Sulfate-dependent Anaerobic Oxidation of Methane as a Generation Mechanism for Calcite Cap Rock in Gulf Coast Salt Domes

Abstract:

Gulf Coast salt domes, specifically their calcite cap rocks, have been widely recognized for their association with significant reserves of crude oil and natural gas. However, the specific microbial reactions that facilitate the precipitation of these cap rocks are still largely unknown. Insight into the mineralization mechanism(s) can be obtained from the specific geochemical signatures recorded in these structures. Gulf Coast cap rocks contain carbonate and sulfur minerals that exhibit variable carbon (d¹³C) and sulfur isotope (δ³⁴S) signatures. Calcite d¹³C values are isotopically depleted and show a large range of values from –1 to –52‰, reflecting a mixture of various carbon sources including a substantial methane component. These depleted carbon isotope compositions combined with the presence of abundant sulfide minerals in cap rocks have led to interpretations that invoke microbial sulfate reduction as an important carbonate mineral-yielding process in salt dome environments. Sulfur isotope data from carbonate-associated sulfate (CAS: trace sulfate incorporated within the carbonate mineral crystal lattice) provide a more direct proxy for aqueous sulfate in salt dome systems and may provide a means to directly fingerprint ancient sulfate reduction. We find CAS sulfur isotope compositions (δ³⁴S_CAS) significantly greater than those of the precursor Jurassic sulfate-salt deposits (which exhibit δ³⁴S values of ~ +15‰). This implies that cap rock carbonate generation occurred via microbial sulfate reduction under closed-system conditions. The co-occurrence of depleted carbonate d¹³C values (< ~30‰) and the enriched δ³⁴S_CAS values are evidence for sulfate-dependent anaerobic oxidation of methane (AOM). AOM, which has been shown to yield extensive seafloor carbonate authigenesis, is also potentially partly responsible for the carbonate minerals of the Gulf Coast calcite cap rocks through concomitant production of alkalinity. Collectively, these data shed new light on a potential hotspot of microbial activity in the deep biosphere.