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

In modern microbial mats, hydrogen sulfide shows pronounced sulfur isotope (δ 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 δ S patterns and bulk δ S values. We report μm-scale δ S patterns in Proterozoic samples with well-preserved microbial mat textures. We show a well-defined relationship between δ S values and sulfide mineral grain size and type. Small pyrite grains (<25 μm) span a large range, tending toward high δ S values (-54.5‰ to 11.7‰, mean: -14.4‰). Larger pyrite grains (>25 μm) have low but equally variable δ S values (-61.0‰ to -10.5‰, mean: -44.4‰). In one sample, larger sphalerite grains (>35 μm) have intermediate and essentially invariant δ 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 δ S values. In another stage, large sphalerite grains with low δ 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 δ S values. We do not see evidence for significant 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 δ S values measured for individual pyrite grains. Fine-scale analyses of δ S patterns can thus be used to extract environmental information from ancient microbial mats and aid in the interpretation of bulk δ S records.