Role of sulfide oxidation in dolomitization: Sediment and pore-water geochemistry of a modern hypersaline lagoon system

Dolomite precipitation has been reported in sediments of marginal-marine hypersaline lagoons near Rio de Janeiro, Brazil, and has been related to microbial sulfate reduction. Our field study of pore-water and sediment geochemistry in two lagoons reveals only modest degrees of SO4 reduction and even H2S oxidation. Significant differences in dolomite contents, salinity, and carbon-sulfur cycling occur in the two lagoons; pore waters from the most dolomite-rich sediments exhibit significantly elevated SO4/Cl ratios relative to seawater. Moreover, the O isotope composition of SO4 in all lagoonal waters has evolved significantly from seawater values. This suggests active sulfur recycling via H2S oxidation, which provides acid that decreases pore-water saturation state for high-Mg calcite and aragonite, thermodynamically favoring the precipitation of dolomite. The open hydrogeo-chemical nature of these lagoons maintains SO4 concentrations below gypsum saturation, allowing dolomite to form from fluids with normal marine Mg/Ca.