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

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 similar to 2.4 Ga 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 parts per thousand range of Delta S-33 in one sample containing detrital pyrite (-3.6 parts per thousand to 11.7 parts per thousand) is larger than previously reported worldwide, and there is evidence for mass-independent sulfur isotope fractionation in authigenic pyrite throughout the section (Delta S-33 from -0.8 parts per thousand to 1.0 parts per thousand). The 90 parts per thousand range in delta S-34 observed (-45.5 parts per thousand to 46.4 parts per thousand) 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 delta S-34 (<-20 parts per thousand) overgrown by euhedral rims with higher Delta S-34 (4-7 parts per thousand) and enrichments in As, Ni, and Co. The preservation of extremely sharp sulfur isotope gradients (30 parts per thousand/<4 mu m) 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. (C) 2011 Elsevier Ltd. All rights reserved.