Environmental insights from high-resolution (SIMS) sulfur isotope analyses of sulfides in Proterozoic microbialites...

In modern microbial mats, hydrogen sulfide shows pronounced sulfur isotope (δ³⁴ S) variability over small spatial scales (~50‰ over <4 mm), providing information about microbial sulfur cycling within different ecological niches in the mat. In the geological record, the location of pyrite formation, overprinting from mat accretion, and post-depositional alteration also affect both fine-scale δ³⁴ S patterns and bulk δ³⁴ S_pyrite values. We report μm-scale δ³⁴ S patterns in Proterozoic samples with well-preserved microbial mat textures. We show a well-defined relationship between δ³⁴ S values and sulfide mineral grain size and type. Small pyrite grains (<25 μm) span a large range, tending toward high δ³⁴ S values (-54.5‰ to 11.7‰, mean: -14.4‰). Larger pyrite grains (>25 μm) have low but equally variable δ³⁴ S values (-61.0‰ to -10.5‰, mean: -44.4‰). In one sample, larger sphalerite grains (>35 μm) have intermediate and essentially invariant δ³⁴ S values (-22.6‰ to -15.6‰, mean: -19.4‰). We suggest that different sulfide mineral populations reflect separate stages of formation. In the first stage, small pyrite grains form near the mat surface along a redox boundary where high rates of sulfate reduction, partial closed-system sulfate consumption in microenvironments, and/or sulfide oxidation lead to high δ³⁴ S values. In another stage, large sphalerite grains with low δ³⁴ S values grow along the edges of pore spaces formed from desiccation of the mat. Large pyrite grains form deeper in the mat at slower sulfate reduction rates, leading to low δ³⁴ S_sulfide values. We do not see evidence for significant ³⁴ S-enrichment in bulk pore water sulfide at depth in the mat due to closed-system Rayleigh fractionation effects. On a local scale, Rayleigh fractionation influences the range of δ³⁴ S values measured for individual pyrite grains. Fine-scale analyses of δ³⁴ S_pyrite patterns can thus be used to extract environmental information from ancient microbial mats and aid in the interpretation of bulk δ³⁴ S_pyrite records.