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A proxy for reconstructing histories of carbon oxidation in the Northeast Pacific using the carbon isotopic composition of benthic foraminifera

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A proxy for reconstructing histories of carbon oxidation in the Northeast Pacific using the carbon isotopic composition of benthic foraminifera

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Content A PROXY FOR RECONSTRUCTING HISTORIES OF CARBON OXIDATION IN THE NORTHEAST PACIFIC USING THE CARBON ISOTOPIC COMPOSITION OF BENTHIC FQRAMINIFERA by Jennifer Carol Holsten A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE (GEOLOGICAL SCIENCES) May 2003 Copyright 2003 Jennifer Carol Holsten R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. UMI Number: 1416558 UMI UMI Microform 1416558 Copyright 2003 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CALIFORNIA 90089-1695 This thesis, written by Jennifer Carol Holsten under the direction o f fte r thesis committee, and approved by all its members, has been presented to and accepted by the Director o f Graduate and Professional Programs, in partial fulfillment o f the requirements fo r the degree o f Master of Science in Geological Sciences y _ _ Director Date M ay -1 6 .,... 7.003 Thesis Committee 7 Chair R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. ACKNOWLEDGEMENTS In completing this thesis I have received the kind support of many individuals. I would like to express my gratitude to my advisor, Lowell Stott, for his valuable insights, suggestions, and continued professional and personal support. This project greatly benefited from the thoughtful comments of William Berelson and Robert Douglas, who gave generously of their time and ideas. Many thanks are due to Miguel Rincon for his teaching and assistance in performing laboratory analyses, and to Masha Prokopenko for her stimulating discussions and an introduction to diffusion-reaction modeling. I also acknowledge the important contributions of the crew and scientists aboard the R/V New Horizon during the CALMEX cruise. This work was supported by a grant from the National Science Foundation to Lowell Stott and William Berelson. I am overwhelmed when I consider the friends and family who deserve to be mentioned on this page. They have made the last few years fun and memorable. Thank you to all who have listened and shared ideas along the way. I am especially grateful to Mike Geller for his endless patience and encouragement from the beginning. Special thanks to Mom, Dad, and Kim, whose love and confidence have been the greatest motivation to accomplish my goals. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. TABLE OF CONTENTS Acknowledgements ii List of Tables v List of Figures vi Abstract x 1. Introduction 1 1.1. Reconstructing Paleoproductivity 3 1.2. Background 6 2. Study Area 10 2.1. Santa Barbara Basin 13 2.2. Santa Monica Basin 14 2.3. Baja California 16 3. Methods 21 3.1. Pore Water IC 0 2 and §1 3 C 21 3.2. Pore Water Methane Concentration 22 3.3. Distribution and §1 3 C ofBenthic Foraminifera 24 4. Results 29 4.1. Santa Monica Basin 29 4.2. Santa Barbara Basin 32 4.3. Soledad Basin 36 4.4. Magdalena Basin 39 4.5. Matzatlan margin 42 iii R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 4.6. San Bias Basin 46 5. Discussion 50 5.1. Benthic Foraminiferal Distribution and 81 3 C Patterns Along the Northeastern Pacific Margin 50 5.2. Benthic Foraminifera As Recorders o f Pore Water 51 3 C 52 5.2.1. B. argentea 53 5.2.2. B. subadvena 55 5.2.3. B. tenuata 57 5.3. Implications for Reconstructing Histories of Carbon Oxidation Using the 81 3 C ofBenthic Foraminifera 61 5.3.1. Diffusion-reaction Modeled Sources o f Pore Water 51 3 C 63 5.3.2. Contribution o f Methane Oxidation to Pore Water TCO2 and 813C 65 5.4. History of Carbon Oxidation in Santa Monica Basin 68 6. Conclusions 73 References 75 iv R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. LIST OF TABLES Table 1. Locations o f CALMEX multi-cores that are analyzed in this study. 11 Table 2. Summary of observed benthic foraminiferal abundance maxima and average measured carbon isotopic compositions along the northeastern Pacific margin. 52 Table 3. Sediment depths o f observed maximum abundance o f B. argentea and the predicted depths o f test precipitation from comparison o f pore water and foraminiferal S1 3 C. 55 Table 4. Sediment depths of observed maximum abundance o f B. subadvena and the predicted depths of test precipitation from comparison o f pore water and foraminiferal 81 3 C. 56 Table 5. Sediment depths o f observed maximum abundance of B. tenuata and predicted horizons o f test precipitation from comparison of pore water and foraminiferal S1 3 C. 59 Table 6. Calculated contributions of CO2 derived from methane (CH4) and organic carbon (Co r g ) oxidation to pore water TCO2. 66 Table 7. Comparison o f calculated pore water S1 3 C values if all available methane is oxidized, values if only organic carbon oxidation occurs, and observed values. 67 v R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. LIST OF FIGURES Figure 1. Dysoxic basins along the northeastern Pacific margin from which multi-cores were collected during the CALMEX cruise for testing proxy carbon oxidation measurements. Figure 2. General dissolved oxygen (left) and temperature and salinity (right) profiles for the northeastern Pacific. Figure 3. Site map of Santa Barbara Basin multi-cores NH01-3-MC13 and NH01-8-MC2. Figure 4. X-radiograph of the upper 19 cm o f multi-core NH01-3-MC13 collected from Santa Barbara Basin showing well-preserved laminated sediments. Figure 5. Bathymetric map of Santa Monica Basin. Figure 6. X-radiographs of the upper 20 cm ofNH 01-l-M Cl (top) and the upper 17 cm ofNH01-2-MCl (bottom) collected from Santa Monica Basin. Figure 7. X-radiograph o f the upper 19 cm ofNH01-10-MCl. Figure 8. Magdalena Basin bathymetry and locations o f CALMEX multi-cores. Figure 9. X-radiograph o f the upper 18 cm ofNH01-12-MCl collected from Magdalena Basin. Figure 10. Matzatlan margin bathymetry and CALMEX multi-core locations. Figure 11. X-radiograph of the upper 20 cm o f multi-core NH01-30-MC1 that was collected from the Matzatlan margin. Figure 12. Bathymetry of San Bias Basin. Figure 13. Scanning electron micrographs o f the benthic foraminiferal species analyzed in the study. Figure 14. TCO2 measurements o f whole core squeezer pore water sampled from NHQ1-2-MC1 (W. Berelson, unpublished data). Figure 15. Pore water S!3C measured from NH01 sites 1 and 2 whole core squeezer samples compared with predicted values. 11 12 13 14 15 15 16 18 18 19 20 20 25 29 29 vi R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 16. Benthic foraminiferal distribution in Santa Monica Basin from NH01-1-MC1 (left) and NH01-2-MC1 (right). 30 Figure 17. Measured 51 3 C values of benthic foraminifera from Santa Monica Basin multi-cores NHO1-1 -MC1 and NHO1 -2-MC1. 31 Figure 18. Comparison of NH01-1-MC1 and NHO 1-2-MC 1 foraminiferal and predicted pore water isotopes. 31 Figure 19. TCO2 measurements of whole core squeezer pore water sampled from NH01-8-MC2 (W. Berelson, unpublished data). 32 Figure 20. Comparison of measured pore water 81 3 C from NH01-8-MC2 and modeled values using -0.7%o and - 1 .4% o bottom water 51 3 C. 32 Figure 21. Measured concentration of methane in Santa Barbara Basin pore water collected via whole core squeezer (NH01-8-MC2). 33 Figure 22. Benthic foraminiferal distribution in Santa Barbara Basin (NH01-3- MC13). 34 Figure 23. Carbon isotopic composition o f foraminiferal calcite in Santa Barbara Basin multi-core NHO 1 -3-MC 13. 35 1 ^ Figure 24. Comparison of B. argentea and B. subadvena 8 C to modeled pore 1 ^ • water 8 C in Santa Barbara Basm. 35 Figure 25. Pore water TCO2 measurements from NH01-10-MC1 (W. Berelson, unpublished data). 36 Figure 26. Measured and predicted pore water 81 3 C from NH01-10-MC1. 36 Figure 27. Pore water methane concentrations in multi-cores (NH01-10-MC1 and NHO 1-11 -MC 1) and gravity core (NHO 1-10-GC1) collected from Soledad Basin. 3 7 Figure 28. Benthic foraminiferal abundance profiles in Soledad Basin (NH01-11- MC1). 38 Figure 29. Measured 81 3 C of benthic foraminifera in Soledad Basin multi-core NH01-11-MC1. 38 1 ^ Figure 30. Comparison of Soledad Basin foraminifera and pore water 8 C. 38 Vll R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 31. Measured TCO2 from NH01-12-MC1 (W. Berelson, unpublished data). 40 Figure 32. Measured and predicted S1 3 C o f pore water TCO2 from NH01-12- MC1. 40 Figure 33. Methane concentrations in pore water samples from a multi-core and gravity core (NHQ1-12-MC2 and GC3) collected from Magdalena Basin. 40 Figure 34. Foraminiferal distribution in Magdalena Basin (NH01-12-MC2). 42 Figure 35. Carbon isotopic composition o f benthic foraminifera from NH01-12- MC2 in Magdalena Basin. 42 Figure 36. Measured TCO2 from NHO 1-30-MC1 (W. Berelson, unpublished data). 43 Figure 37. Measured and predicted pore water §1 3 C from NHO 1-30-MC1. 43 Figure 38. Pore water methane concentrations measured in a multi-core and gravity core (NH01-29-MC1 and GC3) collected from the Matzatlan margin. 44 Figure 39. Benthic foraminiferal distribution at the Matzatlan margin site (NH01-30-MC1). 44 Figure 40. Measured 51 3 C values of benthic foraminifera collected from Matzatlan margin multi-core NHO 1 -3 0-MC 1. 44 Figure 41. Carbon isotopic compositions o f NHO 1-3 0-MC 1 benthic foraminifera 1 ' X and predicted pore water 5 C. 45 Figure 42. Measured TCO2 from NH01-31-MC1 (W. Berelson, unpublished data). 46 Figure 43. Measured and predicted pore water 81 3 C from NH01-31-MC1. 46 Figure 44. Methane concentrations of San Bias Basin (NH01-31-MC1) pore water. 47 Figure 45. Foraminiferal distribution pattern in San Bias Basin (NH01-32-MC1). 48 Figure 46. Measured 81 3 C values of benthic foraminifera collected from San Bias Basin multi-core NH01-32-MC1. 48 Vlll R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 47. Carbon isotopic compositions of benthic foraminifera in San Bias Basin in comparison to pore water values. 49 Figure 48. Pore water TCO2 gradients from six CALMEX sites along the northeastern Pacific margin (W. Berelson, unpublished data). 50 Figure 49. Abundances of living (rose Bengal stained) B. tenuata in San Bias and Santa Barbara Basins. 58 Figure 50. Relationship between the 81 3 C gradient from 0-5 mm and the quantity of carbon oxidation (Berelson and Stott, in press). 69 1 " 3 Figure 51. A8 C values using B. argentea and B. tenuata from Santa Monica Basin center (DOE 26) and slope (EW95 MC4) cores. 70 Figure 52. Observed expansion of the laminated zone outward from the center of Santa Monica Basin. 70 Figure 53. Comparison o f B. tenuata 81 3 C from Santa Monica Basin cores DOE 26 and NHO 1-2-MC 1 (L.D. Stott, unpublished data). 72 ix R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. ABSTRACT This study reports live foraminiferal distribution and S1 3 C, and pore water 1 T TCO2, 8 C, and methane in Santa Monica Basin, Santa Barbara Basin, and along the western Mexican margin. Pore water TCO2 gradients reflect carbon oxidation and CO2 diffusion across the sediment-water interface. Carbonate tests of benthic foraminifera from dysoxic, laminated sediments record the 1 3 C/1 2 C composition of pore water TCO2 . Paleo-pore water 51 3 C profiles can be reconstructed from fossil tests and used to interpret carbon rain and intermediate water ventilation histories. Test calcification occurs at 0-lmm for Bolivina argeniea, 4-6mm for 1 T Buliminella tenuata, and l-3mm for Bolivina subadvena. The 8 C of live B. • I T argentea approximates pore water 8 C at 0-lmm, B. tenuata displays the lowest S1 3 C, and B. subadvena displays intermediate values. A consistent S1 3 C relationship between species implies preferred habitats for calcification. The calcification depths 1 T implied by the 8 C values agree with the depths of maximum live abundance. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 1. INTRODUCTION Benthic foraminifera are abundant and common throughout the world’s oceans and tend to fossilize well, making them a widely available resource for the investigation of both modem and ancient marine environments. The specific behavioral patterns and microhabitat preferences of different benthic foraminiferal species are not well known. Paleoceanographic studies continuously benefit from an improved understanding of foraminiferal ecology, and how changes in their environment are manifested in the geochemistry of the fossilized shell or changes in their abundance and association with other species. Some species have been recognized as opportunistic, exhibiting a rapid response to sudden changes related to flux of organic material to the sediment and dissolved oxygen concentrations in bottom water (Bernhard and Reimers, 1991; Corliss and Silva, 1993; Bernhard, et. al, 1997; Cannariato, et. al, 1999). The tests of benthic foraminifera provide a unique and effective approach for tracing changes in deep-sea environments and global climate on geologic time scales. A large percentage of the ocean’s total marine primary productivity occurs along the ocean margins (Rullkotter, 2000), as does more than 90 percent of organic carbon burial (Hartnett, et. al., 1998). The production, oxidation and sequestration of organic carbon along ocean margins have an important impact on global carbon and oxygen cycles. The formation and preservation of organic carbon and marine carbonate act as a source or sink for carbon within the global carbon cycle, and have an influence on concentrations of atmospheric greenhouse gases such as carbon 1 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. dioxide and methane. Reconstructions o f paleoproductivity records contribute to understanding the interactions between atmospheric and deep-sea carbon reservoirs, global carbon budgets, and climate variability through geologic time. Additionally, it may be that changes in productivity could have been responsible for the observed alternating intervals ofbioturbation and lamination that characterize the Pleistocene sediments of the silled basins in the northeast Pacific (Stott, et. al., 2000). Flushing and ventilation of the basins along the northeastern Pacific margin coincide with upwelling, which is a seasonal phenomenon (Huyer, 1983). The strength o f upwelling has varied in response to changing climate. Changes in the strength, frequency or oxygen content of basin flushing events have been called upon to explain the variable formation and preservation of the laminated sediments through time (Reimers, et. al., 1990; Gorsline, 1992; Christensen, et. al., 1994; Kennett and Ingram, 1995). Previous studies have, for example, proposed that there was higher dissolved oxygen in Pacific Intermediate Water during glacial periods and this was the cause ofbioturbated sediments in Santa Barbara Basin (Kennett and Ingram, 1995; Behl and Kennett, 1996; Cannariato and Kennett, 1999). It has also been suggested that the changes in dissolved oxygen concentrations and associated sedimentation patterns occur on decadal timescales (Gorsline, 1992; Christensen, et. al., 1994; Stott, et. al., 2000; O’Connell, et. al., 2001). In the present study I examined how benthic foraminifera may be used to reconstruct histories of carbon oxidation in a series of silled basins along the northeastern Pacific margin. This information will be used to better assess how 2 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. changing marine productivity and variable intermediate water ventilation have contributed to the history of laminated and bioturbated sediments in the northeast Pacific. 1.1. Reconstructing Paleoproductivity Modem records of surface ocean productivity have been established from data collected seasonally during time-series monitoring studies over years or decades. Sediment trapping results give direct evidence o f seasonal and annual variations in particle fluxes to the sea floor in the northeast Pacific (Honjo, et. al. 1982; Thunell, et. al., 1993; Smith, et. al., 1994; Thunell, 1998). A temporal coupling between surface productivity, the arrival of detritus at the sea floor, and benthic biological activity was documented by Smith, et. al. (1994) using simultaneous sediment trap collection, seasonal sediment core collection, in situ measurements o f benthic oxygen utilization, and time-lapse photography o f the sea floor. Their results indicated an increase in benthic oxygen consumption associated with increased particulate flux to the sea floor and epifaunal organism activity during the spring season. These studies have supplied evidence that both bottom water chemistry and benthic biological activity are sensitive to changes in surface productivity and the amount of particulate material exported to the sea floor. Even so, while increased surface productivity corresponds to greater fluxes of organic material to the sea floor, the quantity of organic carbon that is exported and subsequently buried may be spatially and temporally variable (Hartnett, et. al., 3 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 1998; Thunell, 1998; Van Mooy et. al., 2002). Export increases as primary productivity increases but burial efficiency may decline, inhibiting the accumulation of organic carbon in sediments, which is problematic for paleoceanographic reconstructions (Berelson and Stott, 2003). Increased sea surface temperatures along the northeastern Pacific margin are correlated with decreased upwelling due to seasonal shifts in wind speed and direction (Smith and Eppley, 1982; Huyer, 1983). Reduced upwelling and the resulting warming o f the surface layer have a negative impact on surface productivity, observed as a decline in zooplankton growth (Roemmich and McGowan, 1995). The strong correlation between spring sea surface temperature and upwelling driven productivity has made it possible to estimate productivity changes by using extended records of sea surface temperature. Unfortunately, only relatively short historical data sets are available. A means to examine patterns of productivity over much longer periods of time has not been established. In order to more closely examine the importance o f paleoproductivity on benthic dissolved oxygen concentrations, a series of proxy measurements has been developed for pore water total carbon dioxide (TCO2), which responds to changing carbon oxidation (Stott, et. al., 2000; Stott, et. al., 2002; Berelson and Stott, 2003). The proxies include the carbon isotopic composition (81 3 C) o f pore water dissolved inorganic carbon (DIC), the distribution patterns o f living epifaunal and infaunal benthic foraminifera, Bolivina argentea and Buliminella tenuata, and isotopic composition of foraminiferal calcite. 4 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The pore water TCO2 gradient varies in response to changes in the rate of carbon oxidation and the rate of CO2 diffusion. The 51 3 C of TCO2 in the pore water also varies systematically in response to these factors. The isotopic composition of the pore water is recorded by the 81 3 C of benthic foraminifera at their horizon of maximum abundance, where it is thought that individuals prefer to precipitate their carbonate tests. B. argentea inhabits the upper millimeter (mm) o f sediment, whereas B. tenuata is most abundant at approximately 4-6 mm sediment depth. The difference in 81 3 C o f these two benthic foraminifera can therefore be used to reconstruct the pore water TCO2 gradient, which provides a proxy measurement of carbon oxidation rates. Depending on basin turnover, changes in the rate of carbon oxidation can alter the amount of oxygen available in basin bottom water and the preservation o f laminated sediments. This proxy approach to reconstructing paleoproductivity using the differences in carbon isotope signatures of benthic foraminiferal species provides a method for reconstructing carbon oxidation rate histories. Provided that a sediment sample interval has not been bioturbated and that it contains sufficient quantities of the foraminiferal species for isotopic analyses, an extremely high-resolution carbon oxidation record can be reproduced. It is also possible to combine these paleoproductivity estimates with sea surface temperature reconstructions using isotopic and trace metal paleothermometry to investigate whether past sea surface temperature fluctuations and variable upwelling have impacted carbon export from the surface ocean. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The goal of the present study was to extend the application o f the proxy to a range of environments that differ in their bottom water oxygen concentrations and carbon oxidation along the margin of the northeastern Pacific. In addition, the results from this research provide evidence for a third species of infaunal benthic foraminifera that can be used as a proxy for pore water isotopic composition in reconstruction o f ancient pore water gradients. The proxy is tested at a site in Santa Barbara Basin, two additional sites in Santa Monica Basin, and four sites along the western continental margin of Mexico. Low levels of bottom water dissolved oxygen, high sedimentation rates, and laminated sediments characterize the region (Soutar and Crill, 1977; Lange, et. al., 1995; Hartnett, et. al., 1998). At all sites examined, the greatest abundance o f Bolivina subadvena is found at sediment depths of 1-3 mm, consistently between the maximum abundance horizons of B. argentea and B. tenuata, and reliably records intermediate 81 3 C values regardless of where it is found in the sediment column. Secondly, this study investigated the potential of assessing regional changes in paleoproductivity through reconstruction and comparison o f longer-term (century-scale) carbon oxidation records using longer sediment records. 1.2. Background Organic material is strongly depleted in 1 3 C relative to TCO2 in seawater. Typical 81 3 C values for photosynthetically produced organic matter following a C3 6 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. pathway range from -18 to -2 3 % o relative to PDB1 for mid-latitude samples (McCorkle, et. al., 1985). C3 photosynthetic processes preferentially utilize the lighter carbon isotopes from the ambient pool of dissolved CO2. The preferential uptake of 1 2 C is a diffusion-limited process and the 81 3 C o f marine organic matter depends on the concentration and isotopic composition of aqueous CO2. Rau, et. al. (1991) found that surface ocean TCO2 is inversely related to temperature, and reduced TCO2 corresponds to higher organic 81 3 C. An increase in the rate of primary productivity relative to renewal rates will leave surface waters enriched in 1 T CO2, which can be recorded in organic matter. However, the impact of this on bottom and pore water carbon isotopic compositions is likely to be small. The rain of organic carbon to the sea floor influences the chemistry of sediment pore water as O2 is consumed in the oxidation of organic matter. This process releases isotopically light CO2 to the pore water. Pore water isotopic composition will also be affected to some extent by dissolution of biogenic calcite. However, mass balance considerations imply that the oxidation of organic carbon dominates the pore water 81 3 C profiles due to its much more depleted S1 3 C values. Thus pore waters near the sediment-water interface are expected to be lighter than the overlying bottom water. Because only a small fraction of all organic carbon exported to the sea floor is buried and preserved in the sediments (Rullkotter, 2000), 1 Isotopic compositions are given in comparison to a standard (std) using 8-notation such that: s uc = x 1000 ) std 7 (1 3 C /1 2 C)S R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. the TCO2 gradient, and subsequently the 51 3 C gradient, in the upper section of sediment is largely controlled by the rate at which organic carbon reaches the sea floor and is oxidized. McCorkle, et. al. (1985) estimated organic carbon rain rates from modeled S1 3 C profiles and documented a systematic variation in pore water 81 3 C profiles with varying carbon rain rates. Higher fluxes of carbon to the sea floor result in more negative pore water 51 3 C values. Bacterial decomposition o f buried organic material in the deep anoxic sediments produces methane, which is oxidized in the upper section o f the sediment column or diffuses into the water column. Large accumulations o f methane are also stored in the form o f gas hydrates under high pressure and low temperature conditions, found in offshore sediments along the continental margin and characterized by 81 3 C values o f-57 to -73%o (Kvenvolden, 1995). The production ' [ ' ) 1 * 1 of methane from organic matter preferentially sequesters C, increasing the 8 C of pore water CO2 as a result. In contrast, the oxidation o f methane enriches the pore water in 1 2 C, decreasing the S1 3 C value of pore water CO2 . The calcite precipitated by B. argentea, B. subadvena and B. tenuata may not reflect equilibrium isotopic values. The carbon and oxygen isotopic composition of benthic foraminiferal carbonate is a function of temperature, biologically mediated fractionation of Ca1 2 CC >3 termed “vital effect”, and the geochemistry of the ambient pore water (Grossman, 1987; Mackensen, et. al., 2000; Rathbum, et. al., 2000). The fractionation of oxygen isotopes recorded by foraminifera is temperature dependent, but cannot be used to investigate the microhabitat preferences o f benthic 8 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. foraminifera. Because bottom and pore water temperature at a single site is 1 8 relatively uniform on these small spatial scales, little variation in 8 O is recorded. The influence of vital effects on carbon isotopic composition of foraminiferal calcite may be divided into two subcategories: metabolic and kinetic fractionations (Mackensen, et. al., 2000). Metabolic fractionation is a result of the incorporation of 1 0 respired CO2 into the test, and kinetic fractionation occurs due to variable rates of carbonate precipitation. Vital effects will generally cause slightly more depleted 1 T 8 C values, usually within 2% o of equilibrium values (Grossman, 1987). Corliss (1985) first noted an infaunal microhabitat preference by calcareous benthic foraminifera within deep-sea sediments, finding the greatest number of individuals living in a relatively narrow range o f sediment depths. Later studies have made similar observations for several species o f benthic foraminifera (Corliss and Silva, 1993; McCorkle, et. al., 1997). Comparisons of pore water and foraminiferal isotopic compositions have led to inferences that the horizon of maximum abundance commonly coincides with a preferred sediment depth for calcite test precipitation (McCorkle, et. al., 1990; Rathbum, et. al., 1996; McCorkle, et. al., 1997; Mackensen, et. al., 2000). Many taxa show little variation in their 81 3 C value throughout the 1 T sediment column, however the 8 C values of epifaunal taxa are generally higher I T 1 T than infaunal taxa 8 C. The use of 8 C differences between benthic species to estimate pore water gradients and carbon rain rates was introduced by McCorkle, et. al. (1990,1997) based on data collected from the Pacific and Atlantic Ocean margins. 9 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 2. STUDY AREA The present study is part of an ongoing research effort to better determine how changes in surface ocean productivity and rates of carbon oxidation affect the benthic environments in dysoxic basins. Multi-cores were collected during the CALMEX cruise aboard the R/VNew Horizon in November, 2001 from several marine basins along the margin of the northeastern Pacific (Figure 1, Table 1). The topography o f the region includes tectonically formed silled basins situated along the western continental margin of North America. These locations are noted for containing preserved laminated sediment deposits that commonly accumulate in near-anoxic bottom water conditions associated with overlying oxygen deficient water masses (Soutar and Crill, 1977; Lange, et. al., 1995). Low oxygen conditions prevent the incursion of larger benthic organisms that might otherwise burrow into the sediment, allowing the formation and preservation o f well-laminated sediments. The flow of California Current surface water toward the equator along the western North American margin is regulated by seasonal shifts in the strength and position of the North Pacific High (Huyer, 1983). The seasonal changes drive a cycle of strong offshore transport and upwelling in the summer months, which influences productivity and ventilation of basin bottom water. The California Cooperative Oceanic Fisheries Investigation (CALCOFI) has extensively studied the California Current system, and Lynn and Simpson (1987) present a detailed description of its physical characteristics. Low temperature, low salinity and high oxygen content characterize the surface water carried by the California Current. 10 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. lanta Barbara Basin •N^anta Monica Basin 30° — 27° Soledad Basin Magdalena Basin, Matzatlan Margins San Bias Baslh 114° 111° 105° 120 ° 108° Figure 1. Dysoxic basins along the northeastern Pacific margin from which multi-cores were collected during the CALMEX cruise for testing proxy carbon oxidation measurements. Additional CALMEX sites in the Gulf of California are not shown. Site Core ID Coordinates Depth (m) Santa Barbara Basin NH01-3-MC13 34° 13.61'N 119° 59.42'W 585 NH01-8-MC2 34° 17.83'N 120° 01.36'W 556 Santa Monica Basin NH01-1-MC1 33° 41.06'N 118° 47.24'W 908 NHO 1-2-MC 1 33° 41.71'N 118° 48.17'W 910 Soledad Basin NH01-10-MC1 25° 12.66'N 112° 43.03'W 541 NH01-11-MC1 25° 08.03'N 112° 40.06' W 446 Magdalena Basin NH01-12-MC1 23° 26.60' N 111° 34.20'W 713 NH01-12-MC2 23° 28.27' N 111° 36.70'W 686 Matzatlan Margin NH01-29-MC1 22° 40.01'N 106° 28.70'W 442 NHO 1-3 0-MC 1 22° 38.26' N 106° 31.60' W 600 San Bias Basin NH01-31-MC1 21° 15.32'N 105° 57.42' W 430 NH01-32-MC1 21° 10.22'N 105° 53.47' W 450 Table 1. Locations of CALMEX multi-cores that are analyzed in this study. 11 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Bottom water along the continental margin consists of the California Undercurrent, a poleward flowing water mass originating from eastern equatorial and North Pacific intermediate waters. This subsurface water mass is characterized by high nutrient and low oxygen concentrations. The lowest levels of dissolved oxygen in the California Undercurrent are concentrated over the continental slope (Figure 2) and extremes in low oxygen occur seasonally with increased Undercurrent transport from the south (Lynn and Simpson, 1987). A series of progressively shallower sills intersect the oxygen minimum zone and isolate the continental margin basins, limiting the flow of water between adjacent basins and restricting ventilation by more oxygenated waters, which occurs seasonally to annually (Reimers, et. al., 1990; Christensen, et. al., 1994). Biologic activity and the oxidation of organic matter quickly deplete available dissolved oxygen in already deficient basin bottom and pore waters. 500 '1 0 0 0 •5 o. •S 1500 2000 2500 — Soledad Basin M agdalena, San Bias Basins Santa B arbara B asin ~ M atzatlan m argin - Santa M onica Basin \ 1 2 3 4 5 D issolved O xygen (m L/L) 500 1000 a. ■ S 1500 2000 2500 T em perature (°C) 5 10 15 20 _ - - — Salinity ■ — Tem p / \ / \ - i / \ / / / / / i / i i i i \ \ '( : \ ; 33 33.5 34 34.5 Salinity (PSU) 35 Figure 2. General dissolved oxygen (left) and temperature and salinity (right) profiles for the northeastern Pacific. Each site is ventilated by water from within the oxygen minimum zone at the depth indicated. Data from Live Access to Reference Data from PMEL. 12 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 2.1. Santa Barbara Basin Santa Barbara Basin lies at the northernmost end o f the Southern California Borderland region. A sill at 475 meters water depth separates it from adjacent basins to the west. The geology and physical oceanography of Santa Barbara Basin are described in greater detail by various authors (Hulsemann and Emery, 1961; Sholkovitz and Gieskes, 1971; Reimers, et. al., 1990; Thunell, 1998). The maximum water depth is approximately 590 meters, near the site o f CALMEX multi-core NH01-3-MC13 (Figure 3). The “SBBC” multi-core discussed by Stott, et. al. (2002) and the Ocean Drilling Project (ODP) Site 893 (Shore-based Scientific Party, 1994) were also collected close by. Methane cold seeps have previously been reported in this area o f the basin (Hinrichs, et. al., 2000). An x-radiograph o f this core shows that the basin contains well-laminated sediments (Figure 4). Baihymetry in meters Sans 300 550 500. 119.5°W Figure 3. Site map of Santa Barbara Basin multi-cores NH01-3-MC13 and NH01-8-MC2. Also shown are the locations of center core “SBBC” discussed by Stott, et. al. (2002) and ODP Site 893 (Shore-based Scientific Party, 1994). Contour interval is 100 meters. 13 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4. X-radiograph of the upper 19 cm of multi-core NH01-3-MC13 collected from Santa Barbara Basin showing well-preserved sediment laminations. 2.2. Santa Monica Basin Santa Monica Basin is separated from Santa Barbara Basin by a sill at about 200 meters water depth and has a much deeper sill at 740 meters that restricts inflowing water. The center of the basin has a maximum water depth of 910 meters. The basin has been the subject of numerous studies (Gorsline, 1992; Christensen, et. al, 1994; Stott, et. al., 2000; Berelson and Stott, 2003). The Hueneme-Mugu submarine fan covers a large portion o f the northern end of the basin. Multi-cores NH01-1-MC1 and NH01-2-MC1 were collected from 908 and 910 meters water depth, respectively, in the southern section of the basin plain (Figure 5) within the zone of laminated sediments mapped by Christensen, et. al. (1994). The x- radiographs o f these cores show that both are laminated in the upper section of the sediment column and are interrupted by a large turbidite deposit (Figure 6). 14 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. B athym etry in m eters 838 ,894 .848 Q IH01-2-MC1 NH01-1-MC1 ,9 3 8 119°W Figure 5. Bathymetric map of Santa Monica Basin. Contour interval is 50 meters. Locations of multi-cores NH01-1-MC1 and NH01-2-MC1 are shown. 0 cm 17 cm Figure 6. X-radiographs of the upper 20 cm of NH01-1-MC1 (top) and the upper 17 cm of NH01-2-MC1 (bottom) collected from Santa Monica Basin. The darker section of each core represents a turbidite deposit. 15 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 2.3. Baja California The region along the western Baja California and Mexican continental margins is also affected by the seasonality of the California Current system that allows equatorial surface water to flow further north into the Gulf of California in summer (Orozco, 1993; Thunell, et. al., 1993). The mixing o f distinct water masses from the north and south influences the complex hydrography of southern Baja California and the mouth of the Gulf o f California. The well-defined oxygen minimum zone associated with Pacific Intermediate Water (PIW) affects this area as well. Orozco (1993) indicates that as much as 50 percent of basin volumes within the south to central Gulf of California are occupied by PIW. Ventilation patterns similar to Santa Barbara and Santa Monica Basins have been noted in the Gulf o f California, an indication o f widespread and synchronous changes along the northeastern Pacific margin (Behl and Kennett, 1996). 0 cm 19 cm Figure 7. X-radiograph of the upper 19 cm of NH01-10-MC1. Preserved laminated sediment is clearly visible. 16 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. A relatively shallow sill depth of approximately 200 meters and high sedimentation rates characterize Soledad Basin (Del Viscio, 2001). CALMEX multi-cores collected from the Soledad sites include NH01-10-MC1 from 541 meters and NH01-11-MC1 from 446 meters water depth. An x-radiograph of NH01-10-MC1 clearly shows well-laminated sediments in the basin (Figure 7). Researchers at Oregon State University have studied the geology and structural characteristics of the Magdalena region (Yeats and Haq, 1981) and Deep Sea Drilling Project (DSDP) Leg 63 site 471 was located on the southern end of the Magdalena Fan. DSDP Leg 63 primarily aimed to investigate the oceanographic history of the California Current system and the origin and structural evolution o f the southern and Baja California borderlands, though yielded inconclusive results due to poor preservation o f microfossils and the structural complexity of the region. CALMEX multi-cores were collected from a basin that lies just inshore from the Magdalena Fan (Figure 8). A ridge at 400 meters separates the basin from the open ocean to the west. Multi-cores NH01- 12-MC1 and NH01-12-MC2 were taken from the southern end o f the basin at 713 and 686 meters water depth, respectively, and NH01-13-MC1 was collected from the shallower eastern slope at 399 meters. There were no laminations observed in NH01-13-MC1 as determined by visual inspection at the time of collection and an x-radiograph ofNH01-12-MCl shows that the sediment from this region is indeed bioturbated (Figure 9). 17 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Magdalena Fan ■ 12-1 Figure 8. Magdalena Basin bathymetry and locations of CALMEX multi-cores. Contour interval is 200 meters. NH01-12-MC1 and MC2 were collected in the deep basin center. NH01-13-MC1 is from shallower water along the slope. Map adapted from Ness, et. al. (1981). 0 cm 18 cm Figure 9. X-radiograph of the upper 18 cm of NH01-12-MC1 collected from Magdalena Basin. The sediment has been bioturbated. 18 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The slope of the Matzatlan margin is located to the south of the opening of the Gulf o f California. CALMEX multi-cores were collected from site 29 at 442 meters and site 30 at 600 meters water depth (Figure 10), where the slope intersects the depths o f the oxygen minimum zone. An x-radiograph ofNHOl- 30-MC1 shows that this site contains preserved sediment laminations (Figure 11). San Bias Basin is located to the south o f the Matzatlan margin, off the coast of San Bias, Mexico. It is bounded on the northwest by Las Tres Marias islands and on the southwest by a ridge at 2 0 0 meters water depth, isolating it from the open ocean (Figure 12). CALMEX sites 31 and 32 are located on the western side o f the basin. Multi-core NH01-31-MC1 was taken from 430 meters water depth and NH01-32-MC1 was taken from 450 meters water depth. 24® N ■ 23®N ■ 22®N — f- % Bathymetry in rrsters MEXICO J ^AhO lis-iUCi ’ - 3 0 - i v ^ 1 ,2 , 3 I 108®W 107°W 106°W 105®W Figure 10. Matzatlan margin bathymetry and CALMEX multi-core locations. Contour interval is 400 meters. Map adapted from Larson (1972). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 11. X-radiograph of the upper 20 cm of multi-core NH01-30-MC1 that was collected from the Matzatlan margin. The core contains visible sediment laminations. San Bias . Las X T res 2800* NH01-31-M&1 •N H O I-33-w tl o o Cabo Corrientes A O O Figure 12. Bathymetry of San Bias Basin. Contour interval is 200 meters. Sites of NH01- 31-MC1 and NH01 -32-MC1 are shown. Map adapted from Ness, et. al. (1981). 2 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3. METHODS 3.1. Pore Water T C 0 2 and 5 1 3 C Multi-cores collected for pore water chemistry were taken into the ship’s refrigerated lab area. Pore water samples were collected using a whole core squeezer (Bender, et. al., 1987) to obtain millimeter-scale samples from near the sediment- water interface. At some sites, additional pore water samples from up to 50 centimeters (cm) sediment depth were obtained by expressing pore water from discrete sections of a multi-core within a glove bag filled with an inert atmosphere. The pore water samples for TC02, 8 1 3 C of TC0 2 and methane concentration were collected simultaneously. TC0 2 measurements were made aboard the ship by coulometry (W. Berelson, unpublished data). The squeezed and sectioned pore water samples for isotopic analysis were sealed in glass vials with Teflon-coated silicone septa, leaving no headspace, immediately poisoned with mercuric chloride and kept refrigerated until analysis. For isotopic determination, about 2 milliliters o f pore water was acidified with H3PO4 in a reaction vessel under vacuum. C 0 2 was extracted cryogenically and measured by electronic manometer, then frozen into a glass break-seal tube. Isotopic analyses were made using a VG Prism II isotope ratio mass spectrometer. The precision o f standard water samples analyzed with the pore water samples was ± 0.3%o for S13C. It has been previously established that the carbon isotopic composition of pore water TC0 2 can be modeled using a simple isotope mass balance calculation 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Berelson and Stott, 2003). The oxidation o f marine photosynthate carbon, which 1 T has a 8 C value of-21%o, combined with bottom water TCO2 and its respective 1 q 1 ^ 8 C value results in the observed down core pore water 8 C gradient. This combination can be represented as: (TC02 b .w.)(S1 3 C b .w .) + (TC02 p.w. - TC02 b.w.)(51 3 C o r g ) = (TC02 p.w.)(81 3 C p.w .) where (b.w.) indicates a bottom water value and (p.w.) refers to a pore water value at a chosen sediment depth horizon. Provided that the TCO2 and 8 1 3 C of bottom water and pore water TCO2 are known, the pore water 8 1 3 C can be predicted using the above equation. In most instances, the predicted pore water S1 3 C values determined 1 3 using this simple approach closely approximate the measured 8 C of pore water TC02. 3.2. Pore Water Methane Concentration Measurements o f pore water methane concentrations were made on board ship using a gas chromatograph. Analyses o f methane concentration were also performed on hydrocast samples collected in Niskin bottles throughout the water column (A. Graham, unpublished data). Repeated measurements of serially diluted standards with known methane concentrations were made to construct a linear calibration curve and test the accuracy o f the gas chromatograph. Standards between 0 and 116 ppm by volume (5.2 pmol/L) were used in the calibration. The calibration is linear at higher methane concentrations (D. Capone, personal communication) and we extrapolate the calibration curve for samples greater than 116 ppm. 22 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Pore water samples were expressed into syringes with gas-tight stopcocks at about 0.5 mm intervals in the top 2 to 3 cm of each core. As for pore water TCO2 and 8 13C, additional samples for methane analysis were expressed from discrete sections of deeper sediment. Several milliliters o f the water samples were injected into sample vials sealed with Teflon-coated red rubber septa caps and allowed to equilibrate with a known volume of headspace. An aliquot of gas from the headspace was then injected into the gas chromatograph for methane analysis. Measured headspace concentrations were corrected for background concentrations of methane in the ambient air and the total concentration of methane in the pore water was calculated. Henry’s Law describes the partitioning of a gas between the aqueous and gas phases: Concentration in gas phase H = ----------------------------------------- Concentration in aqueous phase where H is the Henry’s Law coefficient, which is the inverse of the aqueous solubility o f the gas at a given temperature and salinity. The solubility of methane at temperatures from -2 to +30°C and salinities from 0 to 40 parts per thousand is given by Wiesenburg and Guinasso (1979). The Henry’s Law relationship is used to calculate the concentration of methane dissolved in the pore water once equilibrated with headspace. The sum o f this value and the measured concentration of methane in the headspace is the total concentration of methane in the pore water sample. A separate study of down core methane concentrations was conducted by Andrew Graham o f the University of Hawaii. His samples were collected by sealing a poisoned subsample o f the sediment core in a helium-flushed vial with a 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. known volume o f headspace. Methane concentrations were measured at the University o f Hawaii within several months of collection by analyzing the gas contained in the headspace on a gas chromatograph, and calculating pore water concentrations using porosity data. While there is generally good agreement between the two datasets, methane concentrations at sediment depths below the horizon of sulfate depletion in Magdalena and Soledad Basins are quite different. Measurements made aboard ship indicate lower concentrations than those made later by A. Graham in the lab. It is likely that the method of sediment sectioning and squeezing o f pore waters did not prevent methane gas from escaping and therefore our measurements are artificially low. 1 ^ 3.3. Distribution and 8 C of Benthic Foraminifera From each site, a multi-core adjacent to those analyzed for pore water chemistry was extruded and sectioned at approximately 1.1 mm intervals. The samples from the upper 2 cm for most cores were bagged with buffered rose Bengal stain, sealed and refrigerated. In the lab, each sediment sample was wet-sieved over a 63-micron (pm) screen, and the less than 63 pm fraction that passed through the screen was collected in filter paper. The bulk sediment dry weight o f each sample was calculated from the dry weights o f the greater than and less than 63 pm fractions. The dry sample greater than 63 pm was carefully removed from the filters, placed on weighing papers using a dry brush, weighed on an analytical balance, and then placed in glass vials for storage. The less than 63 pm fractions were weighed 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. while still in the filters to avoid losing fine particles adhered to the filter that could not be removed with a dry brush. A split o f the greater than 63 pm fraction was weighed and picked under microscope for individual foraminifera. Figure 13 provides an illustration of each of the three species studied. The test of B. argentea is biserial with overlapping chambers, relatively flat in cross-section, may have a distinct keel, and is often fairly translucent. B. subadvena has a smaller biserial test that is rounded in cross-section and has distinct chambers, the later ones becoming inflated. The test may be slightly twisted. The test o f B. tenuata is identified as being triserial, with distinct chambers that become rapidly inflated and a visible aperture. Figure 13. Scanning electron micrographs of the benthic foraminiferal species analyzed in this study. From left to right: B. argentea with 200 pm scale bar (Bernhard, et. al., 1997), B. subadvena (Matoba and Yamaguchi, 1982), and B. tenuata with 200 pm scale bar (Bernhard, et. al., 1997). Individuals that contained rose Bengal stain were considered recently alive. Counted live individuals were carefully selected as being completely or very nearly completely (only one empty chamber) stained to ensure that reported numbers are reflective o f the live foraminiferal populations. All stained specimens included 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in the counts were archived on paper slides using Tragacanth adhesive. Some specimens that were very thinly calcified broke apart when touched and were not archived. Foraminiferal abundance per gram of bulk sediment was calculated by applying the ratio of >63 pm to <63 pm weight for the entire sample to the weight of the >63 pm split counted for abundance to find the total dry weight of the split, and then normalizing the number of counted individuals to the calculated dry weight of the split. The abundance o f each species per gram sediment was then plotted against the average sediment depth of the corresponding sample interval. The depth horizon o f maximum abundance of each species is determined visually from the graph. Stained individuals were picked from the remainder of the greater than 63 pm population for isotopic analysis. Due to the very small mass of the individual tests, groups of 10-15 B. argentea, 20-30 B. tenuata, and 60-80 B. subadvena per sample were analyzed. Each group was soaked in sodium hypochlorite for 24 hours to remove organic material, thoroughly washed with de-ionized water and methanol, and then dried in an oven at 40°C overnight. The isotopic composition of the cleaned foraminiferal calcite was determined via mass spectrometry. Precision of i ■ j carbonate standards run with the samples was ± 0.1%o for 8 C. Isotopic measurements are reported relative to the Vienna Pee Dee Belemnite (VPDB). Replicate analyses o f NBS19 carbonate were used for calibration of the reference gas and analytical precision was monitored using repeated analysis of ULTISS calcite standard. There are a number of factors that can affect the results of the analyses. In addition to analytical errors in TCO2 extractions and precision of 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. isotopic measurements made by mass spectrometer, potential problems also include precision o f sectioning high porosity sediments near the sediment-water interface at millimeter resolution and the use of rose Bengal stain to distinguish living foraminiferal populations (Bernhard, 1988). In several o f the sectioned cores it was difficult to accurately obtain millimeter-scale sediment samples for foraminiferal analyses. Sediment depths assigned to high porosity sediments near the sediment-water interface were measured as accurately as possible during extrusion. At locations such as Santa Barbara Basin, for example, the sediment-water interface was easily recognized by the presence of undisturbed filamentous bacterial mats that have been widely noted to occur in the deeper parts of the basin (Soutar and Crill 1977; Reimers, et. al., 1990). Other cores that had less recognizable interfaces, like those containing a thin layer of suspended sediments, may have been assigned a sediment depth of 0 mm slightly above or below where the actual sediment-water interface existed. Foraminiferal isotope measurements should not be greatly affected by this potential source of error. However, assignment of sediment depths should be considered in the interpretation of foraminiferal distribution patterns and the comparison of foraminiferal to pore water isotope values. A variety o f methods, including the use o f vital stains, are available to distinguish living from dead foraminifera populations. Rose Bengal is a vital stain that adsorbs onto proteins, turning the cytoplasm a bright pink color (Bernhard, 2000). It has been argued that recognition of living foraminifera via rose Bengal 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. staining may yield an overestimation of actual living populations, as dead individuals that contain preserved organic material will also be stained (Bernhard, 1988; McCorkle, et. al. 1997). Comparisons o f actual to stained living foraminiferal populations indicate that dead protoplasm continues to stain with rose Bengal for more than four weeks (Bernhard, 1988). On these time scales, the actual living position o f individuals in a sediment column may be obscured by bioturbation, burial, or other passive transport. However, in the Borderland and marginal basins bioturbation is not an issue and we would not expect the distribution to be affected within a four-week time period during which the dead protoplasm continues to stain. Rose Bengal stain was chosen for this study because it is a practical method easily used aboard ship to distinguish living individuals in relatively large quantities. The effects of other staining techniques on the isotopic signatures o f foraminiferal calcite are not known (Rathbum, et. al., 2000), and the results obtained using this method may be compared with previous studies that have utilized rose Bengal stain. The vertical distribution patterns observed for each species are consistent in each of the oxygenated environments investigated, demonstrating the reliability of using the rose Bengal staining method in this study. Small differences between actual and observed abundance values may be present, but are not sufficient to conceal the vertical distribution pattern for each o f the benthic species examined. 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4. RESULTS 4.1. Santa Monica Basin Bottom water TCO2 measured in Santa Monica Basin is 2.4 mmol/L and its § 13C is -0.5%o. Both pore water geochemical profiles show steep gradients (Figures 14, 15). Isotopic analyses of squeezed pore water samples yield similar results at both Santa Monica Basin multi-core sites. In the upper 2 cm, the measured pore water S13C gradient is about -2%o. There is good correlation between measured and predicted S 13C of bottom water and pore water in the upper 0.5 cm. Deeper in the sediment, predicted values are more negative than measured. Concentrations of methane in pore waters were not measured in Santa Monica Basin. T C 0 2 (m m ol/L ) • N H 01-1-M C 1 < ■ > N H 01-2-M C 1 ♦ predicted -4 5 1 3C -2 -1 Figure 14. TC02 measurements of whole core Figure 15. Pore water 8I3C measured from NH01 squeezer pore water sampled from NH01-2-MC1 sites 1 and 2 whole core squeezer samples (W. Berelson, unpublished data). compared with predicted values. 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Similar patterns of foraminiferal distribution were found in both cores collected by CALMEX in Santa Monica Basin (Figure 16). Larger populations in NH01-2-MC1 may be a result of transport of foraminifera from the slope to the basin center (R. Douglas, personal communication). B. argentea lives primarily in the upper millimeter, the highest abundance of B. tenuata occurs around 4-5 mm, and B. subadvena inhabits intermediate depths o f 1-3 mm. The results for B. argentea and B. tenuata are in agreement with depth distributions previously established in Santa Monica Basin (Stott, et. al., 2000; Stott, et. al., 2002; Berelson and Stott, 2003). £ ,§ 6 a . o g g •3 > < 10 ,A A \\ - B . t e n u a t a - B ■ a r g e n t e a - B . s u b a d v e n a S S , -5 . o. 4 43 > < 10 B . t e n u a t a ~ B . a r g e n t e a B . s u b a d v e n a 0 20 40 60 80 100 120 140 A bundance per gram sedim ent 0 40 80 120 160 200 240 280 320 A bundance p er gram sedim ent Figure 16. Benthic foraminiferal distribution in Santa Monica Basin from NH01-1-MC1 (left) and NH01-2-MC1 (right). There are clear abundance maxima for B. argentea at 0-1 mm, B. subadvena at 1-3 mm, and B. tenuata at near 4 mm. Results for B. argentea and B. tenuata agree with previous studies of EWING and DOE cores (Berelson and Stott, 2003). The vertical distribution patterns are also visible in the carbon isotope measurements of live individuals made from both NH01 multi-cores (Figure 17). There is a slightly larger range of measured 8 1 3 C values of B. argentea from NH01- 2-MCI. The carbon isotopic compositions o f B. argentea, B. tenuata, and B. 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. subadvena are reasonably uniform regardless of the sediment depth at which the foraminifera were collected, a reflection of a constant sediment depth of calcification for each species. B. argentea records values of about -0.5% o to -1 .1 % o, consistent with pore water S1 3 C in the upper millimeter of sediment, B. subadvena exhibits 1 t slightly more negative values of - 1 .5%o, similar to the 8 C of pore water TCO 2 at 2 mm, and B. tenuata records the lightest isotope values o f -2.5% o, comparable to pore water values at 7 mm (Figure 18). These results replicate the findings of Berelson and Stott (2003) used to develop proxy measurements for carbon oxidation as well as begin the incorporation of a third species of benthic foraminifera as a recorder of pore water isotopic composition. B . t e n u a t a B . s u b a d v e n a B . a r g e n t e a c u 4 I 1 0 1 2 ■ □ & 1-M C1 B subadvena ® 1-M C1 B argentea 1 1 1-M C1 B ten u ata 2-M C1 B subadvena 0 2-M C1 B argentea □ 2-M C 1 B tenuata -3 -2.5 -2 -1.5 -1 -0.5 0 8 UC Figure 17. Measured §BC values of benthic foraminifera from Santa Monica Basin multi cores NHO1 -1 -MC1 and NHO1 -2-MC1. Each species records consistent 81 3 C regardless of position in the sediment column, implying a preferred depth horizon for test calcification. Figure 18. Comparison of NHO 1 - 1 -MC 1 and NHO 1-2-MC 1 foraminiferal and predicted pore water isotopes. The range of measured foraminiferal 5°C over all sediment depths is indicated. The average foraminiferal SBC corresponds to the pore water 5nC value at the horizon of species maximum abundance. 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.2. Santa Barbara Basin Steep pore water geochemical gradients are also observed in Santa Barbara Basin. Bottom water TCO2 from N H 01-8-M C 2 measured about 2.4 mmol/L, similar to Santa Monica Basin (Figure 19). The 8 13C profile indicates a gradient of -6%o in the upper 2 cm of sediment (Figure 20). The measured bottom water value o f- 1 ,4%o is more depleted than that of Santa Monica Basin, and resembles the pore water S13C o f -1.3%o at 1 mm. Stott, et. al. (2002) reported “SBBC” pore water gradients and benthic foraminiferal 8 13C similar to C A LM EX data, but found Santa Barbara Basin IT 1 2 bottom water 8 C more like that of Santa Monica Basin. Predicted pore water 8 C values calculated using a bottom water value of-0.7%o, given by Stott, et. al. (2002), more closely match the observed values than those found using the measured S 13C of -1.4%o. We believe the measured value may reflect either an analytical error or a T C O a (m m ol/L) 0 0.5 1 S ' ei.5 .g o. ■ § 2 3 O J 2.5 3 3.5 N H 01-8-M C 2 predicted (-0.7%o b.w.) predicted (-1 ,4% ob.w.) -12 -10 -2 5i3C Figure 19. TC02 measurements of whole core Figure 20. Comparison of measured pore water squeezer pore water sampled from NHO 1-8-MC2 5 BC from NHO 1-8-MC2 and modeled values (W. Berelson, unpublished data). using -0.7%o and -1 .4%o bottom water 8 C. 32 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. problem with storage, perhaps due to insufficient poisoning of the sample which allowed additional carbon oxidation to occur during storage. Methane concentrations in Santa Barbara Basin pore waters in the upper 2.5 cm of sediment were measured to be less than 1 pmol/L (Figure 21). The resulting gradient is relatively small in this upper section of the sediment column. o 0.5 ■ ? o 6 1 D « O J -a " H 0) B 1.5 c /3 2 2.5 0 0.5 1 1.5 2 C H (famol/L) 4 Figure 21. Measured concentration of methane in Santa Barbara Basin pore water collected via whole core squeezer (NH01-8-MC2). Benthic foraminiferal abundance patterns observed in Santa Barbara Basin are shown in Figure 22. For all species, the horizons of maximum abundance match those observed in Santa Monica Basin and the Santa Barbara Basin “SBBC” multi core. Stained B. tenuata individuals were found over a larger range of sediment depths in Santa Barbara Basin than in Santa Monica Basin, perhaps an indication that this species migrates through the sediment column in this basin or the preservation of organic matter is longer in this basin. An earlier study of live assemblages in the 33 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. center of Santa Barbara Basin by J.M. Bernhard made similar observations of B. tenuata distribution (Stott, et. al., 2002). s ft 'B o. J - \ H. t e n u a t a ~®— B , a r g e n t e a _ « — g s u b a d v e n a 0 1000 2000 3000 4000 5000 6000 7000 8000 A bundance p e r gram sedim ent Figure 22. Benthic foraminiferal distribution in Santa Barbara Basin (NH01-3-MC13). The patterns are consistent with prior results from the center of the basin (Stott, et. al, 2002) and with the CALMEX profiles from Santa Monica Basin. The B. tenuata analyzed from NH01-3-MC13 do not exhibit a narrow range of variability in carbon isotopic composition (Figure 23). Measured 81 3 C values of B. tenuata in this core were not reproducible between or within sample intervals, suggesting that it is not a reliable recorder o f pore water isotopic composition at a relatively fixed depth horizon in Santa Barbara Basin. There was no systematic • I T * variation in 5 C with depth, which could imply variable calcification depths for this species though additional study of B. tenuata in Santa Barbara Basin is necessary to investigate this possibility. In contrast, both B. argentea and B. subadvena record consistent 81 3 C values regardless of the sediment depth at which they are found. Because pore water 51 3 C 34 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. measurements were made at only the shallower site (NH01-8-MC2), a second pore water §1 3 C profile is modeled for the deeper site (NH01-3-MC13) from which the • 1 ^ foraminiferal data was collected (Figure 24). Average foraminiferal 5 C values for B. argentea and B. subadvena are consistent with prior analyses (Stott, et. al., 2002) and coincide with the modeled NH01-3-MC13 pore water 81 3 C at the horizon of observed maximum live abundance. ■ s a 4) > < -3.5 -2.5 -1.5 5 13C ■ B . t e n u a t a ° B . a r g e n t e a 0 B . s u b a d v e n a Figure 23. Carbon isotopic composition of foraminiferal calcite in Santa Barbara Basin multi-core NH01-3-MC13. B. tenuata exhibits a large variability in 5nC values. B. argentea and B. subadvena record more consistent values. Xj a. S 3 -0.5 -12 -10 B . s u b a d v e n a B . a r g e n t e a ~-E3r— 'EM • NH 01-3-M C13 ° N H 01-8-M C2 -2 S13C Figure 24. Comparison of B. argentea and B, subadvena 81 3 C to modeled pore water 51 3 C in Santa Barbara Basin. Both species in NH01 -3- MC13 record values similar to those reported by Stott, et. al. (2002) and reflect the 5°C of ambient pore water at the depth of maximum abundance. 35 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.3. Soledad Basin Pore water TCO2 and 5 13C profiles from Soledad Basin are shown in Figures 25 and 26. There is a steep TC O 2 gradient and an associated S13C gradient of more than -4%o in the upper 2 cm of sediment in Soledad Basin. There is a step in each gradient at 8 mm, possibly indicative of bioirrigation near the sediment-water interface. The measured bottom water § 13C value is -0 .5 % o, which agrees with the value predicted based upon measured bottom water TCO2 . Pore water methane concentrations are shown in Figure 27. Multi-core samples extend to 50 cm, below which are gravity core samples. Concentrations increase from near zero at the sediment-water interface to greater than 4 50 pmol/L below 100 cm sediment depth. There is a change in gradient at approximately 70 cm, where pore water methane concentrations rapidly increase, that is likely a response to sulfate depletion. 0 0.2 0.4 0.6 0.8 o. f U " O 0.8 ~2 D E T 3 4) CO 2.6 2.8 TCO (m m ol/L) 1 1.2 1.4 1.6 * predicted 0 m easured -3 SI3C Figure 25. Pore water TC 02 measurements from Figure 26. Measured and predicted pore water 51 3 C NH01-10-MC1 (W. Berelson, unpublished data). from NHO 1 -10-MC1. 36 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 0 50 e f a 'B C u -8 ioo 'S a > B t 3 « in 150 200 0 100 200 300 400 500 600 CH^ (|j,mol/L) Figure 27. Pore water methane concentrations in multi-cores (NH01-10-MC1 and NH01- 11-MC1) and gravity core (NH01-10-GC1) collected from Soledad Basin. The depth distribution patterns in Soledad Basin show that B. argentea is most abundant near the sediment-water interface, B. subadvena reaches maximum abundance in the upper 3 mm, and B. tenuata is most abundant at close to 4 mm (Figure 28). B. tenuata was found living throughout a larger range of sediment depths, which may be an indication of migration through the sediment column as 1 T was found in Santa Barbara Basin. However, little variation in 8 C values (Figure 29) is suggestive o f a preferred sediment horizon where B. tenuata individuals precipitate their tests. The foraminiferal 51 3 C measurements clearly show the vertical stratification of species in the sediment column: B. tenuata records the lightest 5 C values (-2%o), B. subadvena the intermediate values (-1.5% o), and B. argentea the heaviest 5 13C values (-1.3%o). The average S1 3 C value of each species intersects the pore water 37 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 51 3 C gradient near the horizon of species maximum abundance (Figure 30), an 1 3 indication that all three species approximate pore water 5 C values. 10 \ \ V, \ i 1 p / ' • B . t e n u a t a B . a r g e n t e a - B . s u b a d v e n a S e, J Z D. y T 3 V e > 0 500 1000 1500 2000 2500 3000 3500 4000 A bundance p e r gram sedim ent Figure 28. Benthic foraminiferal abundance profiles in Soledad Basin (NHO 1-11-MC1). B . t e n u a t a B . a r g e n t e a B . s u b a d v e n a -3 -2.5 8 )3C -1.5 Figure 29. Measured 51 3 C of benthic foraminifera in Soledad Basin multi-core NH01-11-MC1. 0 r 0.2 0.4 5 6 0.6 ■ s S • + “> CL y •o 0 .8 + - » c 4) 1.4 1.6 -5 B . s u b a d v e n a B . t e n u a t a \ B . a r g e n t e a - 53- V a-e-i -4 -2 -1 8 13C Figure 30. Comparison of Soledad Basin foraminifera and pore water 8UC. The composition of each species approximates pore water 5I3C at the horizon of maximum abundance: B. argentea records near bottom water values, B. tenuata reflects pore water values at about 4 mm, and B. subadvena records intermediate values. 38 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 4.4. Magdalena Basin There were no visible laminations observed in the Magdalena Basin sediments at the time o f core collection or in the x-radiograph ofNH01-12-MCl. The pore water geochemical profiles from the Magdalena Basin site (Figures 31, 32) show that the pore water collected from these cores may have been bioirrigated. The pore water TCO2 and 81 3 C show very small gradients with increased sediment depth, an indication that the pore water was vertically mixed. The measured pore water S1 3 C values of -2.2% o are uniformly more negative than the values predicted based upon the measured TCO 2 values and a bottom water value of-0.5%o (Figure 32). It is possible that the pore water samples were insufficiently poisoned at the time of collection, allowing continued carbon oxidation to occur during sample storage 1 - 2 before analysis. The more negative pore water 8 C values are not observed in the benthic foraminifera collected from this site, to be discussed. Pore water methane concentration increases with sediment depth, exceeding 450 pmol/L below 160 cm sediment depth (Figure 33). Concentrations are very close to zero near the sediment-water interface. There is a sharp increase in the gradient at 125 cm. Above this horizon, measurements were made from multi-core samples and below it are gravity core samples. The steepening gradient most likely reflects the depletion of sulfate in the pore water. 39 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Sediment depth (cm) 0.5 1 1.5 2 2.5 2.2 2.4 2.6 2.8 3 3.2 TCO^ (m m ol/L) Figure 31. Measured TC 02 from NH01-12-MC1 Figure 32. Measured and predicted d1 3 C of pore (W. Berelson, unpublished data). water TC02 from NH01-12-MC1. ® m easured predicted 55 1.5 0 it 50 100 ■o 150 u 200 250 300 100 200 300 400 C H 4 (p m ol/L ) 500 Figure 33. Methane concentrations in pore water samples from a multi-core and gravity core (NH01-12-MC2 and GC3) collected from Magdalena Basin. 40 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The foraminiferal abundance observed in Magdalena Basin is shown in Figure 34. B. argeniea was found primarily closest to the sediment-water interface, B. subadvena at around 2 mm, and B. tenuata lived deepest in the sediment column. The maximum abundance of B. tenuata was found at a slightly greater depth, close to 6 mm. The isotopic compositions of foraminiferal tests are also distributed in accordance with the vertical depth distribution. B. argentea records the most positive values, B, tenuata the most negative, and B. subadvena the intermediate (Figure 35). The 51 3 C of B. argentea is similar to that of bottom water, and the b1 3 C of B. subadvena and B. tenuata resemble those found at other CALMEX sites with pore water TCO 2 gradients that respond to changes in organic carbon oxidation. Unfortunately, despite good results for both benthic foraminiferal distribution and isotopic composition, the validity of the proxy cannot be sufficiently tested at this location. Although bioturbation and bioirrigation in Magdalena Basin prevent the immediate presentation o f complete evidence in favor o f applying the proxy, it does not entirely exclude the possibility o f future use of the proxy in the basin. Live foraminiferal depth distributions and isotopic compositions for B. argentea, B. tenuata, and B. subadvena remain consistent with the patterns observed at other sites where the proxy has been shown to provide reliable estimates of pore water geochemical gradients. It would be worthwhile to pursue the collection of additional multi-cores from Magdalena Basin to test the proxy. 41 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. c u • S u > < 10 -A o r * - / \ t e n u a t a -® — B . a r g e n t e a B . s u b a d v e n a 100 200 300 400 A bundance p e r gram sedim ent 500 Figure 34. Foraminiferal distribution in Magdalena Basin (NH01-12-MC2). The vertical distribution patterns in this core are comparable to those found in the other sites investigated. > • K •3 4 -3 B . t e n u a t a B . a r g e n t e a B . s u b a d v e n a -2.5 -1.5 S I3C -0.5 Figure 35. Carbon isotopic composition of benthic foraminifera fromNH01-12-MC2 in Magdalena Basin. All three species record uniform S1 3 C values regardless of position in the sediment column. 4.5. Matzatlan Margin Bottom water TCO2 measured at the Matzatlan margin site was about 2.2 1 ^ mmol/L (Figure 36). Pore water extractions yielded questionable 5 C values, possibly due to longer storage before analyses or insufficient poisoning of the samples. If a loss of CO2 occurred through the Teflon septa during sample storage, 1 3 the lighter isotopes would preferentially escape and a more positive pore water 5 C value would result. However, this is the opposite of the depleted isotopic values observed in the squeezed pore water samples (Figure 37). The observed depletion might be due to continued oxidation of organic carbon during sample storage despite the addition of mercuric chloride to each sample at the time of collection. Because 42 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. of these complications, predicted values of pore water 51 3 C will be used for the c o m p a r i s o n of foraminiferal and pore water isotopic compositions at the Matzatlan site. 0.5 •3 a. e 1.5 2.5 0.5 5 1 o. .8 T 3 00 1.5 2.5 predicted m easured 2.15 2.2 2.25 2.3 2,35 2.4 2.45 2.5 T C 0 2 (m m ol/L ) -2.5 -2 -1.5 -1 -0.5 0 Figure 36. Measured TC 02 fromNH01-30-MCl Figure 37. Measured and predicted pore water (W. Berelson, unpublished data). 51 3 C from NHO1 -30-MC1. 51 3 C is predicted using measured T C 02 values, assuming -0.5%o bottom water and -21%o organic carbon. To produce a predicted pore water isotope gradient via isotope mass balance, 1 ^ the measured TCO2 values were used in conjunction with an assumed 5 C of organic carbon of-21%o and a bottom water value of-0.5%o. The validity of the modeled values is tested by comparison with the 51 3 C profiles from the other sites investigated, as well as with the benthic foraminiferal 51 3 C measured at this site. The predicted profile exhibits familiar steep geochemical gradients in the upper 2 cm and the S1 3 C values resemble the foraminiferal 8!3C measured from the Matzatlan margin, to be discussed. 43 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The measured concentrations of methane in Matzatlan pore water samples are shown in Figure 38. The observed gradient is relatively small, as near zero concentrations at the interface increase only to about 5.5 pmol/L at 430 cm in the sediment. 100 200 S 300 400 500 3 4 5 CH G unol/L) Figure 38. Pore water methane concentrations measured in a multi-core and gravity core (NH01-29-MC1 and GC3) collected from the Matzatlan margin. p g . e. > < 10 - B , t e n u a t a ~ B . a r g e n t e a B . s u b a d v e n a 100 200 300 400 500 600 700 A bundance per gram sedim ent 800 Figure 39. Benthic foraminiferal distribution at the Matzatlan margin site (NH01-30-MC1). 0 0.5 ^ 1 & 1 -g R . I S qj i ■ - > -O 2 2.5 3 3.5 4 B . t e n u a t a B . a r g e n t e a B . s u b a d v e n a -1.5 5 l3C -0.5 Figure 40. Measured 5ljC values of benthic foraminifera collected from Matzatlan margin multi-core NH01-30-MC1. 4 4 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. B. argentea was found to be living in the upper millimeter of the sediment column, most B. subadvena individuals were found close to 2 mm sediment depth, and the maximum abundance of B. tenuata was around 4 to 5 mm (Figure 39). The vertical stratification of these species is comparable to the other sites and is again visible in the foraminiferal isotopic compositions. The most positive S1 3 C values (about -0,2% o) are recorded by B. argentea, the intermediate values (-0.6% o) are recorded by B. subadvena, and B. tenuata has the most negative (-l% o) values 1 1 (Figure 40). The narrow range of measured 8 C values measured in each species is reflective of a preference for test calcification at a specific sediment depth horizon. Predicted pore water 51 3 C agrees with foraminiferal values at the depth horizon of maximum abundance for each species (Figure 41). All three benthic species of foraminifera reliably record the S1 3 C of ambient pore water. B . s u b a d v e n a B . t e n u a t a j B . a r g e n t e a I I ( 0 0.5 S ' o £ 1 a < D " O G M L5 < s > 2 2.5 -3 -2.5 -2 -1.5 -1 -0.5 0 5 i3C Figure 41. Carbon isotopic compositions ofNH01-30-MCl benthic foraminifera and predicted pore water 8 ljC. The foraminifera record SL ’C values similar to pore water at the horizon of species maximum abundance. 45 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 4.6. San Bias Basin The bottom water TCO2 value is about 2.2 mmol/L in San Bias Basin (Figure 42). The measured pore water isotopic composition of the San Bias Basin samples are considered unreliable for the same reasons previously described. For this reason, pore water §1 3 C values predicted from the TCO2 measurements (Figure 43) will also be used to compare foraminifera and pore water isotopic composition at this site. Bottom water 8I3C is estimated to be about -0.5% o in San Bias Basin. The TCO2 gradient corresponds to a steep 51 3 C gradient of more than -4.5% o in the upper 2 cm of sediment. Low pore water methane concentrations are observed near the sediment-water interface, with a steepening of the methane gradient at about 25 cm sediment depth (Figure 44). The profile is similar to Matzatlan margin (N H 01-29) pore water methane concentrations in the upper 50 cm of sediment. 0.5 • O 1 -a v 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 T C O (m m ol/L ) 0.5 4 3 a. 1.5 predicted m easured -5 -3 -2 SI3C Figure 42. Measured TCO2 from NHO1-31 -MC1 Figure 43. Measured and predicted pore water (W. Berelson, unpublished data). 5 C from NHO 1 -31 -MC 1. Assumed values of -0.5%o for bottom water and -21%o for organic carbon were used to calculate predicted values. 4 6 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. C H (uraol/L ) 4 Figure 44. Methane concentrations of San Bias Basin (NHO 1-31-MCI) pore water. Because the sediments near the sediment-water interface in the cores collected from San Bias contained high percentages of water, obtaining millimeter- scale sections of sediment was difficult. The upper 2.5 millimeters of each San Bias core was sectioned and stained as one sample. As a result, the maximum abundance horizons of B. argentea and B. subadvena are combined into the same sample interval and are indistinguishable from one another (Figure 45). The maximum abundance of B. tenuata occurred at near 5 mm sediment depth, though a high abundance was also observed near the sediment-water interface. Despite the difficulty in sectioning the high porosity sediment at millimeter resolution, the preferred vertical sequence o f the three species is still visible in the distribution of foraminiferal carbon isotope measurements (Figure 46). Two B. tenuata 8 13C measurements of-1.2%o and -1.4%o are omitted due to a mass spectrometry problem involving a single batch of samples. All other foraminiferal 47 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. S C samples from San Bias Basin were analyzed at later dates in conjunction with new standards. B. argentea records the most positive S1 3 C (-0.7% o), reflective of its preference for a near-epifaunal habitat. Intermediate values of-l.l% o are recorded by B. subadvena and the most negative S!3C values (-1 .6%o) are recorded by B. tenuata, indicative of infaunal habitats that are vertically stratified according to species. Pore water S1 3 C values at the depth of species maximum abundance correspond to measured values of B. argentea, B. subadvena, and B. tenuata (Figure 47). These benthic foraminifera reliably record the S1 3 C of ambient pore water at the sediment horizon where they precipitate their carbonate tests. & J L £ 5 a < L > " U c 0) C j Q c 3 g 7 /7 'Ti 10 B . t e n u a t a — ® — 5. a r g e n t e a B . s u b a d v e n a 0 200 400 600 800 1000 1200 1400 1600 A bundance p e r grain sedim ent 0 1 S S 2 ,g o. T 3 G U g ■ 3 4 B 0 i ! ' ’ 1 1 ■ ■ C O O G • T - 0 o a c 0 a ■ B . t e n u a t a ° B . a r g e n t e a B , s u b a d v e n a , i , . . . -2 -1.5 -0.5 5 13C Figure 45. Foraminiferal distribution pattern in Figure 46, Measured §,3C values of benthic San Bias Basin (NH01-32-MC1). The maximum foraminifera collected from San Bias Basin multi abundance of B. subadvena, usually 1-3 mm, core NH01-32-MC1. appears to be the same as B. argentea due to difficulty in sectioning at millimeter resolution. 48 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. B . s u b a d v e n a B . t e n u a t a \ B . a r g e n t e a Figure 47. Carbon isotopic compositions of benthic foraminifera in San Bias Basin in comparison to predicted pore water values. The foraminiferal 8nC correspond to pore water S1 3 C at the horizon of species maximum abundance. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5. DISCUSSION 1 ^ 5.1. Benthic Foraminiferal Distribution and 8 C Patterns Along the Northeastern Pacific Margin The relationships between the 51 3 C signatures of three species of benthic foraminifera, B. argentea, B. subadvena, and B. tenuata, and the S1 3 C of ambient pore water TCO2 are investigated at six sites along the northeastern Pacific margin that vary in bottom water oxygen concentration and carbon accumulation. The range of measured pore water TCO2 gradients along the margin (Figure 48), which is controlled by changes in the rates of carbon oxidation and CO2 diffusion, illustrates these differences between the six locations. In general, a steeper pore water gradient is indicative of increased carbon oxidation and depletion o f dissolved oxygen in bottom water, though CO2 diffusion may also vary slightly. 0 o o S * NHO 1-2 < NHO 1-3 * NH01-8 * NHOl-lO ■ NH01-12 NHO 1-30 0 NHO 1-31 4 2 2.5 3 3.5 4 4.5 5 TCO (mmol/L) Figure 48. Pore water TC02 gradients from six CALMEX sites along the northeastern Pacific margin (W. Berelson, unpublished data). A range of environments that differ in bottom water oxygen concentration and carbon accumulation are represented. 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Comparison o f the depth distribution patterns o f the three species at sites along the northeastern Pacific margin indicates that the positions o f living B. argentea, B. subadvena, and B. tenuata within the sediment column are recurrent (Table 2). The maximum abundance is very often not a sharp peak at a specific depth horizon, but rather high abundances persist through a narrow range of sediment depths. B. argentea prefers a habitat near the sediment-water interface, usually in the upper millimeter of sediment. The infaunal species B. subadvena is consistently found at intermediate depths of 1-3 mm. Similar distribution patterns of B. subadvena are also observed in basins within the Gulf of California (R. Douglas and F. Staines-Urias, personal communications). Stained B. tenuata individuals are observed to be most abundant at depths ranging from 3-6 mm. The relative vertical sequence of the three species remains constant at all sites. The overall range of sediment depths in which the three species abundance maxima occur and the distances between the abundance maximum of B. subadvena and those of B. argentea and B. tenuata are slightly variable between sites. These variations are small, only 1 - 2 mm, and are within the uncertainties in obtaining sectioned sediment samples at the millimeter scale and assigning depths of maximum live foraminiferal populations using rose Bengal staining. Thus, the sediment depth horizon of maximum abundance for each o f the three benthic foraminiferal species is considered constant, regardless of these very small discrepancies between sites. The consistent depth stratification of B. argentea, B. subadvena, and B. tenuata within the sediment column is also evident in their measured carbon isotopic 51 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. compositions, which are summarized in Table 2. With the exception of B. tenuata in 1 T Santa Barbara Basin, each of the three species records uniform 8 C values regardless of position in the sediment column, indicative of a preferred sediment depth horizon for test precipitation. B. argentea consistently records the most positive 8 C values that may resemble those close to bottom water, reflective of its preferred habitat near the sediment-water interface. B. subadvena records uniform intermediate 8 1 3 C values at each site, concurrent with its intermediate position in the sediment column. Generally, the most negative 8 1 3 C is recorded by B. tenuata, in agreement with a preferred environment deeper in the sediment column. B. argentea Depth 51 3 C (mm) (% o ) B. subadvena Depth S,3 C (mm) (% o ) B. tenuata Depth S1 3 C (mm) (% o ) Santa Barbara Basin 0-1 -1.199 1-2 -2.366 3.5-4.5 -2.139 Santa Monica Basin 0-1 -0.773 1-2 -1.384 3.5-4 -2.432 Soledad Basin 0-1 -1.318 1-3 -1.468 3-5 -1.969 Magdalena Basin 0-1 -0.614 1-2 -1.356 4-6 -1.543 Matzatlan margin 0-1 -0.204 1-2 -0.631 3-5 -1.044 San Bias Basin 0-2.5 -0.711 0-2.5 -1.093 4-6 -1.666 Table 2. Summary of observed benthic foraminiferal abundance maxima and average measured carbon isotopic compositions along the northeastern Pacific margin. | 3 5.2. Benthic Foraminifera As Recorders of Pore Water 8 C The carbon isotopic composition data generated from five o f the six investigated sites indicate that the 8 1 3 C recorded by benthic foraminifera, 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. excluding B. tenuata in Santa Barbara Basin, resembles the 8 1 3 C of pore water TCO2 in a narrow range o f sediment depths. In many instances, this range is similar to the horizon of maximum abundance for each species. The correlation between the measured 51 3 C o f foraminiferal carbonate and pore water TCO2 cannot be confirmed at the sixth site, Magdalena Basin, due to bioturbation and bioirrigation. The §1 3 C values recorded in the carbonate tests o f B. argentea, B. subadvena, and B. tenuata 11 may be a reliable approximation of ambient pore water 8 C. It is possible that the Sl3C recorded by a benthic foraminiferal species consistently overestimates or underestimates the 8 1 3 C o f pore water TCO2, or that the observed position of maximum live abundance does not always represent a preferred habitat for test calcification. For these reasons, in evaluating the potential use of benthic I o I o foraminifera as recorders of pore water 8 C, it is important to consider the 8 C signatures o f each species independently compared to ambient pore water. The differences between the carbon isotopic compositions (A8 13C) of epifaunal and 1 T infaunal benthic foraminiferal species that accurately record the 8 C of pore water TCO2 may be used to reconstruct pore water TCO2 gradients in a range of environments along the margin o f the northeastern Pacific that vary in bottom water oxygen concentration and carbon accumulation. 5.2.1. B. argentea The results o f Santa Monica Basin B. argentea isotope measurements duplicate those o f Berelson and Stott (2003). The measured 8 1 3 C values ranged 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. within about -l%o o f bottom water (-0.5%o), similar to upper millimeter pore water values. In Santa Barbara Basin, B. argentea uniformly records 8 1 3 C values that are substantially more negative (~l%o) than bottom water and more closely resemble the 813C of pore water in the upper millimeter o f sediment. In Soledad Basin, B. argentea repeatedly records a 8 1 3 C value approximately 0.8%o lighter than that of bottom water. At this site, these foraminiferal values intersect the pore water 81 3 C profile at a slightly greater sediment depth o f ~3 mm. On the Matzatlan margin and in San Bias Basin it is difficult to compare B. argentea and pore water 51 3 C values because only predicted pore water data are available. A bottom water 8 1 3 C value must be assumed in order to model pore water 813C values from measured TCO2 concentrations, thus fixing the x-intercept of the modeled profile and potentially obscuring the actual similarity or difference between the foraminiferal and pore water 51 3 C. Using the pore water S13C profiles modeled with a bottom water value o f-0.5%o, the Matzatlan margin B. argentea appear to record slightly heavier values than bottom water. The San Bias Basin B. argentea appear to record slightly lighter values than bottom water. The small discrepancy 1 - 2 between foraminiferal and modeled pore water 8 C at both sites is within 0.2 to 0.3%o and might be explained by an inaccurate estimate o f the bottom water S1 3 C 1 ^ used in the model or the margin of error in the foraminiferal 8 C measurement. At both sites, B. argentea seems to record 8 °C values like those ofbottom water or upper millimeter pore water, though additional isotopic analyses o f Matzatlan margin and San Bias Basin bottom and pore waters are needed to confirm this. 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Santa Barbara Santa Monica Soledad Matzatlan San Bias Basin Basin Basin margin Basin Observed 0 - 1 mm 0 - 1 mm 0 - 1 mm 0 - 1 mm 0-2.5 mm Predicted 1 mm 0 mm 3 mm 0 - 1 mm 0 - 1 mm Table 3. Sediment depths of observed maximum abundance of B. argentea and the predicted depths of test precipitation from comparison of pore water and foraminiferal 81 3 C. The correlation between the B. argentea S1 3 C and pore water 8 1 3 C values occurs at a near constant sediment depth (Table 3) along the northeastern Pacific margin. With the exception of Soledad Basin, B. argentea precipitates its test with a 8 1 3 C like that of pore water TCO2 within the upper millimeter of sediment. This is coincident with the observed horizon of maximum living (rose Bengal stained) abundance for this species, implying that B. argentea prefers to precipitate its test in the upper millimeter of the sediment column. As such, this species can reasonably be applied as a proxy measure of ambient pore water carbon isotopic composition near the sediment-water interface in a variety of environments situated along the northeastern Pacific margin. 5.2.2. B. subadvena In Santa Monica Basin, B. subadvena records 8 1 3 C values that correspond to the pore water profile at 1.5 mm sediment depth, in agreement with the horizon of maximum abundance at this site. Santa Barbara Basin B. subadvena have S1 3 C signatures that are predicted by the pore water profile to occur at 2 mm sediment depth, comparable to the horizon o f species maximum live abundance that is 1 observed at 1-2 mm in this basin. The Soledad Basin B. subadvena 8 C 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. values intersect the pore water profile at close to 4 mm while the maximum live abundance is observed at 1-3 mm sediment depth. The Matzatlan margin B. 1 T I T • subadvena 8 C results correspond to the pore water 8 C profile at approximately 1.5 mm sediment depth and the San Bias foraminiferal data is similar to pore water S1 3 C at 3 mm. Santa Barbara Santa Monica Soledad Matzatlan San Bias Basin Basin Basin margin Basin Observed 1 - 2 mm 1 - 2 mm 1-3 mm 1 - 2 mm 0-2.5 mm Predicted 2 mm 1.5 mm 4 mm 1.5 mm 3 mm Table 4. Sediment depths of observed maximum abundance of B. subadvena and the predicted depths of test precipitation from comparison of pore water and foraminiferal 8I3 C. The range of measured 8 1 3 C values of B. subadvena was very small for each site, which implies test precipitation at a single depth horizon. The sediment depths 1 T at which the foraminiferal data match the pore water 8 C are similar to the observed horizons of maximum living (rose Bengal stained) abundance for B. subadvena, consistently found to occur within 1-3 mm at all sites (Table 4). In Soledad Basin the 8 1 3 C signature of B. subadvena is representative of slightly deeper pore water 1 T carbon isotopic composition according to the pore water 8 C profile, which predicts that test precipitation occurs at close to 4 mm sediment depth. This small difference between the observed horizon of maximum live abundance and the predicted depth of test precipitation might be attributed to the uncertainties in sectioning the multi core at 1 mm resolution, isotopic analyses o f the carbonate or pore water, or the bottom water 8 1 3 C value used in the model. Additionally, Soledad Basin is 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. characterized by a higher rate o f carbon oxidation than the other sites investigated, which may be an influence on the microhabitat preference of B. subadvena. At several locations along the margin o f the northeastern Pacific, the observed maximum live abundance of B. subadvena constantly occurs at a sediment depth horizon o f 1-3 mm. The 8 1 3 C recorded by B. subadvena also consistently corresponds to the S1 3 C of ambient pore water TCO2 within nearly the same sediment depth interval, an indication that this species prefers this environment for 1 ^ the precipitation o f its carbonate test. Therefore, it is reasonable to use the 8 C of B. subadvena as a proxy estimate o f ambient pore water carbon isotopic composition at intermediate sediment depths of 1-3 mm in a range o f environments that vary in their bottom water dissolved oxygen concentrations and rates of carbon accumulation. 5.2.3. B. tenuata Two o f the dysoxic sites studied, Santa Barbara and San Bias Basins, have slightly different B. tenuata distribution patterns than the other four locations. An abundance maximum of rose Bengal stained individuals was found at 4-5 mm sediment depth in both of these basins, though high abundances equal to the subsurface maximum are also observed near the sediment-water interface (Figure 49). This pattern of an infaunal habitat coupled with an epifaunal habitat preference may be a response to extremely low oxygen conditions. The presence of living B. tenuata throughout a wide range o f sediment depths in Santa Barbara Basin has 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. previously been noted (Stott, et. al., 2002), implying that individuals migrate considerably within the sediment column. is s San B ias B asin Santa B arbara B asin 10 0 200 400 600 800 1000 A bundance per gram sedim ent Figure 49. Abundances of living (rose Bengal stained) B. tenuata in San Bias and Santa Barbara Basins. It is possible that B. tenuata individuals prefer to precipitate their tests in a specific infaunal habitat, and then migrate toward more favorable oxygen levels or other living conditions. In this case, the 8 1 3 C signature of each species should remain relatively constant, reflecting the 8 1 3 C of ambient pore water at the preferred depth horizon for test precipitation. In the “SBBC” core, uniform B. tenuata S1 3 C suggests an apparent calcification depth coincident with the abundance maximum at approximately 4 mm despite the possibility of significant migration. However, in contrast, the isotopic compositions observed in CALMEX samples do not imply a specific depth horizon for precipitation of B. tenuata tests. Measured B. tenuata 8 1 3 C 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in Santa Barbara Basin were highly variable in the upper 4 ram o f sediment and none were reproducible. It is unlikely that analytical error is responsible for the non-uniform B. tenuata SI3C measurements, as duplicate samples were run on different dates in conjunction with replicate standards and other foraminiferal carbonate samples. It is possible that individual B. tenuata tests included in the first analyses of Santa Barbara Basin samples were insufficiently cleaned and contained remnants of organic material or other contaminants that affected the 8 1 3 C results. The duplicate samples were prepared under a microscope to ensure that no dirty or unidentifiable shell fragments were included in the analysis. Santa Barbara Santa Monica Soledad Matzatlan San Bias Basin Basin Basin margin Basin Observed 4-5 mm 4-5 mm 3-5 mm 3-5 mm 4-6 mm Predicted None 7 mm 6 mm 4-5 mm 4-5 mm Table 5. Sediment depths of observed maximum abundance of B. tenuata and predicted horizons of test precipitation from comparison of pore water and foraminiferal 5I3 C. At the other four sites, B. tenuata records consistent 8 1 3 C values regardless of sediment depth, indicative o f a preferred depth horizon for test precipitation. While this preferred horizon is generally similar to the depth of maximum species abundance, the position of equivalent pore water 8 1 3 C values occasionally indicates test precipitation deeper in the sediment (Table 5). Santa Monica Basin B. tenuata have 8 1 3 C values that match pore water values closer to 7 mm, though stained individuals were observed to be most abundant at 4-5 mm sediment depth. In 59 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Soledad Basin, the 8 1 3 C of B. tenuata is the same as pore water 8 1 3 C at about 6 mm sediment depth, whereas the greatest live abundance is found at 3-5 mm. The Matzatlan margin and San Bias Basin B. tenuata S1 3 C results are each in agreement with pore water values at 4-5 mm, comparable to the horizons of maximum species abundance found at those locations. At all sites investigated along the margin o f the northeastern Pacific, the greatest abundance of living B. tenuata was found primarily between 4-6 mm sediment depth. In most cases, the 8 1 3 C values recorded in the foraminiferal tests are uniform and intersect the pore water 8 1 3 C profiles within a similar range of sediment depths, suggestive o f a preferred environment for carbonate secretion. The slight offset ( 1 - 2 mm) between the depths of observed maximum live abundance and predicted test precipitation in Santa Monica and Soledad Basins are within the margins o f error in sampling the sediment into 1 mm sections and analyzing the 1 ■ y carbon isotopic compositions of carbonate and pore water. Variable 8 C values measured in Santa Barbara Basin do not indicate test precipitation at a preferred sediment depth horizon for B. tenuata. The 8 1 3 C recorded by B. tenuata can be used as a proxy measurement of deeper infaunal ambient pore water carbon isotopic composition at 4-6 mm sediment depth in a range of environments along the margin of the northeastern Pacific, not including Santa Barbara Basin. 60 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.3. Implications for Reconstructing Histories o f Carbon Oxidation Using the 5 1 3 C o f Benthic Foraminifera At several sites along the margin o f the northeast Pacific, the maximum abundance of B. argentea is repeatedly within the upper millimeter o f sediment, which agrees with the average depth of test precipitation at 0 . 8 mm predicted by pore water 8 13C. B. subadvena records the most uniform 8 °C values o f the three species, illustrated by the very narrow range of measured values throughout the sediment column at each site. It is observed to live primarily at intermediate depths of 1-3 mm in the sediment column and is predicted by pore water 51 3 C values to precipitate its test at an average o f 2.4 mm. Likewise, the greatest numbers of B. tenuata are found at about 4-6 mm sediment depths, and the average depth of test precipitation 1 3 predicted from pore water 8 C is 5.5 mm. 1 ^ While the depth o f ambient pore water associated with the 8 C of 5. argentea, B. subadvena, and B. tenuata is not always precisely the same as the depth of maximum live species abundance, for all three species it is within 1 - 2 mm deeper in the sediment column which can be explained by uncertainties in sectioning the sediment at the 1 mm scale and the isotopic analyses o f pore water TCO2 and foraminiferal carbonate. None o f the species systematically over- or underestimate the pore water S1 3 C at the sites investigated. The isotopic analyses o f benthic foraminifera and pore water combined with species distribution patterns show that the 8 1 3 C o f B. argentea, B. subadvena and B. tenuata provide reasonable estimates of the 8 1 3 C of ambient pore water TCO2 in a range o f environments along the 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. northeastern Pacific margin that vary in bottom water oxygen content and carbon accumulation, excluding B. tenuata in Santa Barbara Basin. The carbon isotopic composition of pore water TCO2 is the result of a combination o f the 8 1 3 C o f TCO2 in bottom water and the 51 3 C of TCO2 added from sources with distinct isotopic compositions. The analyses o f pore water made at several o f the CALMEX sites indicate measurable quantities o f methane. A pore water methane profile dominated by diffusion will have a constant gradient, whereas a profile influenced by diffusion and reactions will have a variable gradient. A steeper methane gradient was observed at some sites, notably Soledad and Magdalena Basins, which could reflect oxidation of methane from the pore water rather than diffusion through the sediment column or a stronger source o f methane from deeper sediment horizons. Methane can be anaerobically oxidized in the upper section of the sediment column by sulfate-reducing marine microbes: CH4 +SO 4' -»HCO~ +HS~ + h 2o according to Boetius, et. al. (2000). Because methane typically has a 8 1 3 C o f-57 to -73%o (Kvenvolden, 1995), this process o f methane oxidation would preferentially 1 - j add C to ambient pore water in the upper portion of the sediment column, 1 3 potentially causing the 8 C signatures observed in the benthic foraminiferal tests to reflect isotopically lighter pore water that is not attributable solely to the oxidation of photosynthetically derived carbon. Conversely, the production of methane at depths below the horizon o f sulfate depletion would sequester 12C, leaving the pore water DIC enriched in 1 3 C and increasing the 8 1 3 C values recorded by benthic foraminifera. 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Previous study of the pore water isotope profiles from Santa Barbara Basin (Stott, et. al., 2002) and the other CALMEX locations has not revealed evidence of an influence from methane derived CO2 on pore water or foraminiferal isotopic compositions. The predicted pore water §1 3 C profiles, which attribute pore water CO2 entirely to the oxidation of photosynthetically derived carbon, closely match the measured 81 3 C profiles. To test this observation, the isotopic composition of the source of CO2 added to the pore water can be estimated using a diffusion-reaction model for pore water TCO2 profiles as described by Stott, et. al. (2002). In addition, the contributions of the oxidation of organic carbon and methane to the pore water TCO2 pool, and subsequent effects on the S1 3 C of pore water TCO2, can be estimated via isotope mass balance calculations. 5.3.1. Diffusion-Reaction Modeled Sources o f Pore Water bI3C The 51 3 C o f the CO2 added to pore water in Santa Barbara Basin is estimated to be -15%o based on a diffusion-reaction model for pore water TCO2, heavier than the value o f-17%o calculated by Stott, et. al. (2002) using SBBC core data. This discrepancy in calculated source values might be due to a difference in the depth scales of the measured pore water profiles used in the model. The measured profile from the SBBC core extends to 50 cm sediment depth, whereas the CALMEX pore water measurements in Santa Barbara Basin include only the upper 2 cm o f the sediment column, leaving the deeper portion o f the profile unconstrained by the model. As the TCO2 and 81 3 C gradients in the upper 2 cm are alike in both cores, 63 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the SBBC pore water TCO2 data provides estimated TCO2 values for the deeper section of the NH01-8-MC2 profile and the modeled source of 81 3 C in Santa Barbara Basin is recalculated to be -19%o. 1 " X The diffusion-reaction model was also used to find source 8 C values for Soledad and Santa Monica Basins. The former contains higher concentrations of pore water methane relative to other CALMEX sites and the latter also contains significant amounts of methane. The closest match between the pore water profile predicted by the model and the observed 51 3 C values requires a carbon isotopic source of -21%o in both basins. All o f the source S1 3 C values produced by the diffusion-reaction model resemble the typical mid-latitude 81 3 C values (-18 to -23%o) of biologically produced organic material presented by McCorkle, et. al. (1985). The results of the calculations are expected to be significantly different if there were a substantial influence of the oxidation or production of methane on the measured isotopic compositions of pore water TCO2 or benthic foraminiferal carbonate. The estimated source isotopic compositions are appreciably heavier than methane-derived CO2 values that vary from -5 7 to -73%o (Kvenvolden, 1995) and are more indicative of CO2 produced from the oxidation of organic carbon. The results o f the diffusion- reaction model indicate that the isotopic compositions of pore water TCO2 and benthic foraminifera primarily respond to the oxidation of organic carbon and are not significantly influenced by methane oxidation or production. 64 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. To further constrain the potential influence o f small amounts o f methane oxidation on the 813C o f pore water TCO2, two end member scenarios are examined. Pore water 51 3 C values that include the addition of CO2 derived from methane oxidation are compared with pore water values that result from the oxidation o f photosynthetically produced carbon. 5.3.2. Contribution o f Methane Oxidation to Pore Water TCO2 and bI3C Two end member contributions of methane oxidation to pore water TCO2 can be considered in terms of isotope mass balance calculations: 1) all methane diffuses out of the sediments without participating in chemical reactions that affect pore water TCO2, and 2) all methane produced in the sediment column is oxidized and added to 1 -j the pore water TCO2 pool. In the first case, the 8 C o f pore water TCO2 is not influenced by the 1 2 C addition from methane. If the second is true, methane greatly influences the 8°C of pore water TCO2. The methane concentration at the horizon o f sulfate depletion is taken to reflect the source value. If it is assumed that the pore water methane gradients reflect the complete oxidation of all methane produced deep in the sediment column, the contribution o f organic carbon oxidation to pore water TCO2 is calculated using an isotope mass balance equation: (81 3 C p w )(TC0 2 p w ) = (S1 3 C bw)(TC02 bw ) + (ACH4)(81 3 C cm) + (TC02 0 r g)(51 3 C o r g ) where (pw) refers to measured pore water values at a sediment depth horizon, (bw) indicates a bottom water value, and (org) represents organic carbon values. The 65 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. amount of methane oxidized is given by ACH4, the difference between measured concentrations at the depth of the pore water values and below the sulfate-reduction zone. The results of this isotope mass balance for the amount o f CO2 derived from organic carbon oxidation (TCO2 o r g ) in Santa Barbara, Soledad and Magdalena Basins are given in Table 6 . Santa Monica Basin values cannot be calculated for lack of observed pore water methane concentrations, nor can values for Matzatlan 1 ' I margin or San Bias Basin without observed pore water 8 C values. Because the pore water methane concentrations along the northeastern Pacific margin are all micromolar, the complete oxidation o f methane does not influence pore water TCO2 concentration, which is millimolar. As shown in Table 6 , even when all available methane is oxidized, the ratio of CO2 derived from organic carbon oxidation to that from methane oxidation is consistently much greater than 1 . Contribution to Pore Water CO2 Sediment depth (cm) a c h 4 (mmol) TCO2 org (mmol) Co r/C H 4 Santa Barbara Basin 0-39 0.02 0.4 20 39-65 0.02 10.2 510 Soledad Basin 0-46.5 0.006 0.52 87 46.5-185 0.51 18.1 35 Magdalena Basin 0-46 0.01 0.52 52 46-280 0.46 40.3 88 Table 6. Calculated contributions of CG2 derived from methane (CH 4 ) and organic carbon (C0 rg) oxidation to pore water TC02. The high ratios of Co rg to CH 4 contributions indicate that organic carbon oxidation dominates the addition of C 02 to the pore water. 6 6 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Although the concentrations of methane do not influence pore water TCO2 concentrations, the negative 8 1 3 C value o f the methane might have a significant effect on the §1 3 C of pore water TCO2 . The isotope mass balance calculations are repeated to find the 8 1 3 C of pore water TCO2 using a measured pore water TCO2 value, assuming that all available methane is oxidized and that remaining CO2 added to the pore water is derived from organic carbon oxidation. The calculated pore water §1 3 C values that include the oxidation o f all methane present are compared to calculated pore water values that attribute all CO2 added to the pore water to organic carbon oxidation in Table 7. Pore Water & 13C(%o) Sediment depth (cm) Calculated if available CH4 oxidized Calculated if only Corg oxidized Observed Santa Barbara Basin 1 -3.68 -3.66 -3.67 (NH01-8-MC2) 39 -17.68 -17.62 none Soledad Basin 1.5 -5.39 -5.13 -4.04 (NH01-10-MC1) 46.5 -18.81 -18.76 -18.73 Magdalena Basin 1 -0.92 -0.92 -2 . 2 (NH01-12-MC1,2) 46.5 -19.89 -19.88 -19.89 Table 7. Comparison of calculated pore water 81 3 C if all available methane is oxidized, values if only organic carbon oxidation occurs, and observed values. The oxidation of all available methane in the sediment column does not have a significant impact on the 51 3 C of pore water TC 02. The 8 1 3 C o f pore water TCO2 does not change appreciably even when there is 1 T complete methane oxidation in the sediment column. Pore water 8 C is not affected by the addition o f methane derived CO2 at the methane concentrations present today. The calculated values o f each site do not differ beyond the uncertainty in carbon 67 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. isotopic analyses. The calculated values are also generally similar to the 11 observed value at each depth horizon at each site. The observed pore water 8 C value in Magdalena Basin is much more negative than either calculated 81 3 C value, and most likely reflects continued carbon oxidation during storage, as previously discussed. Small discrepancies between the observed and calculated values can be attributed to the margins of error in sediment core sectioning, whole core squeezing and measurements of isotopic composition. The amount o f methane present in pore waters at the sites investigated cannot contribute significant concentrations of CO2 to the pore water TCO2 pool, and its effect on the carbon isotopic composition o f pore water TCO2 is negligible. The carbon isotopic composition of pore water TCO2 that is recorded by benthic foraminifera primarily responds to changes in the oxidation o f photosynthetically produced organic carbon. The conclusion drawn from this investigation is that n reconstructions of pore water TCO2 gradients using the 5 C signatures o f B. argentea, B. subadvena, and B. tenuata can be used to interpret changes in carbon oxidation along the northeastern Pacific margin. 5.4. History o f Carbon Oxidation in Santa Monica Basin The pore water 81 3 C gradient is indicative of the pore water TCO2 gradient, which is controlled by the rate o f carbon oxidation occurring at the sea floor. Berelson and Stott (2003) established a relationship between the AS1 3 C o f benthic 1 T foraminifera, i.e. the pore water 8 C gradient, and carbon oxidation in Santa 68 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Monica Basin (Figure 50). In locations where benthic foraminiferal species have been shown to be reliable recorders of the carbon isotopic composition of ambient pore water, such as Santa Monica Basin, longer histories o f carbon oxidation may be produced. 7.0 «.0 5.0 " O s y 4.0 ■3 J, 3 - ° o 2.0 1.0 O .f l 0.50 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 ASUC (0-5 mm) Figure 50. Relationship between the 51 3 C gradient from 0-5 mm and the quantity of carbon oxidation (Berelson and Stott, 2003). Extending the proxy record in Santa Monica Basin demonstrates a spatial and temporal change in carbon oxidation over the past 500 years (Berelson and Stott, 2003). The DOE 26 core was collected from the center of the basin and is laminated through the past 400 years. The EW95 MC4 core is from the slope and is laminated beginning in the 1940s. The values and trends of A51 3 C between B. argentea and B. tenuata are similar when both cores are laminated (Figure 51). This implies that both sites are recording similar changes in carbon oxidation through time. Over the last three centuries, there is a progressive increase in the pore water gradient, concurrent with an expansion o f the laminated zone outward from the center of 69 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. the basin which was documented by Christensen, et. al. (1994) (Figure 52). The A51 3 C record from the last century in the basin shows a decrease in carbon oxidation between 1900-1920 followed by an increase until the late 1970s, at which time carbon oxidation began to decline and the laminated zone began to retreat. 2 1.8 DOE 26 1.6 vo 1.4 1.2 EW95 MC4 1500 1600 1700 1800 1900 2000 Year AD Figure 51. A51 3 C values using B. argentea and B. tenuata from Santa Monica Basin center (DOE 26) and slope (EW95 MC4) cores. The shaded section indicates where both cores are laminated simultaneously (Berelson and Stott, 2003). H $ e 2 0 ’ !J9«00' i lg 0 4 0 ' M ° 00‘ R u « n * m e ' Santa Sorboro C h a n n e l DEPTH m METERS 180 M CONTOUR iNTERVAl C a n y o n J\ \ w o n y o n POINT D O M E ANACAPA J S L A N O / V ^ ~Canyon O u m e \ Y ( T T x T X \ ) . u s " ' 33°<tO' 3 1 2 0 0 31199 25494 25505® ; / s ' ; ( r % \ \ ... 119«00' v Sonic M onica '3 V * V ? " * 0 " ,\ a Y f r ~ \ ' O . , \ \ \ w Y \\ R * ‘ |o i x l « \ J c<-25StO „ C T „..0 ' »3i V \ N > l / . Redondo Fan '//V v . 34«00' 33‘MG' Figure 52. Observed expansion of the laminated zone outward from the center of Santa Monica Basin. Ages are based on 2 1 0 Pb dating at core locations shown (Christensen, et. al., 1994). 70 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The addition of a second extended B. tenuata record from Santa Monica Basin using CALMEX multi-core NH01-2-MC1 (L.D. Stott, unpublished data) confirms the longer trends observed in the DOE 26 data (Figure 53). These cores were collected from the deep southern section o f the basin plain, within the zone of laminated sediments (Christensen, et. al., 1994). While the spatial and temporal correlation between the EW95 MC4 and DOE 26 sites is visible only during the last century, the B. tenuata 51 3 C data from NH01-2-MC1 extends the record of concurrent changes in carbon oxidation in different areas of Santa Monica Basin through the last 500 years. Combining long-term records o f benthic carbon oxidation in low oxygen environments with records of organic carbon burial allows the reconstruction of organic carbon rain to the sea floor. Previous work in Santa Monica Basin has shown that changes in the amount o f carbon rain reaching the sea floor is coupled with primary productivity (Berelson and Stott, 2003) and subsequent variations in benthic carbon oxidation will affect the concentration of dissolved oxygen in bottom waters. As such, changes in bottom water ventilation along the northeastern Pacific margin are not necessary to explain the variable formation and preservation of laminated and bioturbated sediments. 71 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 2000 1900 1800 1 3 1700 1600 1500 1400 -♦ — DOE 26 o- — NH01-2-MC1 1300 •2 -1.5 1 -2.5 ■ 3 51 3 C (% o ) Figure 53. Comparison of B. tenuata 81 3 C from Santa Monica Basin cores DOE 26 and NH01-2-MC1 (L.D. Stott, unpublished data). Both cores were collected from the deep center of the basin plain and record similar carbon isotope values and trends over time. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 6. CONCLUSIONS The depth distribution patterns and isotopic composition of three benthic foraminifera, B. argentea, B. subadvena, and B. tenuata, reflect the specific microenvironments in which they live. B. argentea inhabits the upper millimeter of the sediment column, B. subadvena is most abundant at depths o f 1-3 mm, and B, tenuata prefers a slightly deeper infaunal habitat at 4-6 mm. In most instances, these three species of benthic foraminifera record unvarying 51 3 C values, suggestive o f a narrow range o f depth horizons for calcification o f carbonate tests. The foraminiferal 8 1 3 C values are similar to the S1 3 C of pore water TCO2 at the sediment depth horizon of species maximum abundance. B. argentea records 5I3C values that are similar to the 8 1 3 C of pore water TCO2 at an average o f 0 . 8 mm sediment depth, coincident with its habitat preference. B. subadvena 8 1 3 C is representative of pore water at an average of 2.4 mm sediment depth, which agrees with its observed living 1 3 maximum abundance. B. tenuata is unreliable as a recorder of pore water 8 C in Santa Barbara Basin, but at all other sites reflects pore water isotopic composition at an average o f 5.5 mm, similar to the sediment depth at which most living individuals are found. This study verifies previously published calibrations in Santa Monica Basin and extends the application of pore water S1 3 C reconstructions to four other locations along the northeastern Pacific margin: Santa Barbara Basin, Soledad Basin, the Matzatlan margin, and San Bias Basin. Benthic foraminifera in Magdalena Basin display depth distribution and isotopic composition patterns that are comparable 73 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. to those o f the other sites though cannot be compared to pore water geochemical profiles due to bioturbation and bioirrigation. The results o f diffusion-reaction modeling o f pore water TCO2 gradients and isotope mass balance calculations indicate that carbon dioxide derived from the oxidation of methane does not have a significant influence on the observed carbon isotopic compositions of pore water TCO2 or benthic foraminiferal carbonate in Santa Barbara, Soledad, or Magdalena Basins. The differences between the carbon isotopic compositions o f benthic foraminifera living at specific sediment depths are indicative o f the amount of carbon oxidation. The proxy is applicable in environments with different bottom water oxygen concentrations and carbon accumulation along the northeastern Pacific margin. Testing of the proxy for benthic carbon oxidation at different locations demonstrates its reliability in a range of oxygenated environments and its potential for future regional correlations of paleoproductivity records. Surface productivity and the oxidation o f organic carbon at the sea floor are important controls of bottom water dissolved oxygen concentrations, and Holocene and Pleistocene records of bioturbated and laminated sediments in this area can be interpreted to reflect changes in these processes. With future testing, the proxy measurements o f pore water S1 3 C and TCO2 gradients using the 51 3 C of benthic foraminiferal carbonate may also be applicable at other dysoxic sites with associated accumulations o f preserved laminated sediments. 74 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. REFERENCES Behl, R.J. & Kennett, J.P. (1996). Brief interstadial events in the Santa Barbara Basin, NE Pacific, during the past 60 kyr. Nature, 379, 243-246. Bender, M., Martin, W., Hess, J., Sayles, F., Ball, L. & Lambert, C. (1987). A whole-core squeezer for interfacial pore water sampling. Limnology and Oceanography, 32, 1214-1225. Berelson, W.M. & Stott, L.D. (2003). Productivity and organic carbon rain to the California margin seafloor: Modem and paleoceanographic perspectives. Paleoceanography 18, 10.1029/2001P A000672. Bernhard, J.M. (1988). Postmortem vital staining in benthic foraminifera: Duration and importance in population and distributional studies. Journal of Foraminiferal Research, 18, 143-146. Bernhard, J.M. (2000). Distinguishing live from dead foraminifera: Methods review and proper applications. Micropaleontology, 46, supplement 1, 38-46. Bernhard, J.M. & Reimers, C.E. (1991). Benthic foraminiferal population fluctuations related to anoxia: Santa Barbara Basin. Biogeochemistry, 15, 127-149. Bernhard, J.M., Sen Gupta, B.K. & Bome, P.F. (1997). Benthic foraminiferal proxy to estimate dysoxic bottom-water oxygen concentrations: Santa Barbara Basin, U.S. Pacific continental margin. Journal o f Foraminiferal Research, 27, 301-310. Boetius, A., Ravenschlag, K., Schubert, C.J., Rickert, D., Widdel, F., Gieseke, A., Amann, R., Jorgensen, B.B., Witte, U., & Pfannkuche, O. (2000). A marine microbial consortium apparently mediating anaerobic oxidation of methane. Nature, 407, 623-626. Cannariato, K.G. & Kennett, J.P. (1999). Climatically related millennial-scale fluctuations in strength of California margin oxygen-minimum zone during the past 60 k.y. Geology, 27, 975-978. Cannariato, K.G., Kennett, J.P. & Behl, R.J. (1999). Biotic response to late Quaternary rapid climate switched in Santa Barbara Basin: Ecological and evolutionary implications. Geology, 27, 63-66. 7 5 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Christensen, C.J., Gorsline, D.S., Hammond, D.E. & Lund, S.P. (1994). Non-annual laminations and expansion of anoxic basin-floor conditions in Santa Monica Basin, California Borderland, over the past four centuries. Marine Geology, 116,399-418. Corliss, B.H. (1985). Microhabitats of benthic foraminifera within deep-sea sediments. Nature, 314, 435-438. Corliss, B. & Silva, K.A. (1993). Rapid growth of deep-sea benthic foraminifera. Geology, 21, 991-994. Del Viscio, J.G. (2001). Decadal-scale climate variability in the northeast Pacific oxygen minimum zone; Evidence from carbonate and organic carbon records. Abstracts with Programs - Geological Society o f America, 33, 231. Grossman, E.L. (1987). Stable isotopes in modem benthic foraminifera: A study of vital effect. Journal o f Foraminiferal Research, 17, 48-61. Gorsline, D.S. (1992). The geologic setting o f Santa Monica and San Pedro Basins, California Continental Borderland. Progress in Oceanography, 30, 1-36. Hartnett, H.E., Keil, R.G., Hedges, J.I. & Devol, A.H. (1998). Influence o f oxygen exposure time on organic carbon preservation in continental margin sediments. Nature, 391, 572-574. Hinrichs, K.-U., Summons, R.E., Orphan, V., Sylva, S.P. & Hayes, J.M. (2000). Molecular and isotopic analyses o f anaerobic methane-oxidizing communities in marine sediments. Organic Geochemistry, 31,1685-1701. Honjo, S., Manganini, S.J., & Cole, J.J. (1982). Sedimentation of biogenic matter in the deep ocean. Deep Sea Research I, 29, 609-625. Hulsemann, J. & Emery, K.O. (1961). Stratification in recent sediments o f Santa Barbara Basin a controlled by organisms and water character. Journal of Geology, 69, 279-290. Huyer, A. (1983). Coastal upwelling in the California Current system. Progress in Oceanography, 12, 259-284. Kennett, J.P. & Ingram, B.L. (1995). A 20,000-year record of ocean circulation and climate change from the Santa Barbara Basin. Nature, 377, 510-514. Kvenvolden, K.A. (1995). A review of the geochemistry o f methane in natural gas hydrate. Organic Geochemistry, 23, 997-1008. 76 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Lange, C.B., Schimmelmann, A. Yasuda, M.K. & Berger, W.H. (1995). Paleoclimatic significance of marine varves off Southern California. In P. Wigand & M. Rose (Eds.), Proceedings o f the Southern California climate symposium: Trends and extremes o f the past 2000 years, Technical Reports - Natural History Museum o f Los Angeles County (1999) (pp. 45-92). Los Angeles: Natural History Museum of Los Angeles County. Larson, R.L. (1972). Bathymetry, magnetic anomalies, and plate tectonic history of the mouth o f the Gulf o f California. Geological Society o f America Bulletin, 83, 3345-3360. Live Access to Reference Data at PMEL [Data file]. Available from National Oceanic and Atmospheric Administration Pacific Marine Environmental Laboratory website, http://ferret.wrc.noaa.gov/las/main.pl. Lynn, R.J. & Simpson, J.J. (1987). The California Current system: The seasonal variability of its physical characteristics. Journal o f Geophysical Research, 9 2 ,12947-12966. Mackensen, A., Schumacher, S., Radke, J. & Schmidt, D.N. (2000). Microhabitat preferences and stable carbon isotopes o f endobenthic foraminifera: Clue to quantitative reconstruction of oceanic new production? Marine Micropaleontology, 40, 233-258. Matoba, Y. & Yamaguchi, A. (1982). Late Pliocene-to-Holocene Benthic Foraminifers of the Guaymas Basin, Gulf o f California: Sites 477 through 481. In J.R. Curray, D.G. Moore, et. al. (Eds.), Initial Reports o f the Deep Sea Drilling Project, 64 (Part 2) (pp. 1027-1056). Washington: U.S. Govt. Printing Office. McCorkle, D.C., Corliss, B.H. & Famham, C.A. (1997). Vertical distributions and stable isotopic compositions of live (stained) benthic foraminifera from the North Carolina and California continental margins. Deep-Sea Research I, 44, 983-1024. McCorkle, D.C., Emerson, S.R. & Quay, P.D. (1985). Stable carbon isotopes in marine pore waters. Earth and Planetary Science Letters, 74, 13-26. McCorkle, D.C., Keigwin, L.D., Corliss, B.H., & Emerson, S.R. (1990). The influence o f microhabitats on the carbon isotopic composition of deep-sea benthic foraminifera. Paleoceanography, 5,161-185. 77 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Ness, G.E., Sanchez, O., Couch, R.W. & Yeats, R.S. (1981). Bathymetry and oceanic crustal ages in the vicinity o f the mouth of the Gulf of California, illustrated using Deep Sea Drilling Project leg 63 underway geophysical profiles. In R.S. Yeats, B.U. Haq, et. al. (Eds.), Initial Reports o f the Deep Sea Drilling Project, 63 (pp. 919-923). Washington: U.S. Govt. Printing Office. O’Connell, S., Ortiz, J., van Geen, A., & Zheng, Y . (2001). Rapid changes in organic carbon preservation in sediments from coastal Baja California are linked to global climatic changes. Abstracts with Programs - Geological Society o f America, 33, 225. Orozco, E.T. (1993). Analisis volumetrico de las masas de agua del Golfo de California. Thesis for Master of Sciences. Centro de Investigation Cientifica y de Education Superior de Ensenada, Ensenada, Baja California, 80 pp. Rathbum, A.E., Corliss, B.H., Tappa, K.D., & Lohmann, K.C. (1996). Comparisons of the ecology and stable isotope compositions of living (stained) benthic foraminifera from the Sulu and South China Seas. Deep-Sea Research I, 43, 1617-1646. Rathbum, A.E., Levin, L.A., Held, Z., & Lohmann, K.C. (2000). Benthic foraminifera associated with cold methane seeps on the northern California margin: Ecology and stable isotope composition. Marine Micropaleontology, 38, 247-266. Rau, G.H., Froelich, P.N., Takahashi, T., & Des Marais, D.J. (1991). Does sedimentary organic 8 1 3 C record variations in Quaternary ocean [C C > 2(aq)]? Paleoceanography, 6 , 335-347. Reimers, C.E., Lange, C.B., Tabak, M. & Bernhard, J.M. (1990). Seasonal spillover and varve formation in the Santa Barbara Basin, California. Limnology and Oceanography, 35, 1577-1585. Roemmich, D., & McGowan, J. (1995). Climatic warming and the decline of zooplankton in the California current. Science, 267,1324-1326. Rullkotter, J., (2000). Organic matter: The driving force for early diagenesis. In H.D. Schulz & M. Zabel (Eds.), Marine Geochemistry (pp. 129-172). Berlin Heidelberg NY: Springer. Sholkovitz, E.R., & Gieskes, J.M. (1971). A physical-chemical study of the flushing o f the Santa Barbara Basin. Limnology and Oceanography, 16,479-489. 78 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Shore-based Scientific Party (1994). Site 893. In J.P. Kennett, J.G. Baldauf, et. al. (Eds.), Proceedings of the Ocean Drilling Program, Initial Reports, 146 (Part 2) (pp. 15-50). College Station, IX: Ocean Drilling Program. Smith, K.L., Kaufinann, R.S. & Baldwin, R.J. (1994). Coupling o f near-bottom pelagic and benthic processes at abyssal depths in the eastern North Pacific Ocean. Limnology and Oceanography, 39,1101-1118. Smith, P.E., & Eppley, R.W. (1982). Primary production and the anchovy population in the southern California bight: Comparison o f time series. Limnology and Oceanography 27, 1-17. Soutar, A. & Crill, P.A. (1977). Sedimentation and climatic patterns in the Santa Barbara Basin during the 19th and 20th centuries. Geological Society o f America Bulletin, 88,1161-1172. Stott. L.D., Berelson, W., Douglas, R., & Gorsline, D. (2000). Increased dissolved oxygen in Pacific intermediate waters due to lower rates of carbon oxidation in sediments. Nature, 407, 367-370. Stott, L.D., Bunn, T., Prokopenko, M., Mahn, C., Gieskes, J., & Bernhard, J. (2002). Does the oxidation of methane leave an isotopic fingerprint in the geologic record? Geochemistry, Geophysics, Geosystems, 3, 10.1029/2001GC000196. Thunell, R., Pride, C., Tappa, E. & Muller-Karger, F. (1993). Varve formation in the Gulf of California: Insights from time series sediment trap sampling and remote sensing. Quaternary Science Reviews, 12, 451-464. Thunell, R.C. (1998). Particle fluxes in a coastal upwelling zone: Sediment trap results from Santa Barbara Basin, California. Deep Sea Research II, 45, 1863-1884. Van Mooy, B.A.S., Keil, R.G., & Devol, A.H. (2002). Impact of suboxia on sinking particulate organic carbon: Enhanced carbon flux and preferential degradation of amino acids via denitrification. Geochimica et Cosmochimica Acta, 6 6 , 457-465. Weisenburg, D.A. & Guinasso, N.L. (1979). Equilibrium solubilities of methane, carbon monoxide, and hydrogen in water and seawater. Journal o f Chemical and Engineering Data, 24, 356-360. 79 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Yeats, R.S., & Haq, B.U. (1981). Deep-sea drilling off the Califomias: Implications o f Leg 63. In R.S. Yeats, B.U. Haq, et. al. (Eds.), Initial Reports o f the Deep Sea Drilling Project, 63 (pp. 949-961). Washington: U.S. Govt. Printing Office. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission.

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Creator Holsten, Jennifer Carol (author)

Core Title A proxy for reconstructing histories of carbon oxidation in the Northeast Pacific using the carbon isotopic composition of benthic foraminifera

School Graduate School

Degree Master of Science

Degree Program Geological Sciences

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

Tag geochemistry,Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Stott, Lowell (committee chair), Berelson, William (committee member), Douglas, Robert (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-305051

Unique identifier UC11337519

Identifier 1416558.pdf (filename),usctheses-c16-305051 (legacy record id)

Legacy Identifier 1416558.pdf

Dmrecord 305051

Document Type Thesis

Rights Holsten, Jennifer Carol

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 au...

Repository Name University of Southern California Digital Library

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