PII S0016-7037(99)00292-6 Microbial sulfate reduction rates and sulfur and oxygen isotope fractionations at oil and gas seeps in deepwater Gulf of Mexico PAUL AHARON* and BAOSHUN FU Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA (Received March 22, 1999; accepted in revised form July 5, 1999) Abstract—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 (Bathy- modiolus 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 mol SO 4 2- cm -3 day -1 ). Sulfur and oxygen isotope fractionation factors are highest in the reference sediment ( S = 1.027;  O = 1.015) but substantially lower in the seep sediments ( S = 1.018 to 1.009;  O = 1.006 to 1.002) and are controlled primarily by kinetic factors related to sulfate reduction rates. Kinetic effects also control the  34 S/ 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  34 S/ 18 O ratios are diagnostic of either microbial sulfate reduction at a fixed  34 S/ 18 O ratio of 4/1 or lower ratios caused by SO 4 –H 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 sedi- mentary sulfate reconstructed on the basis of  34 S and  18 O compositions. Copyright © 2000 Elsevier Science Ltd 1. INTRODUCTION Dissolved sulfate is abundant in seawater (28 mM/L) and its microbial reduction is one of the dominant processes in the early diagenesis of marine sediments (Goldhaber and Kaplan, 1974). Modern marine sedimentary environments sustaining intense microbial sulfate reduction include stratified inland seas (e.g., Black Sea, Deuser, 1970; Sweeney and Kaplan, 1980), fjords (e.g., Saanich Inlet, Nissenbaum et al., 1972), continental shelves (Jørgensen, 1982), organic-rich deltas (Lin and Morse, 1991), and hydrothermal sediments (Jørgensen et al., 1992). In all these cases organic carbon of either terrestrial and/or marine origin serves as the primary electron-donor and metabolic substrate for sulfate anaerobic respiration: SO 4 2- + 2(CH 2 O) = H 2 S + 2HCO 3 - . (1) Hydrogen sulfide derived from microbial sulfate reduction has also been recently documented in cold submarine seeps along active (e.g., Sagami Bay, Japan, Masuzawa et al., 1992) and passive (e.g., North Sea, Dando et al., 1991) margins where light aliphatic hydrocarbons, primarily methane, serve as the reduced carbon source (Aharon, 1999): SO 4 2- + CH 4 = H 2 S + CO 3 2- + H 2 O. (2) In turn, the H 2 S produced within the sediment by free-living microbial communities serves as nourishment for chemosyn- thetic symbions living within the tissues of benthic fauna inhabiting the interface between underlying anoxic sediments and the overlying oxygen-rich bottom waters. A known feature of sulfate reduction is the measured decline of dissolved SO 4 coupled with the increase of dissolved H 2 S with depth in interstitial fluids. Another consequence of sulfate reduction is the enrichment of both 34 S and 18 O in the dissolved sulfate because bacteria discriminate to various degrees against 34 S and 18 O isotopes (Harrison and Thode, 1958; Mizutani and Rafter, 1969). Studies reporting sulfur isotope fractionations caused by microbial sulfate reduction in natural settings are rare, and determinations of both oxygen and sulfur isotopes of the residual sulfate are even fewer. Exceptional are the studies of Zak et al. (1980) and Fritz et al. (1989) which compared sulfur and oxygen isotope fractionations in deep-sea sediments and landfill sites, respectively. Knowledge of isotope fractionations accompanying micro- bial sulfate reduction are important because both 34 S and 18 O concentrations have been used as indicators of the origin and geochemical history of the sulfate (Fritz et al., 1989). However, laboratory experiments using pure cultures failed to resolve the question whether or not the oxygen isotope fractionation is controlled either by kinetic or thermodynamic processes be- cause the experiments were difficult to control and the reactions were claimed to be inhibited by bacterial poisoning with excess H 2 S (Lloyd, 1968; Mizutani and Rafter, 1969, 1973; Fritz et al., 1989). It is therefore of interest to assess the isotope fraction- ations in a relatively simple natural marine environment where microbial sulfate reduction and concomitant organic matter decomposition are dominant. Moreover, it is of importance to contrast microbial sulfate reduction rates and isotope fraction- ations in deepwater sediments harboring chemosynthetic or- ganisms vis-a-vis shallow water photosynthetic sediments. In this study we explore the factors controlling isotope * Author to whom correspondence should be addressed. Pergamon Geochimica et Cosmochimica Acta, Vol. 64, No. 2, pp. 233–246, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 + .00 233