Microbial sulfate reduction rates and sulfur and oxygen isotope fractionations at oil and gas seeps in deepwater Gulf of Mexico

Sulfate reduction and anaerobic methane oxidation are the dominant microbial processes occurring in hydrate-bearing sediments at bathyal depths in the Gulf of Mexico where crude oil and methane are advecting through fault conduits to the seafloor. The oil and gas seeps are typically overlain by chemosynthetic communities consisting of thiotrophic bacterial mats (Beggiatoa spp.) and methanotrophic mussels (Bathymodiolus spp.), respectively. Cores were recovered with a manned submersible from fine-grained sediments containing dispersed gas hydrates at the threshold of stability. Estimated sulfate reduction rates are variable but generally are substantially higher in crude oil seeps (up to 50 times) and methane seeps (up to 600 times) relative to a non-seep reference sediment (0.0043 $μ$mol SO42− cm−3 day−1). Sulfur and oxygen isotope fractionation factors are highest in the reference sediment ($\alpha$S = 1.027; $\alpha$O = 1.015) but substantially lower in the seep sediments ($\alpha$S = 1.018 to 1.009; $\alpha$O = 1.006 to 1.002) and are controlled primarily by kinetic factors related to sulfate reduction rates. Kinetic effects also control the $δ$34S/$δ$18O ratios such that slow microbial rates yield low ratios whereas faster rates yield progressively higher ratios. The seep data contradict previous claims that $δ$34S/$δ$18O ratios are diagnostic of either microbial sulfate reduction at a fixed $δ$34S/$δ$18O ratio of 4/1 or lower ratios caused by SO4–H2O equilibration at ambient temperatures. The new results offer a better understanding of methane removal via anaerobic oxidation in the sulfate reduction zone of hydrate-bearing sediments and have significant implications regarding the origin and geochemical history of sedimentary sulfate reconstructed on the basis of $δ$34S and $δ$18O compositions.