IN SITU SULFUR ISOTOPE EVIDENCE FOR LOW ATMOSPHERIC OXYGEN AND HIGH SEAWATER SULFATE IN PROTEROZOIC GLACIOGENIC SEDIMENTS OF THE TUREE CREEK GROUP, WESTERN AUSTRALIA

The lower Turee Creek Group in Western Australia includes a series of diamictites in the ~2.4 Ga Meteorite Bore Member that, along with similar deposits in North America and South Africa, may record the onset of global glaciation and rising atmospheric oxygen. We report in situ sulfur isotope measurements by secondary ion mass spectrometry (SIMS) of individual pyrite grains preserved in a series of glaciogenic mudstones and sandstones as well as cherts and banded iron formations from two exposures of the lowermost Turee Creek Group. We used two different analytical conditions to perform sulfur isotope measurements: 1) ³²S, ³³S and ³⁴S were analyzed using a 10 µm primary Cs⁺ ion beam with an average spot-to-spot reproducibility of 0.20‰ for δ³⁴S and 0.09‰ for Δ³³S (2 SD), and 2) ³²S and ³⁴S were analyzed using a 3 µm primary Cs⁺ beam with an average spot-to-spot reproducibility of 0.72‰ for δ³⁴S (2 SD). Among pyrite grains that we classify as unambiguously authigenic, δ³⁴S ranges from –38.2 to 46.4‰ (VCDT) with an average of 0.0‰, and Δ³³S ranges from –1.65 to 2.44‰ with an average of 0.09‰. Two samples contain pyrite grains with rounded margins and sometimes pitted surfaces in association with detrital quartz grains of similar morphology, and we interpret these grains as detrital pyrite. Among these grains, δ³⁴S ranges from –32.5 to 17.0‰ with an average of 0.7‰, and Δ³³S ranges from –3.62 to 11.56‰ with an average of 1.95‰. Measurements with a ~3 µm spot size reveal δ³⁴S variability of up to 74‰ among grains <2 mm apart and up to 40‰ in single grains <70 mm in diameter. In some cases, secondary pyrite with δ³⁴S of approximately 5‰ occurs as a rim around a visible core of primary pyrite with δ³⁴S of approximately –30‰. Together these observations reflect an ocean and atmosphere in transition. The degree of δ³⁴S variability in authigenic pyrite suggests that seawater sulfate concentration had increased beyond the threshold for limitation of isotopic fractionation associated with bacterial sulfate reduction, yet atmospheric oxygen remained low enough to preserve detrital pyrite and moderate mass-independent sulfur isotope fractionations during the onset of early Proterozoic glaciation.