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

Journal Article · Sat Jan 01 04:00:00 EST 2000 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/S0016-7037(99)00292-6· OSTI ID:20014546

Aharon, P; Fu, B

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. Sulfur and oxygen isotope fractionation factors are highest in the reference sediment but substantially lower in the seep sediments and are controlled primarily by kinetic factors related to sulfate reduction rates. Kinetic effects also control the {delta}{sup 34}S/{delta}{sup 18}O ratios such that slow microbial rates yield low ratios whereas faster rates yield progressively higher ratios. The seep data contradict previous claims that {delta}{sup 34}S/{delta}{sup 18}O ratios are diagnostic of either microbial sulfate reduction at a fixed {delta}{sup 34}S/{delta}{sup 18}O ratio of 4/1 or lower ratios caused by SO{sub 4}-H{sub 2}O 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 {delta}{sup 34}S and {delta}{sup 18}O compositions.

• Research Organization: Louisiana State Univ., Baton Rouge, LA (US)
• OSTI ID: 20014546
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 2 Vol. 64; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
03 NATURAL GAS
GULF OF MEXICO
ISOTOPE RATIO
METHANE
NATURAL GAS DEPOSITS
OXIDATION
PETROLEUM DEPOSITS
REDUCTION
SEEPS
SULFATE-REDUCING BACTERIA
SULFATES