Multiple Sulfur Isotope Fractionations during Thermal Decomposition of and Thermochemical Sulfate Reduction by Natural Organic Compounds

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Intellectual merit: Anomalous isotope fractionations of sulfur (AIF-S) signatures preserved in pre-2.4 Ga sedimentary rocks have been used to drive the debate about when the Earth first had an oxygenated atmosphere. The mechanism for these sulfur isotope anomalies has been postulated to be produced by UV photolysis of volcanic SO2 gas in an anoxic atmosphere. Recent investigators, however, have recognized that the UV photolysis of SO2 by a broadband UV lamp, which closely simulates the UV spectra of sunlight, does not produce many of the AIF-S signatures in natural samples. The PI and colleagues reported the presence of distinct AIF-S signatures in the H2S and Cr-reducible sulfur species (probably disulfide) that were generated from relatively high temperature (150-200°C) reactions between crystals of amino acids and sodium sulfate in the presence of H2O, and suggested that thermochemical sulfate reduction (TSR) by organic matter in sediments might have produced some of the AIF-S signatures in pre-2.4 Ga sedimentary rocks (Watanabe et al., 2009). This discovery is significant, because, until this study, UV photolysis of SO2 was the only known cause for AIF-S. The TSR model was criticized by many researchers who claimed the process was not applicable to natural systems, in which the organic compounds are complex. Research proposed in this proposal will systematically investigate: (i) the multiple sulfur isotopic characteristics of the gaseous, liquid and solid products (e.g., H2S, Cr-reductive S, organic S compounds) and (ii) chemical characteristics (e.g., C/H/S/N ratios and compound ratios) of heavy hydrocarbon (oil) and light hydrocarbon (gas) that were generated during (a) thermal decomposition of, and (b) TSR by, the remnants of cyanobacteria and immature kerogen from the Eocene Green River Formation.The results would provide insight into (a) why AIF-S signatures are observed in both organic-rich, and some organic-poor rocks (e.g., BIFs and carbonates), (b) why AIF-S apparently disappeared in younger rocks; and (3) how the AIF-S record in sedimentary rocks can be linked to the environmental, chemical, and biological evolution of early Earth.Broader Impacts: The proposed project will a graduate student as a part of his Ph.D. thesis research and two undergraduate students (one per year) as a research assistants. The PI plans to participate in the WISE program: Women in the Sciences and Engineering Institute with a graduate researcher. Future students and young scientists will benefit from the new data and ideas generated from this project.