.01 780 5.44 ± 0.00 P4 20 0.14 ± 0.01 40 0.09 ± 0.01 55 0.12 ± 0.00 75 0.26 ± 0.00 100 1.97 ± 0.01 150 2.39 ± 0.02 200 3.70 ± 0.04 300 4.55 ± 0.01 P4 500 4.80 ± 0.11 750 4.93 ± 0.15 1000 5.20 ± 0.11 P5 30 0.12 ± 0.00 40 0.08 ± 0.00 50 0.08 ± 0.01 75 0.17 ± 0.04 100 0.95 ± 0.01 125 1.57 ± 0.02 222 Station Depth (m) [Zn] (nmol L -1 ) 150 1.71 ± 0.02 200 3.15 ± 0.02 300 4.04 ± 0.01 500 5.38 ± 0.01 750 6.15 ± 0.06 1000 6.77 ± 0.04 1500 7.71 ± 0.05 P6 20 0.04 ± 0.01 30 0.03 ± 0.02 45 0.17 ± 0.00 75 0.93 ± 0.00 100 1.53 ± 0.00 125 2.41 ± 0.01 150 2.12 ± 0.00 200 3.75 ± 0.02 300 3.96 ± 0.05 500 6.22 ± 0.03 750 6.54 ± 0.03 1000 6.14 ± 0.05 1500 8.14 ± 0.04 2000 8.32 ± 0.01 P8 20 0.22 ± 0.00 33 0.20 ± 0.01 50 0.03 ± 0.01 70 0.09 ± 0.02 100 1.06 ± 0.02 125 2.11 ± 0.01 150 2.37 ± 0.01 200 3.25 ± 0.01 500 6.69 ± 0.00 750 6.35 ± 0.04 1000 6.15 ± 0.03 1500 8.74 ± 0.00 2000 8.28 ± 0.00 223 Fig.5-1. Map of the cruise track along Line P in the North East Pacific off of Seattle that was sampled in May 2012 aboard R/V Thompson. P1 P5 P6 P4 P8 P3 P2 P8 224 Fig.5-2. Currents in the upper 350 m during our transect along Line P. Data collected by Acoustic Doppler Current Profiler (ADCP). 225 Fig.5-3. Upper 2500m oceanographic sections of (A) temperature and (B) salinity along Line P. A. B. 226 Fig.5-4. Upper 2500m oceanographic sections of (A) oxygen and (B) nitrite along Line P. A. B. 227 Fig.5-5. Upper 2500m oceanographic sections of (A) nitrate; (B) phosphate and (C) silicate along Line P. A. C. B. 228 Fig.5-6. Upper 2500m oceanographic sections of (A) total dissolved Fe; (B) total dissolved Cu; (C) total dissolved Mn and (D) total dissolved Zn along Line P. A. D. C. B. 229 Fig.5-7. Upper 2500m oceanographic sections of (A) particulate Fe; (B) particulate Cu; (C) particulate Mn and (D) particulate Zn along Line P. Partculate metals have been measured by B. Twining (in prep.). D. C. A. B. 230 Fig.5-8. Total dissolved Zn concentrations vs. silicate from all stations along Line P. Zn vs Si Si ( mol/L) 0 20 40 60 80 100 120 140 160 180 Zn (nmol/L) 0 2 4 6 8 10 Zn (nM) = 0.054 Si ( M) - 0.144 R 2 = 0.944 231 Fig.5-9. Total dissolved Zn concentrations vs. phosphate from all stations along Line P. Zn vs P P ( mol/L) 0 1 2 3 4 Zn(nmol/L) 0 2 4 6 8 10 y = 2.573x - 3.40 R 2 = 0.894 232 Summary We have investigated the impact of low oxygen, reducing conditions prevalent within the OMZ and the influence of eastern boundaries as a source of redox sensitive, bioactive trace metals such as Fe, Mn and Cu, in the three primary OMZs of the world’s oceans – the Eastern tropical South Pacific, eastern tropical north pacific and in the Arabian Sea. Key findings from this study have been summarized below. Fe in the OMZs Vertical profiles of total dissolved iron (DFe), Fe(II) and hydrographic parameters were obtained along three near shore transects across the Peruvian continental shelf in October- November 2005 and in five stations at the Costa Rica Upwelling Dome during June-July 2010. One of the primary findings in our study of the behavior and distribution of Fe within these OMZs was the presence of a well- defined Fe(II) peak coincident with the secondary nitrite maximum (SNM). Along our transects off of Peru, all stations were well-defined by the presence of a double peak in DFe and Fe(II) coincident with a double SNM or broad SNM peak at the OMZ core. Such a peak in Fe(II) coincident with the SNM has been previously reported by several other researchers and its presence in reducing regimes during our study implies that it might be a permanent feature present at low oxygen depths. Across the continental shelf off the Peruvian upwelling regime, large variability in DFe and Fe(II) concentrations were observed near the bottom closer to the sediments depending on the size of the shelf. In nearshore stations off the broad continental shelf along the northern and central transects, exceedingly high DFe (~ 50 - 75 nmol L -1 ) and Fe(II) concentrations (~ 90 nmol L -1 ) were measured while lower values (~ 4-6 nmol L -1 ; <1 nmol L -1 ) were observed at nearshore stations off the narrow southern shelf. The Fe(II):DFe ratio coincident with the SNM 233 decreased as we moved southwards away from the upwelling core towards the narrow continental shelf. Elevated surface concentrations of both DFe and Fe (II) were observed at the offshore stations in the southern transect and probably denote the difference in Fe uptake, based on primary productivity and carbon uptake rates. These high Fe (II) concentrations associated with the shallow transects hint towards a strong role of reduced Fe in maintaining a constant supply of bioavailable Fe along with the importance of the continental shelf width as a sizeable source of DFe into a coastal, high productivity upwelling system. Mn in the OMZs Vertical profiles for total dissolved Mn (DMn), nitrite and hydrographic data were obtained in the eastern tropical South Pacific in October 2005 and February 2010. Transects in October 2005 sampled the shelf slope interaction (75 o W–79 o W) within the Peruvian EEZ during the highest upwelling period while transects in February 2010 sampled the region of active denitrification (80 o W–100 o W) outside of the Peruvian EEZ during period of highest primary productivity. Mn was enriched within both the OMZs, but the distribution paths differed significantly from Fe probably reflecting slower Mn oxidation kinetics. Mn distribution in near shore transects off the Peruvian coast seemed to be due to both- advective processes from continental margins and in-situ processes. In most stations, the DMn concentrations were elevated throughout the water column with a slight increase in the OMZ. Near shore values seem to be highly influenced by advection from coastal sediments which lead to higher values throughout the water column. The more off shore values seem to be a result of both in-situ and advection processes. In the northern off-shore transect during the ETSP cruise in 2010, stations closest to the shelf exhibit high DMn concentrations at all depths, reflecting lateral advection from the margin and 234 partially obscuring the feature present within the secondary nitrite maximum. Interestingly, a distinct “DMn plume” is evident in the southern section (AT-15-61), indicating that a secondary nitrite maximum is not a prerequisite for sharp subsurface maxima in DMn, in contrast to total dissolved Fe. Our results indicate that Mn is primarily decoupled from Fe, which shows strong gradients decreasing from east to west consistent with inputs from the eastern boundary. In contrast, DMn concentrations were lower over the shelf in 2005 (~1.8 nmol L -1 ) and increased off shore in 2010 (~2 nmol L -1 ). DMn concentrations over the Peruvian shelf showed no increase towards the bottom and there was little correlation with dissolved oxygen and dissolved Fe concentrations. During upwelling conditions in October 2005, surface Mn concentrations were lower over the northern broad shelf (1.6 nmol L -1 ) as compared to that over the southern narrow shelf (3.4 nmol L -1 ). Results suggest that Mn is controlled by a combination of oxidative and reductive processes, benthic and atmospheric inputs that are poorly constrained. Copper in the OMZs To compare and assess the extent of the influence of oxygen minimum conditions combined with eastern boundary inputs from near shore reducing sediments, total dissolved copper (DCu) distributions were studied and compared in an offshore transect across the Arabian Sea OMZ (September 2007) and along three near shore transects off the Peruvian coast (October 2005). A pronounced minimum in the upper thermocline, either coincident or just below the PNM, was observed at most of the stations during our study. This Cu feature is probably maintained by a combination of biological uptake by ammonia oxidizers and nitrate reducers in the upper water column along with general scavenging with subsequent incorporation into and removal by zooplanktonic fecal material. 235 A strong drawdown of DCu within the OMZ coincident with the SNM was observed in off shore stations at both our study sites. In our transect through the Arabian Sea OMZ, a distinct draw down in DCu concentrations ( ~ 0.8 nmol L -1 ) was observed at mid-depths coincident with the secondary nitrite maximum (SNM) while no such feature was present in stations outside the denitrification zone. A similar feature was also observed in off shore stations along our three transects off the Peruvian OMZ. Although, clear drawdown of DCu in the OMZ was noticed in off shore stations from the Arabian Sea transect and the Peruvian margin transects, no such clear correlation between DCu concentrations and SNM was observed in near shore stations right off the shelf (Stations 21, 22, 23, & 29). The low concentrations of DCu coincident with the SNM could thus impose significant constraints on the rates of significant OMZ processes such as the N cycle, especially in regions which are known to have a disproportionately large impact on the biogeochemical cycling of nitrogen in the oceans. Overall surface DCu concentrations in near shore transects off the Peruvian shelf were higher (~ 0.9 – 2.2 nM) than those measured in our offshore transect in the Arabian Sea (~0.7 – 1.9 nM). Also in our transects off of Peru, higher surface DCu concentrations over the southern narrow shelf (~ 1.2 nmol L -1 ) as compared to that over the northern broad Peruvian shelf (~ 0.97 nmol L -1 ) were attributed to the presence of copper smelters along the Peruvian and Chilean coasts that could impact particle concentrations in the aerosols, especially on the southern region of the Peruvian coast. Overall high DCu values in stations right off the Peruvian shelf could be attributed to DCu influx from biogenically enriched sediments. DCu decreased as we moved off shore towards the western Arabian Sea and in remote, open ocean waters, such surface water enrichment was not 236 observed. The absence of a pronounced surface maximum at stations closer to the Omani coast is indicative of regions of high biological productivity associated with upwelling. Line P and metals In the transect along our non-OMZ site in the NE Pacific, one of the most striking differences in DFe & DMn distribution was the presence of high Mn: Fe ratios off the continental shelf as compared to those obtained in transects off the Peruvian coast. Surprisingly low Mn:Fe ratios were obtained along our three near shore Peruvian transects and these values have been attributed to the complex redox cycling of Fe and Mn in the reducing sediments. Along the continental margin underlying the Peruvian OMZ, reducing OMZ conditions overlying the organic rich sediments results in the “Fe trapping” while Mn diffuses off shore into the water column, thereby resulting in lower DMn values along the Peruvian continental shelf. Higher Mn: Fe ratios were observed in near shore stations along Line P, perhaps due to the absence of any such complex redox cycling in continental sediments. DCu depth profiles along Line P exhibit general features of a nutrient like element and agree with previous data from the central North Pacific. However, in the absence of a SNM along Line P, no draw down of DCu was observed at mid-depths similar to DCu distributions within the Arabian Sea OMZ.

Abstract (if available)

Abstract This thesis explores the behavior and distribution of key redox sensitive elements - Fe and Mn under spatially varied suboxic conditions along eastern boundary upwelling regions as compared to that of the non-redox sensitive, bioactive trace metal - Cu. The response of these metals was then analyzed in a different non-oxygen minimum zone (OMZ) setting along Line P to compare and analyze the differences in distribution, if any caused by the suboxic conditions. Fe, Cu and Mn were investigated in the three major OMZs of the world’s oceans -- namely, the eastern tropical south Pacific off the coasts of Peru and Chile, the Arabian Sea and in the Costa Rica Upwelling Dome in eastern tropical north Pacific. For the non-OMZ sampling site, we sampled across a dynamic, high productivity region in the North East sub-arctic Pacific along Line P. ❧ Total dissolved Fe, Cu and Mn concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS). Fe (II) concentrations were determined using an automated flow injection analysis system. Results from the Peruvian OMZ indicate that Mn is largely decoupled from Fe. While Fe concentrations were very high on the shelf, it decreased drastically offshore and was coupled to redox conditions. In contrast, Mn concentrations were lower over the shelf and were often higher offshore, especially in surface waters. Results suggest that Mn is efficiently transported away from the highly reducing conditions of the shelf because of slow oxidation kinetics -- in contrast to Fe. In nearshore stations, off the broad continental shelf along the northern and central transects off Peru, exceedingly high Fe were measured with most of the dissolved Fe present as Fe(II) below the oxycline. Along the narrower southern Peruvian shelf, dissolved Fe concentrations were 10-fold lower. ❧ Cu distribution in the OMZs showed some interesting features observed for the first time. In transects through the Arabian Sea OMZ and off of Peru, a distinct draw down in Cu concentrations was observed at mid-depths coincident with the secondary nitrite maximum (SNM) while no such feature was present in stations outside the denitrification zone. Distributions along Line P suggest that one of the most striking differences in Fe & Mn distribution was the presence of high Mn: Fe ratios off the continental shelf along Line P as compared to those obtained in transects off the Peruvian coast. Complex redox cycling of Fe and Mn in the reducing sediments along the continental margin underlying the Peruvian OMZ results in the “Fe trapping” while Mn diffuses off shore into the water column, thereby resulting in lower DMn values along the Peruvian continental shelf. No distinct subsurface Fe plume was present throughout the transect along Line P. Cu depth profiles along Line P exhibit general features of a nutrient like element and agree with previous data from the central North Pacific. However, in the absence of a SNM along Line P, no draw down of Cu was observed at mid-depths similar to Cu distributions within the Arabian Sea OMZ. ❧ Overall, this thesis adds to our understanding of the effect of redox conditions within the suboxic zones on redox sensitive elements – Fe and Mn. It also furthers the knowledge of the behavior and distribution of important, biologically relevant trace metals -Fe, Mn and Cu in the world’s oceans.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Anaerobic iron cycling in an oxygen deficient zone

PDF

B-vitamins and trace metals in the Pacific Ocean: ambient distribution and biological impacts

PDF

The distribution and speciation of copper across different biogeochemical regimes

PDF

The distributions and geochemistry of iodine and copper in the Pacific Ocean

PDF

Concentration and size partitioning of trace metals in surface waters of the global ocean and storm runoff

PDF

The distribution of B-vitamins in two contrasting aquatic systems, and implications for their ecological and biogeochemical roles

PDF

Discerning local and long-range causes of deoxygenation and their impact on the accumulation of trace, reduced compounds

PDF

Going with the flow: constraining the lateral advection of redox-active metals from continental margins under differing oxygen regimes

PDF

Investigating the global ocean biogeochemical cycling of alkalinity, barium, and copper using data-constrained inverse models

PDF

Thermal acclimation and adaptation of key phytoplankton groups and interactions with other global change variables

PDF

Distribution and impact of algal blooms leading to domoic acid events in southern California

PDF

The marine biogeochemistry of nickel isotopes

Asset Metadata

Creator Vedamati, Jagruti (author)

Core Title Comparative behavior and distribution of biologically relevant trace metals - iron, manganese, and copper in four representative oxygen deficient regimes of the world's oceans

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Ocean Sciences

Publication Date 07/30/2013

Defense Date 05/09/2013

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

Tag Copper,denitrification,Iron,manganese,OAI-PMH Harvest,oxygen minimum zones,trace metals

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Moffett, James W. (committee chair), Capone, Douglas G. (committee member), Hammond, Douglas E. (committee member), McKemy, David (committee member), Sañudo-Wilhelmy, Sergio A. (committee member)

Creator Email jagruti.vedamati@gmail.com,vedamati@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-305640

Unique identifier UC11293511

Identifier etd-VedamatiJa-1880.pdf (filename),usctheses-c3-305640 (legacy record id)

Legacy Identifier etd-VedamatiJa-1880.pdf

Dmrecord 305640

Document Type Dissertation

Format application/pdf (imt)

Rights Vedamati, Jagruti

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA