Sulfur isotope studies of sulfide oxidation

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Sulfur is a common redox-active element in Earth's oceans that is cycled by a variety of chemical and biological processes. Its most common form in seawater is as the sulfate ion, which is also its most oxidized form. In marine sediments and also in some parts of deep basins this sulfate is reduced to sulfide. This reaction is driven by the metabolic activity of single-celled prokaryotes and accounts for as much as 50% of the total sedimentary organic carbon that is reoxidized to carbonate/bicarbonate/carbonic acid/carbon dioxide. A majority (~80-95%) of sulfide produced this way is reoxidized by biological and abiological processes. The remaining sulfide is fixed, primarily by reaction with iron to form the sulfide mineral pyrite, generating a buried reservoir of reduced sulfur that balances almost 30% of the long-term oxygen production today. This grant supports a study of the rates and isotopic signatures associated with the abiological pathways of sulfide oxidation in seawater. The isotopic signatures in sulfur compounds produced by biological and abiological pathways are different, and calibrations of these isotope effects in the context of well-characterized reaction rates can be used to trace oxidation pathways in modern and ancient environments. This information will provide fundamental data that will be used to calibrate geochemical models that describe carbon burial and a long term oxygen production and also to disentangle the distribution of biological and abiological oxidation processes in marine environments. This grant also supports a Ph.D. student and includes plans to develop a program to assist teachers and students at local Title I elementary school with the development of their STEM program.