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

学术价值：保存在2.4 Ga前沉积岩中的硫异常同位素分馏（AIF-S）特征被用来推动关于地球何时首次出现含氧大气的争论。这些硫同位素异常的机制被认为是由缺氧大气中火山二氧化硫气体的紫外光解作用产生的。然而，最近的研究人员已经认识到，通过宽带紫外灯对SO2进行紫外光解，这种灯非常接近地模拟了太阳光的紫外光谱，并没有在自然样品中产生许多AIF-S特征。PI及其同事报告了在H2O存在下氨基酸晶体与硫酸钠之间相对高温（150-200°C）反应产生的H2S和cr -可还原硫（可能是二硫化物）中存在明显的AIF-S特征，并表明沉积物中有机质的热化学硫酸盐还原（TSR）可能在2.4 Ga前沉积岩中产生了一些AIF-S特征（Watanabe et al., 2009）。这一发现意义重大，因为在这项研究之前，SO2的紫外光解是AIF-S的唯一已知原因。TSR模型受到许多研究人员的批评，他们声称该过程不适用于有机化合物复杂的自然系统。本提案中提出的研究将系统地调查：(i)始新世绿河组蓝藻残余物和未成熟干酪根在(a)热分解和(b) TSR过程中产生的重烃（油）和轻烃（气）的气态、液态和固态产物（如H2S、cr还原S、有机S化合物）的多重硫同位素特征和（ii）化学特征（如C/H/S/N比和复合比）。这一结果将有助于深入了解(a)为什么在富有机质和一些贫有机质岩石（如bif和碳酸盐岩）中都观察到AIF-S特征；(b)为什么AIF-S在较年轻的岩石中明显消失；(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。更广泛的影响：拟建项目将由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。PI计划与一名研究生一起参加WISE项目：科学与工程研究所的女性。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.