Masters Thesis

Sulfate-dependent Anaerobic Oxidation of Methane as a Generation Mechanism for Calcite Cap Rocks of Gulf Coast Salt Domes, Usa

Gulf Coast salt domes occur in association with significant reserves of carbonate minerals, crude oil, natural gas, and elemental sulfur. As with other diagenetic carbonate minerals, cap rock calcites may form from microbial degradation of organic compounds. However, the specific natures of these microbial reactions remain largely unknown in salt dome systems. Here, carbon (δ13C) and sulfur (δ34S) geochemical signatures recorded in cap rock calcites are used to identify mineral-yielding processes in Gulf Coast salt domes. Calcite δ13C values reported here and elsewhere exhibit 13C-depleted compositions and express a large range from –55‰ (VPDB) to near neutral values, reflecting a mixture of carbon sources including a substantial methane component. Sulfur isotope data from carbonate-associated sulfate (δ34SCAS) exceed those of minor anhydrite associated with the Jurassic salt deposits (~ +15‰ VCDT), implying carbonate generation via microbial sulfate reduction under closed-system conditions. The co-occurrence of low carbonate δ13C values and high δ34SCAS values are evidence for sulfate-dependent anaerobic oxidation of methane (AOM) as the formation mechanism for Gulf Coast carbonate. Collectively, these data shed new light on a potential hotspot of microbial activity in the deep biosphere with potential environmental and economic ramifications.

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