Multiple Sulfur Isotopes in Neoproterozoic Pyrite: Implications for Links Between the Sulfur Cycle and Atmospheric Oxygen

English Abstract:

The size of the seawater sulfate reservoir (SSR) depends on the oxidation state of Earth’s surface because riverine sulfate supply is controlled by oxidative weathering of sulfide minerals on the continents and burial of sulfur as pyrite is controlled by the amount of dissolved oxidants in the ocean. The sulfur isotopic composition of pyrite can provide information on sulfate availability and isotopic properties during bacterial sulfate reduction (BSR). However, only pyrites that formed in direct communication with the SSR, i.e. in an euxinic water column or at the sediment-water interface, will reliably record its properties. We present a pyrite-multiple sulfur isotope record coupled to a numerical model from a Neoproterozoic succession in Svalbard, which was deposited on a thermally subsiding continental shelf between 835 and 620 Ma. δ³⁴S values throughout the succession are highly variable and range from -30 to +40‰, akin to published data from other coeval basins. These data suggest substantial variation in sulfate availability to BSR. However, redox proxies and pyrite contents suggest that BSR during deposition of pre-Sturtian units was restricted to pore waters. In contrast, BSR likely occurred at the sediment-water interface during deposition of the interglacial and post-Marinoan units. The interglacial units show δ³⁴S values up to +40‰, equal to estimates of coeval seawater sulfate, suggesting collapse of the SSR while post-Marinoan δ³⁴S values are lower than estimates for contemporaneous seawater sulfate, indicating a larger SSR. The post-Marinoan samples follow a Rayleigh-like trend in Δ³³S-δ³⁴S space. Modeling suggests that sulfate is not the limiting factor during BSR and that some of the H₂S generated during BSR was lost, probably by migration into the overlying water column. A larger post-Marinoan SSR compared to the interglacial period is consistent with a rise in atmospheric O₂ concentration, supporting recent trace metal studies.