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

¹ Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
² Geological Institute, ETH-Zentrum, CH-8092 Zürich, Switzerland
³ Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA

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 hydrogeochemical nature of these lagoons maintains SO₄ concentrations below gypsum saturation, allowing dolomite to form from fluids with normal marine Mg/Ca.

Key Words: dolomite • isotope geochemistry • hydrogen sulfide • hypersaline environment • pore water • magnesium calcite

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