Abstract
In modern microbial mats, hydrogen sulfide shows pronounced sulfur isotope (δ34 S) variability over small spatial scales (~50‰ over <4mm), 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 Spyrite 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 (<25μm) span a large range, tending toward high δ34 S values (-54.5‰ to 11.7‰, mean: -14.4‰). Larger pyrite grains (>25μm) have low but equally variable δ34 S values (-61.0‰ to -10.5‰, mean: -44.4‰). In one sample, larger sphalerite grains (>35μm) 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 Ssulfide 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 Spyrite patterns can thus be used to extract environmental information from ancient microbial mats and aid in the interpretation of bulk δ34 Spyrite records.
Highlights
Microbial mats are multi-layered aggregations of phylogenetically and metabolically diverse microorganisms
Microbial mats can be preserved in the geological record as microbialites, which are ubiquitous through geological time (Grotzinger & Knoll, 1999) and have the potential to provide a rich suite of information about the evolution of microbial metabolisms and ancient environmental conditions
To evaluate how fine-scale δ34Ssulfide patterns in modern microbial mats are preserved in sulfides in microbialites, we present fine-scale δ34S values of sulfide minerals from two microbialites from the Sukhaya Tunguska Formation, Siberia (Mesoproterozoic–Neoproterozoic transition; Sergeev, Knoll, & Petrov, 1997) and one microbialite from the Draken Formation, Spitsbergen (Neoproterozoic; Knoll, Swett, & Mark, 1991)
Summary
Microbial mats are multi-layered aggregations of phylogenetically and metabolically diverse microorganisms. In some organic-rich modern marine sediments, pore water δ34Ssulfide values are low near the top of the zone of sulfide accumulation and increase with depth This pattern is a result of closed-system Rayleigh isotope fractionation effects where δ34Ssulfide values evolve toward δ34S values of the overlying sulfate reservoir as the sulfate reservoir is progressively consumed. We evaluate fine-scale δ34Spyrite patterns in the context of petrographic information and elemental mapping and compare the results to δ34Spyrite patterns from modern microbial mats (Fike et al, 2008, 2009) and microbial consortia (Wilbanks et al., 2014) This approach enables us to propose a framework for understanding the pyrite formation dynamics that influence bulk δ34Spyrite values in microbialites, an approach that can be extended to other types of sedimentary materials in order to gain additional insights into the spectrum of biogeochemical sulfur cycling associated with major.
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