The 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 of Earths oxygenation based on sulfur isotope fractionations to fill the gap with S isotope data for sedimentary 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 of postdoctoral, 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.
