Sulfate reduction refers to microbially mediated diagenetic processes that deplete sulfate within interstitial pore waters. In anoxic marine sediments, sulfate reduction is generally the most important remineralization process (e.g., Reeburgh, 1983), with the net reaction adding dissolved carbon dioxide (
CO2, or dissolved inorganic carbon, DIC) and sulfide to pore waters. Sulfate depletion typically occurs when microbes utilize interstitial sulfate that is ultimately derived from overlying seawater, to oxidize sedimentary organic matter (SOM) in the net reaction:
2 HCO3- + H2S, (1)
which also releases ammonium and phosphate to pore waters as net by-products of SOM degradation (Richards, 1965). An additional sink for sulfate occurs near the base of the sulfate reduction zone, or sulfate-methane interface (SMI), in a net process called anaerobic methane oxidation (AMO):
HCO3- + HS- + H2O (2)
(Reeburgh, 1976). Although sulfate and methane are apparently co-consumed in the net reaction, the pathway for this process is controversial (e.g., Hoehler et al., 1994).
In most marine sediments, oxidation of SOM (Eq. 1) is the predominant reaction that controls interstitial sulfate concentrations and gradients. However, in methane-charged sediments like those overlying gas hydrate deposits, AMO may become an important process that affects interstitial sulfate concentration gradients. Borowski et al. (1996) linked interstitial sulfate depletion in Carolina Rise and Blake Ridge (CR-BR) (offshore southeastern United States) sediments to upward methane flux and underlying methane inventory. In such methane-rich settings, AMO potentially consumes a significant portion of the interstitial sulfate pool, depleting sulfate more rapidly than oxidation of SOM alone. The underlying methane concentrations regulate the upward methane flux (usually through diffusion) and thus also control the rate of AMO. Steep sulfate gradients (and perhaps their linearity) and shallow depths to the sulfate-methane interface are a consequence of the increased influence of AMO within these continental rise sediments (Borowski, 1998).
We analyze geochemical evidence from Ocean Drilling Program (ODP) drilling on the Blake Ridge (Leg 164) to assess the importance of AMO as a mechanism influencing sulfate depletion. The data presented here are unique because of high-resolution sampling (~1- to 1.5-m intervals) through the upper methanogenic zone and entire sulfate reduction zone. The results substantiate inferences drawn from measurement of sulfate gradients in sediments collected previously by piston coring (Borowski et al., 1996, 1997; Borowski, 1998). Sulfate gradients are steep (and linear) within these methane-rich, continental-rise sediments, and supporting concentration and isotopic data used in simple mixing and diagenetic models demonstrate that AMO is a significant sink for sulfate. We infer that such a magnified role for AMO influences the magnitude and shapes of sulfate gradients in other marine, methane-rich settings associated with gas hydrates.
