Meeting Date: Monday , October 2nd, 2017
New Location: Dave & Buster’s, 20 City Blvd West, Orange CA
Sean Loyd, Ph.D., Associate Professor
Cal State University, Fullerton
Salt dome cap rock calcite formation
by the anaerobic oxidation of methane
Significant hydrocarbon deposits accumulate in traps associated with salt domes. Whereas some of this hydrocarbon is extracted for economic uses, a significant amount is degraded in the subsurface as a result of microbial processes that yield mineral precipitates as byproducts. Salt domes of the Gulf of Mexico Basin typically exhibit extensive deposits of calcite that form atop the salt structure as cap rock. Current cap rock formation models invoke salt-hosted evaporite mineral dissolution as a source of sulfate, which microbes utilize to degrade hydrocarbon. Despite model inferences, the specific natures of these microbial reactions remain largely uncharacterized. We find carbonate-associated sulfate sulfur isotope compositions (δ34SCAS) that exceed those of salt-hosted sulfate minerals, implying cap rock carbonate generation via microbial sulfate reduction under closed-system conditions. The widespread co-occurrence of low carbonate carbon isotope compositions (δ13Ccarb) and high δ34SCAS values indicate that sulfate-dependent anaerobic oxidation of methane (AOM) led to calcite formation. Therefore AOM may serve as an important, unrecognized methane sink that reduces methane emissions from the subsurface in salt dome settings.