Continental margin sediments contain a large reservoir of methane and thus represent a potentially significant contribution to the global methane budget. This is particularly true of methane hydrate-bearing sediments. Despite this large reservoir, the oceans as a whole contribute only about 2% of the annual global flux of methane to the atmosphere (Cicerone and Oremland, 1988). This is largely due to anaerobic methane oxidation (AMO), which consumes upwardly diffusing methane before it can escape the sediments and contribute to the global flux (Reeburgh and Alperin, 1988).
A wealth of geochemical evidence now supports the conclusion that anaerobic methane oxidation is important in marine sediments. The first such evidence came from geochemical models that showed that "concave up" methane concentration profiles observed in many marine sediments can only be accounted for by consumption of methane in anoxic depth intervals (Barnes and Goldberg, 1976; Martens and Berner, 1977; Reeburgh, 1976; Scranton, 1988). The model predictions have been corroborated by radiotracer experiments that show that 14CH4 is converted to 14CO2 in anoxic systems (Alperin and Reeburgh, 1984; Devol, 1983; Hoehler et al., 1994; Iversen and Jørgensen, 1985; Reeburgh, 1980). Distributions of naturally occurring stable isotopes provide further evidence of methane oxidation. The presence of 2H- and 13C-enriched CH4 is consistent with kinetic fractionation arising from methane consumption (Albert and Martens, 1995; Alperin et al., 1988; Oremland and DesMarais, 1983; Whiticar and Faber, 1986), whereas the presence of 13C-depleted CO2 implies oxidation of an isotopically "light" substrate (methane) (Blair and Aller, 1995). Finally, time-series incubations have shown decreasing methane concentrations in contained anoxic samples (Lidstrom, 1983).
Despite the lack of a concretely proven mechanism for anaerobic methane oxidation, several features are consistently observed. Most importantly, from the standpoint of marine sediments, is an apparent link to sulfate reduction:
In many marine sediments where the process has been characterized, a secondary maximum in sulfate reduction rates coincides with the peak in methane oxidation rates. Often, the methane oxidation rate represents a relatively small fraction of the secondary maximum in sulfate reduction rates (Alperin and Reeburgh, 1984; Devol, 1983; Devol and Ahmed, 1981), suggesting that most of the electrons used to reduce sulfate are derived from organic matter fermentation. In at least one instance, however, methane apparently fueled as much as 80% of the secondary maximum in sulfate reduction rates (Iversen and Jørgensen, 1985). The contribution of anaerobic methane oxidation to the overall depth-integrated sulfate reduction rate is typically quite low.
Most studies of anaerobic methane oxidation have focused on nearshore sedimentary environments that are relatively rich in organic matter. Ocean Drilling Program (ODP) Leg 164 thus provided a unique opportunity to study the process in more oligotrophic continental margin sediments. The goal of this study was to characterize anaerobic methane oxidation in sediments overlying methane hydrates on the Blake Outer Ridge. The question was addressed using three independent techniques:
With the exception of tracer-based methane oxidation rates (for which the detection limit was too high), each independent line of evidence indicated the occurrence of methane oxidation in the zone of overlap between methane and sulfate. Although these results bear qualitative analogy to many of the previous nearshore studies, the rates and depth scales involved differ by orders of magnitude.
