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

• Published in: Geobiology Vol. 16; no. 1; pp. 17 - 34
• Main Authors: Gomes, M. L., Fike, D. A., Bergmann, K. D., Jones, C., Knoll, A. H.
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01-01-2018; Wiliey
• Subjects: Accretion; Desiccation; Ecological distribution; Environmental information; Environmental Sciences & Ecology; Fossils; Fractionation; Geology; Hydrogen sulfide; Hydrogen sulphide; Information processing; Isotopes; Life Sciences & Biomedicine - Other Topics; Microbial mats; Microbiota; Microenvironments; Niches; Oxidation; Oxidoreductions; Pore water; Precambrian; Pyrite; Sphalerite; Sulfate reduction; Sulfates; Sulfides; Sulfides - analysis; Sulfur; Sulfur isotopes; Sulfur Isotopes - analysis; Sulphides; Sulphur; Water depth
• ISSN: 1472-4677; 1472-4669
• DOI: 10.1111/gbi.12265

In modern microbial mats, hydrogen sulfide shows pronounced sulfur isotope (δ34S) 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 δ34S patterns and bulk δ34Spyrite values. We report μm‐scale δ34S patterns in Proterozoic samples with well‐preserved microbial mat textures. We show a well‐defined relationship between δ34S values and sulfide mineral grain size and type. Small pyrite grains (<25 μm) span a large range, tending toward high δ34S values (−54.5‰ to 11.7‰, mean: −14.4‰). Larger pyrite grains (>25 μm) have low but equally variable δ34S values (−61.0‰ to −10.5‰, mean: −44.4‰). In one sample, larger sphalerite grains (>35 μm) have intermediate and essentially invariant δ34S 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 δ34S values. In another stage, large sphalerite grains with low δ34S 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 δ34Ssulfide values. We do not see evidence for significant 34S‐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 δ34S values measured for individual pyrite grains. Fine‐scale analyses of δ34Spyrite patterns can thus be used to extract environmental information from ancient microbial mats and aid in the interpretation of bulk δ34Spyrite records.