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Selenium As Paleo-Oceanographic Proxy: a First Assessment

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Selenium As Paleo-Oceanographic Proxy: a First Assessment SELENIUM AS PALEO-OCEANOGRAPHIC PROXY: A FIRST ASSESSMENT A Thesis Presented to The Academic Faculty By Kristen Mitchell In Partial Fulfillment of the Requirements for the Degree Master of Science in the School of Earth and Atmospheric Science Georgia Institute of Technology May 2011 SELENIUM AS PALEO-OCEANOGRAPHIC PROXY: A FIRST ASSESSMENT Approved by: Dr. Philippe Van Cappellen School of Earth and Atmospheric Sciences Georgia Institute of Technology Dr. Ellery Ingall School of Earth and Atmospheric Sciences Georgia Institute of Technology Dr. Paul R. D. Mason Faculty of Geosciences Utrecht University Date Approved: March 28, 2011 ACKNOWLEDGEMENTS I thank my advisor Dr. Philippe Van Cappellen for discussions and support and keeping me moving. I would also like to thank my committee members, Dr. Paul Mason and Dr. Ellery Ingall for suggestions and discussions. I would also like to thank Dr. Thomas Johnson for his support and guidance with analytical techniques without which this work could not have been completed. I would like to thank those who contributed samples and their insights into the sample locations Dr. Ellery Ingall, Dr. Timothy Lyons, Dr. Benjamin Gill, Dr. Gert-Jan Reichart, Julia Diaz, and Jeremy Owens. iii TABLE OF CONTENTS ACKNOWLEDGEMENTS ......................................................................................................................... iii LIST OF TABLES ....................................................................................................................................... vi LIST OF FIGURES .................................................................................................................................... vii SUMMARY ................................................................................................................................................. ix CHAPTER 1. INTRODUCTION ............................................................................................................ 1 CHAPTER 2. REVIEW OF SELENIUM BIOGEOCHEMISTRY ......................................................... 5 2.1 Selenium Cycling ............................................................................................................................. 5 2.2 Redox pathways ............................................................................................................................... 7 2.2.1 Major redox states ......................................................................................................................... 7 2.2.2 Assimilatory reduction .................................................................................................................. 8 2.2.3 Dissimilatory reduction ................................................................................................................. 8 2.2.4 Volatilization ............................................................................................................................... 11 2.2.5 Microbial Oxidation .................................................................................................................... 12 2.2.6 Reduction by iron (II) bearing minerals ...................................................................................... 12 2.2.7 Reduction by aqueous sulfide and iron sulfide ........................................................................... 13 2.2.8 UV transformations of Se ............................................................................................................ 14 2.3 Selenium stable isotope fractionation ............................................................................................ 14 2.3.1 Isotope fractionation theory......................................................................................................... 14 2.3.2 Biotic fractionation ...................................................................................................................... 16 2.3.3 Abiotic fractionation ................................................................................................................... 18 2.3.4 Selenium isotope fractionation in hydrothermal systems ............................................................ 19 2.3.5 Observed fractionations in shales and marine sediments ............................................................ 19 CHAPTER 3. MATERIALS AND METHODS .................................................................................... 20 3.1 Marine shales, mudstones, and ocean anoxic events ..................................................................... 20 3.2 Modern oligotrophic plankton ....................................................................................................... 21 3.3 Black Sea: modern euxinic basin ................................................................................................... 21 3.4 Arabian Sea: modern oxygen minimum zone (OMZ) ................................................................... 23 3.5 Demerara Rise and Cape Verde Basin: OAE2 ............................................................................... 24 3.6 Posidonia Shale: (Early) Toarcian OAE ........................................................................................ 25 3.7 New Albany Shale: stratified basin ................................................................................................ 26 3.8 Alum Shale: Upper Cambrian SPICE ............................................................................................ 26 3.9 Plankton, sediment and rock digests .............................................................................................. 27 iv 3.10 Analytical techniques ..................................................................................................................... 28 3.10.1 Selenium concentrations ............................................................................................................. 28 3.10.2 Selenium isotopic ratios .............................................................................................................. 28 CHAPTER 4. RESULTS ....................................................................................................................... 29 4.1 Average concentrations and isotopic compositions ....................................................................... 29 4.2 Black Sea ....................................................................................................................................... 32 4.3 Arabian Sea .................................................................................................................................... 32 4.4 Demerara Rise and Cape Verde Basin ........................................................................................... 33 4.5 Posidonia Shale .............................................................................................................................. 35 4.6 New Albany Shale ......................................................................................................................... 35 4.7 Alum Shale .................................................................................................................................... 36 CHAPTER 5. DISCUSSION ................................................................................................................. 37 CHAPTER 6. CONCLUSIONS ............................................................................................................. 44 REFERENCES ........................................................................................................................................... 45 v LIST OF TABLES Table 1 Reaction products of selenite reduction with iron minerals ........................................................... 12 Table 2 Selenium isotope fractionations associated with biotic reduction of selenium oxyanions ............ 17 Table 3 Abiotic selenium isotope transformations and associated isotope fractionations .......................... 18 Table 4 Selenium isotopic compositions of geological samples ................................................................. 20 Table 5 Bottom water of oxygen levels and corresponding facies. ............................................................ 21 Table 6 Operating parameters for total Se concentration measurements. ................................................... 28 vi LIST OF FIGURES Figure 1 Compilation of 82/76Se isotope fractionation factors (ε; grey triangles) reported as 80/76Se 82/76 by Johnson and Bullen (2004) and converted to Se using the relationship ε80/76=2/3 ε82/76, and 82/76Se isotopic compositions (δ; solid circles) of natural materials. The latter include rocks, sediments, ore deposits, plankton and natural waters. ................................................................ 4 Figure 2 Phylogenetic tree showing all known Se reducing microorganisms that couple Se oxyanion reduction to growth (J.S. Geelhoed, personal communication, 2007). Species names are shown in italics. .................................................................................................................... 9 Figure 3 Sample locations and associated oceanic anoxic events. OMZ = oxygen minimum zone. .......... 22 Figure 4 Black Sea: sites 9 and 14 are deep basin euxinic depositional environments, the other sites correspond to oxic settings on the shelf (sites 3 and 4) and in the Bay of Sinop (site 16). For further descriptions of the sites, see (Lyons, 1991). ....................................................................
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