A Time-Series Geochemical Study of the ca. 2.5 Ga Batatal Formation in Brazil: Sulfur and Carbon Isotopic Insights into Environmental Conditions before the Great Oxidation Event
Kaufman, Alan J
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Neoarchean metasedimentary rocks from the Batatal Formation, Brazil were studied using petrographic, elemental and stable isotopic techniques to provide a better understanding of coupling between the atmosphere-ocean system and biogeochemical cycles prior to the Great Oxidation Event. Multiple sulfur isotope data from both shale and carbonate lithofacies confirms global preservation of mass-independent fractionations, thought to be produced through photochemical reactions in an oxygen-free Neoarchean atmosphere. Isotopic differences between lithofacies within the shallow marine Batatal Formation, and beyond in deeper subtidal environments from correlative successions in Western Australia and South Africa, suggest a strong influence of environmental conditions on the preservation of distinct &Delta<super>33</super>S signatures. To explain the novel isotopic data, it is proposed the Batatal Formation was deposited in a shallow water evaporitic environment, which resulted in higher sulfate concentrations and greater sulfur isotope fractionation associated with the activity of sulfate-reducing bacteria. Higher temperatures associated with such environments may have also promoted carbon dioxide limitation resulting in lesser carbon isotopic fractionation by photoautotrophs.