A Stratigraphically Resolved Sulfur Isotope Record of the Oxygenation of Earth's Atmosphere and its Correlation with C, Fe, and Mo Geochemical Cycles in the Early Paleoproterozo

Andrey BekkerEAR-0545484在2.47Ga之前的黄铁矿和重晶石中发现了硫同位素的非质量依赖分馏(Farquhar等人，2000年)，这似乎与无氧大气中的光化学过程有关(Farquhar等人，2001年)，为大气氧化还原状态提供了一个新的代表。现有数据表明，最古老的古元古代冰川作用在2.45-2.32Ga之间，在此之前，大气中的氧气含量上升。从富含甲烷的太古宙大气层的温室条件，到氧气充沛、甲烷贫乏的寒冷条件，随后的氧化作用和冰川作用可能会跟踪这一转变。然而，在2.47Ga形成的S同位素中具有明显非质量依赖信号的最年轻的沉积黄铁矿与冰前黄铁矿之间仍然存在明显的地层和时间差异。我们建议测量一个基于硫同位素分馏的高分辨率地球氧合地层记录，以填补S同位素数据对西澳大利亚Brockman超层序中上部与条带状铁建造夹层的页岩中沉积黄铁矿的空白。Brockman超层序的中上部沉积于24635~24493 Ma之间，形成于2.48~2.45Ga地幔喷发事件的晚期。我们的初步数据表明，在这些单元中缺乏非质量依赖的分馏。如果得到证实，这些数据将(1)将大气氧气上升的时间间隔定为20 Ma；(2)为模拟大气氧气上升是渐进的还是突然的提供数据；以及(3)允许对氧合作用与构造岩浆事件、气候变化以及大气和海洋化学成分的其他变化以及碳生物地球化学循环的关系进行地层控制。我们将分别记录西澳大利亚地质调查局和皮尔巴拉铁矿公司储存的DDHWW1和DD98SGP001钻芯，测量高分辨率的地层S同位素记录，并对钻芯页岩中有机质中的碳同位素进行辅助分析。S和碳同位素分析将在地球物理实验室进行。我们将向我们的合作者Olivier Rouxel(WHOI)、Tim Lyons和Clint Scott(UCR)、Ariel Anbar(亚利桑那州立大学)和Eirik Kogue stad(GSU)分发等分钻芯样本进行协调分析，以获得大气氧气含量上升的独立证据。O.Rouxel将分析来自页岩的沉积黄铁矿、带状铁建造中的氧化铁矿物和大量页岩样品的铁同位素，以评估铁旋回对早古元古代海洋氧化还原状态变化的响应。T.Lyons和他的博士生C.Scott将测量氧化还原敏感元素(Mo、Re和U)的含量、C和S的浓度以及Fe的形态，以了解在陆地氧化风化的影响下海水成分是如何变化的。A.安巴尔将分析钼同位素，E.克罗格斯塔德将分析页岩中的铀和铅同位素以及稀土元素，以限制大气和海洋的氧化还原状态。需要资金支持岩心录井和从澳大利亚钻探岩心储存库取回样品，分析S和碳同位素，并将样品分发给我们的同事。广泛的影响：该项目将通过来自CIW、UCR、亚利桑那州立大学、格拉斯哥州立大学和西澳大利亚大学的博士后、研究生和本科生的参与，以及美国和澳大利亚几个机构在地质子学科上的合作，促进培训和学习。这一结果将有利于科学教育，因为大气氧含量上升的问题已经引起了公众的注意(例如，Kerr，2005)，这不仅将作为对现有大气氧含量上升假设的一次检验，还将为发展新的或完善的理论提供一个框架。我们的共同努力不仅将为专注于大气和海洋氧化还原状态的多学科研究树立榜样，而且还将为大气氧气开始上升的时间间隔提供大气和海洋氧化还原状态的定性和定量限制。

Andrey BekkerEAR-0545484The discovery of non-mass dependent fractionation of sulfur isotopes in pre-2.47 Ga pyrite and barite (Farquhar et al., 2000), plausibly related to photochemical processes in an oxygen-free atmosphere (Farquhar et al., 2001) provided a new proxy for the atmospheric redox state. Available data suggest that the oldest Paleoproterozoic glaciation, constrained between 2.45 and 2.32 Ga, was preceded by the rise of atmospheric oxygen. The succession of oxygenation followed by glaciation might track a shift from the greenhouse conditions of a methane-rich, Archean atmosphere to the chill of oxygenated, methane-poor conditions. Yet, an apparent stratigraphic and temporal gap remains between the youngest sedimentary pyrites with a clear non-mass dependent signal in S isotopes formed at 2.47 Ga and pre-glacial pyrites with no non-mass dependent signal. We propose to measure a high resolution stratigraphic record ofEarths oxygenation based on sulfur isotope fractionations to fill the gap with S isotope data forsedimentary pyrites from shales interlayered with banded iron formations within the middle and upper parts of the Brockman Supersequence, Western Australia. The middle and upper parts of the Brockman Supersequence were deposited between 2463 5 Ma and 2449 3 Ma during the late stage of a 2.48-2.45 Ga mantle plume breakout event. Our preliminary data indicate a lack of non-mass dependent fractionation in these units. If confirmed, these data will (1) bracket the rise of atmospheric oxygen to an interval of 20 Ma; (2) provide data to model whether the rise of atmospheric oxygen was gradual or abrupt; and (3) allow a stratigraphic control on the relationship of oxygenation to tectonomagmatic events, climatic changes and other changes in chemical composition of the atmosphere and ocean and C biogeochemical cycle.We will log drill cores DDH WW1 and DD98SGP001, stored by the Geological Survey of Western Australia and Pilbara Iron Co., respectively, measure a high resolution, stratigraphic S isotope record, and carry out supporting analyses of C isotopes in organic matter in drill core shales. S- and C-isotope analyses will be made at the Geophysical Laboratory. Aliquots of well-characterized drill core samples will be distributed for coordinated analyses to our collaborators Olivier Rouxel (WHOI), Tim Lyons and Clint Scott (UCR), Ariel Anbar (ASU), and Eirik Krogstad (GSU) to obtain independent evidence for the rise of atmospheric oxygen. O. Rouxel will analyze Fe isotopes of sedimentary pyrites from shales, Fe oxide minerals in banded iron formations and bulk shale samples to evaluate the response of the Fe cycle to changes in the oceans redox state in the Early Paleoproterozoic. T. Lyons and his Ph.D. student C.Scott will measure the content of redox-sensitive elements (Mo, Re, and U), C and S concentrations, and Fe speciation to understand how seawater composition changed under the impact of the advent of oxidative terrestrial weathering. A. Anbar will analyze Mo isotopes and E. Krogstad will analyze U and Pb isotopes and REE in shales to constrain redox state of the atmosphere and ocean. Funds are requested to support core logging and sample retrieval from Australian drill core repositories, analysis for S- and Cisotopes, and distribution of samples to our colleagues. No direct support of our colleagues research is requested.Broader Impact: The project will promote training and learning through involvement ofpostdoctoral, graduate, and undergraduate students from CIW, UCR, ASU, GSU, and University of Western Australia and collaboration between several institutions in US and Australia across geological sub-disciplines. The results will benefit science education because the question of the rise of atmospheric oxygen has already captured public attention (e.g. Kerr, 2005) and will serve not only as a first order test of existing hypotheses for the rise of atmospheric oxygen but also will provide a framework for the development of new or refined theories. Our combined efforts will not only set an example for multidisciplinary studies focused on the redox state of the atmosphere and ocean but, also, will provide qualitative and quantitative constraints on the redox state of the atmosphere and ocean for the time interval when atmospheric oxygen started to rise.