Oxidation of pyrite during extraction of carbonate associated sulfate |
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| Authors: | Pedro J Marenco Frank A Corsetti Douglas E Hammond Alan J Kaufman David J Bottjer |
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| Institution: | 1. Department of Earth Sciences, University of Southern California, Los Angeles, CA, 90089-0740, USA;2. Department of Geology, University of Maryland, College Park, MD, 20742-4211, USA;1. Department of Earth & Planetary Sciences and Geotop, McGill University, Montréal, QC, Canada;2. Department of Geological Sciences, Stanford University, Stanford, CA, USA;3. CSIRO Mineral Resources, Australian Resources Research Centre, Kensington, WA 6151, Australia;4. Northern Territory Geological Survey, Darwin, NT 0800, Australia;5. Département des sciences de la Terre et de l''atmosphère/Geotop, Université du Québec à Montréal, Montréal, QC, Canada;1. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China;2. Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA;3. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, CAS, Nanjing 210008, China;4. Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China;5. Wuhan Institute of Geology and Mineral Resources, Wuhan 430205, China;6. Department of Earth Sciences, University of California, Riverside, CA 92521, USA;7. Department of Geosciences and Earth System Science Center, Pennsylvania State University, University Park, PA 16802, USA;1. Department of Earth and Planetary Sciences and GEOTOP, McGill University, Montreal, Quebec, Canada;2. Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada;3. Department of Earth Sciences, University of California, Riverside, CA, USA;4. Institute of Geology, Karelian Research Center, RAS, Petrozavodsk, Russia;1. Geology Department, Amherst College, Amherst, MA 01002, USA;2. Dept of Earth & Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA;1. Earth and Planetary Sciences, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA;2. Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA |
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| Abstract: | The sulfur isotopic composition of carbonate associated sulfate (CAS) has been used to investigate the geochemistry of ancient seawater sulfate. However, few studies have quantified the reliability of δ34S of CAS as a seawater sulfate proxy, especially with respect to later diagenetic overprinting. Pyrite, which typically has depleted δ34S values due to authigenic fractionation associated with bacterial sulfate reduction, is a common constituent of marine sedimentary rocks. The oxidation of pyrite, whether during diagenesis or sample preparation, could thus adversely influence the sulfur isotopic composition of CAS. Here, we report the results of CAS extractions using HCl and acetic acid with samples spiked with varying amounts of pyrite. The results show a very strong linear relationship between the abundance of fine-grained pyrite added to the sample and the resultant abundance and δ34S value of CAS. This data represents the first unequivocal evidence that pyrite is oxidized during the CAS extraction process. Our mixing models indicate that in samples with much less than 1 wt.% pyrite and relatively high δ34Spyrite values, the isotopic offset imparted by oxidation of pyrite should be much less than ? 4‰. A wealth of literature exists on the oxidation of pyrite by Fe3+ and we believe this mechanism drives the oxidation of pyrite during CAS extraction, during which the oxygen used to form sulfate is taken from H2O, not O2. Consequently, extracting CAS under anaerobic conditions would only slow, but not halt, the oxidation of pyrite. Future studies of CAS should attempt to quantify pyrite abundance and isotopic composition. |
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