We report for the first time micrometer-scale correlation of geologic and microbial processes in modern marine stromatolites. Precipitation of micritic laminae in these stromatolites was studied by comparing microstructure, as observed in petrographic thin sections, with microbial sulfate-reduction activity. Two-dimensional mapping of sulfate-reduction rates was implemented by incubating a vertical section of a stromatolite face on silver foil coated with 35SO42−. Our results show that sulfate-reduction activity is high in zones of CaCO3 precipitation and indicate that microbial activity produces lithified micritic laminae near the surface of the stromatolites. Similarities with micritic laminae in ancient stromatolites suggest that sulfate reduction may also have been an important mechanism of carbonate precipitation in these fossilized structures.

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