Constraining atmospheric oxygen and seawater sulfate concentrations during Paleoproterozoic glaciation: In situ sulfur three-isotope microanalysis of pyrite from the Turee Creek Group, Western Australia

摘要

Previous efforts to constrain the timing of Paleoproterozoic atmospheric oxygenation have documented the disappearance of large, mass-independent sulfur isotope fractionation and an increase in mass-dependent sulfur isotope fractionation associated with multiple glaciations. At least one of these glacial events is preserved in diamictites of the ～2.4Ga Meteorite Bore Member of the Kungarra Formation, Turee Creek Group, Western Australia. Outcrop exposures of this unit show the transition from the Boolgeeda Iron Formation of the upper Hamersley Group into clastic, glaciomarine sedimentary rocks of the Turee Creek Group. Here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution, as well as the occurrence of detrital pyrite in the Meteorite Bore Member. The 15.3‰ range of Δ~(33)S in one sample containing detrital pyrite (-3.6‰ to 11.7‰) is larger than previously reported worldwide, and there is evidence for mass-independent sulfur isotope fractionation in authigenic pyrite throughout the section (Δ~(33)S from -0.8‰ to 1.0‰). The 90‰ range in δ34S observed (-45.5‰ to 46.4‰) strongly suggests microbial sulfate reduction under non-sulfate limiting conditions, indicating significant oxidative weathering of sulfides on the continents. Multiple generations of pyrite are preserved, typically represented by primary cores with low δ~(34)S (<-20‰) overgrown by euhedral rims with higher δ~(34)S (4-7‰) and enrichments in As, Ni, and Co. The preservation of extremely sharp sulfur isotope gradients (30‰/<4μm) implies limited sulfur diffusion and provides time and temperature constraints on the metamorphic history of the Meteorite Bore Member. Together, these results suggest that the Meteorite Bore Member was deposited during the final stages of the "Great Oxidation Event," when pO_2 first became sufficiently high to permit pervasive oxidative weathering of continental sulfides, yet remained low enough to permit the production and preservation of mass-independent sulfur isotope fractionation.