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

学术价值：保存在2.4Ga之前的沉积岩中的硫的异常同位素分馏(AIF-S)特征已经被用来推动关于地球最初何时有氧气大气的争论。这些硫同位素异常的机制被认为是火山SO2气体在缺氧大气中的紫外线光解所产生的。然而，最近的研究人员已经认识到，宽带紫外灯对SO2的紫外光分解非常接近阳光的紫外光光谱，在自然样品中并不能产生许多AIF-S特征信号。Pi及其同事报告了在可还原的硫化氢和铬的硫物种(可能是二硫化物)中存在明显的AIF-S特征，这些硫物种是在H2O存在下的相对较高的温度(150-200摄氏度)下氨基酸和硫酸钠晶体之间的反应产生的，并提出沉积物中有机物的热化学硫酸盐还原作用可能在2.4Ga之前的沉积岩中产生了一些AIF-S特征(Watanabe等人，2009年)。这一发现意义重大，因为在这项研究之前，SO2的紫外线光解是AIF-S唯一已知的原因。TSR模型受到了许多研究人员的批评，他们声称这一过程不适用于自然系统，因为自然系统中的有机化合物很复杂。这项建议中提出的研究将系统地研究：(1)气、液、固相产物(如H_2S、铬还原S、有机S化合物)的多重硫同位素特征；(2)在(A)热分解过程中产生的重烃(油)和轻烃(气)的化学特征(如C/H/S/N比和化合物比)；绿河组蓝藻和未成熟干酪根的残留物。这一结果将为以下问题提供深入的认识：(A)为什么在富含有机质的岩石和一些有机贫乏的岩石(如BIF和碳酸盐)中都观察到AIF-S特征，(B)为什么AIF-S明显消失在较年轻的岩石中；以及(3)沉积岩中的AIF-S记录如何与早期地球的环境、化学和生物演化联系起来。广泛的影响：拟议的项目将一名研究生作为其博士论文研究的一部分，两名本科生(每年一名)作为研究助理。国际和平研究所计划参加WISE方案：科学和工程研究所的妇女和一名研究生研究员。未来的学生和年轻科学家将从这个项目产生的新数据和想法中受益。

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.