Behavior of Marine Sulfate in the Early Paleozoic: Extending the Trace Sulfate Proxy

知识价值：在过去的十年中，对全球碳和硫循环的详细调查增加了我们对生物圈氧化作用的了解。地球的渐进氧化？在有机质埋藏的驱动下，美国的生物圈被解释为导致了光解质量无关的s同位素比率的均匀化，随后是海洋硫酸盐浓度的增加和海洋硫的氧化还原循环的增强。许多这些进展都依赖于新的分析能力的发展，例如34S/32S和33S/32S的同时测量，或者使用不同的地球化学代理，例如碳酸盐相关硫酸盐的s同位素分析，或CAS。特别是CAS数据集的可用性导致了许多建模工作，这些工作旨在更好地了解硫循环动力学及其与海洋c同位素记录的联系。其中最复杂的模型利用了同位素变化的时间依赖方程，其中S-（或C-）同位素变化的速率不仅反映了输入和输出通量，还反映了海洋储层的大小，这可以缓冲同位素变化的速率。不幸的是，对这一不断增长的CAS记录的解释以及C-S联系在前寒武纪受到以下因素的限制：(1)我们对代表大部分保存的地层记录的限制性盆地和表海环境中CAS同位素组成的潜在变异性的理解相对较差；(2)对其他研究良好的序列的地质年代学控制通常较差；(3)C和S储层的大小差异极大。这影响了同位素记录的灵敏度，因此限制了它们的解释。然而，目前的数据表明，到前寒武纪末期，海洋碳和硫酸盐储层的规模应该可以优化保存在各自同位素系统中的信息。为了更好地了解CAS在沉积体系中的行为以及C和S体系之间的联系，我们对阿根廷Precordillera和西纽芬兰的晚寒武世和中晚奥陶世地层进行了综合野外、地球化学和年代学研究。这些地方(1)保留了开放的海洋演替，这应该限制其同位素记录的局部重叠；(2)代表了对理解从元古代到显生宙过渡中碳-硫同位素旋回演化至关重要的时间间隔。(3)提供机会确定CAS s同位素记录及其随时间变化的模拟是否能够区分可能由海洋环流和相关营养限制循环驱动的海洋氧合的小尺度变化。最后，阿根廷部分的大量膨润土提供了独特的机会来解释C-S记录，并在高分辨率年表的背景下约束提出的建模。更广泛的影响：该项目旨在通过扩大田纳西大学碳酸盐基础领域和地球化学研究的广度，并让不同教育水平的学生参与科学探究的过程，在促进教学、培训和学习的同时推进发现。该项目支持一名女博士生，并将通过为至少两名学生提供研究机会，延续pi让本科生参与研究的悠久历史（5年内9次，发表5篇同行评议的出版物）。最后，该项目将通过实地研究和在美国对阿根廷博士生Fernando Gomez进行地球化学培训来支持国际科学合作。除了该提案所支持的更广泛的影响外，P.I.目前还参与了nsf资助的一项旨在加强田纳西州农村中学地球科学教育的GK-12项目。PI已经利用最初的实地季节作为勘探地质时间的个案研究，目前正在利用这个项目培训当地科学教师Jay Bachman，使其了解地球科学中使用的岩石学和地球化学技术。PI期望在项目期间继续进行这些活动。此外，地质年代学和地球化学分析的结果将有助于为地球历史创建一个详细的、综合的时间尺度。

Intellectual Merit: In the last decade, detailed investigation of the global carbon and sulfur cycles has increased our understanding of biospheric oxygenation. Progressive oxygenation of the Earth?s biosphere, driven by burial of organic matter, is interpreted to have resulted in the homogenization of photolytic mass-independent S-isotope ratios, followed by an increase in marine sulfate concentration and the enhanced redox cycling of marine sulfur. Many of these advances have relied on the development of new analytical capabilities, such as the simultaneous measurement of 34S/32S and 33S/32S, or the use of different geochemical proxies, such as the S-isotope analysis of carbonate-associated sulfate, or CAS. Availability of a CAS dataset, in particular, has resulted in a number of modeling efforts that seek to better understand sulfur cycle dynamics and its linkages with the marine C-isotope record. The most sophisticated of these models utilizes a time-dependent equation for isotopic change wherein the rate of S-(or C-) isotope change reflect not only the input and output fluxes, but also marine reservoir size, which acts to buffer the rate of isotopic change. Unfortunately, interpretation of this growing CAS record, and thus C-S linkages, is limited in the Precambrian by (1) our relatively poor understanding of potential variability of CAS isotopic compositions in restricted basins and epeiric sea environments that represent much of the preserved stratigraphic record, (2) the typically poor geochronologic control of otherwise well-studied successions, and (3) the extreme differences in the size of both C and S reservoirs, which affects the sensitivity of isotopic records and therefore limits their interpretation. Current data suggests, however, that by the end of the Precambrian, marine carbon and sulfate reservoirs reached sizes that should optimize the information preserved in their respective isotopic systems.In order to better understand the behavior of CAS in sedimentary systems and the linkages between C and S systems, we propose an integrated field, geochemical, and geochronological study of Late Cambrian and Mid-Late Ordovician strata of the Argentine Precordillera and Western Newfoundland. These localities (1) preserve open marine successions that should limit local overprinting of their isotopic records, (2) represent time intervals critical to understanding the evolution of C-S isotopic cycles in the transition from the Proterozoic to Phanerozoic worlds, (3) provide the opportunity to determine if S-isotope records from CAS and their timedependent modeling can distinguish small-scale changes in marine oxygenation potentially driven by oceanographic circulation and related cycles of nutrient limitation. Finally, numerous bentonites within the Argentine section provide the unique opportunity to interpret C-S records and constrain proposed modeling within the context of a high-resolution chronology.Broader Impacts: This project aims to advance discovery while promoting teaching, training, and learning by expanding the breadth of carbonate-based field and geochemical research at the University of Tennessee and involving students from differing educational levels in the process of scientific inquiry. The project supports one female Ph.D. student, and will continue the PIs strong history of involving undergraduates in research (9 in 5 years, leading to 5 peer-reviewed publications) by providing research opportunities for at least two students. Finally, the project will support international scientific collaboration through field research and U.S.-based geochemical training of Argentinian Ph.D. student Fernando Gomez. In addition to broader impacts supported by this proposal, the P.I. is a current participant in an NSF-funded GK-12 project aimed enhancing earth science education in rural TN middle schools. The PI has already used the initial field season as a case-study in exploring geologic time, and is currently using this project to train a local science teacher, Jay Bachman, on petrographic and geochemical techniques used in the geosciences. The PI expects to continue such activities through the duration of the project. Furthermore, results of geochronological and geochemical analysis will add to the EARTHIME initiative of creating a detailed, integrated time-scale for Earth history.