Microscale observations of sulfate reduction: Correlation of microbial activity with lithified micritic laminae in modern marine stromatolites

We report for the first time micrometer-scale correlation of geologic and m icrobial processes in modern marine stromatolites. Precipitation of micriti c laminae in these stromatolites was studied by comparing microstructure, a s observed in petrographic thin sections, with microbial sulfate-reduction activity. Two-dimensional mapping of sulfate-reduction rates was implemente d by incubating a vertical section of a stromatolite face on silver foil co ated with (SO42-)-S-35. 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 su lfate reduction may also have been an important mechanism of carbonate prec ipitation in these fossilized structures.