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 (δ34S) variability over small spatial scales (~50‰ over 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 s...

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Bibliographic Details
Published in:Geobiology 2018-01, Vol.16 (1), p.17-34
Main Authors: Gomes, M. L., Fike, D. A., Bergmann, K. D., Jones, C., Knoll, A. H.
Format: Article
Language:English
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
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Summary:In modern microbial mats, hydrogen sulfide shows pronounced sulfur isotope (δ34S) variability over small spatial scales (~50‰ over 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.
ISSN:1472-4677
1472-4669
DOI:10.1111/gbi.12265