The results provide three independent forms of evidence to indicate the occurrence of anaerobic methane oxidation: diagenetic modeling, stable isotope, and radiotracer. Each of these places the zone of methane oxidation in an ~2-m interval that coincides with the overlap of sulfate- and methane-containing pore waters. Further, the model-predicted methane oxidation and tracer-based sulfate reduction rates suggest a similar magnitude for AMO rates.
In a qualitative sense, the behavior of methane oxidation in Blake Ridge sediments is very similar to what has previously been observed for nearshore environments; however the absolute scale on which the process occurs differs markedly. Where methane oxidation has previously been characterized in sediment environments, it generally occurs within a zone of, at most, a few tens of centimeters in thickness and at depths seldom greater than 2 mbsf (Alperin and Reeburgh, 1984; Devol, 1983; Hoehler et al., 1994; Iversen and Jørgensen, 1985; Reeburgh, 1980). In the Blake Ridge sediments, the AMO zone itself spans nearly 2 m and is located more than 20 mbsf. In addition, the methane oxidation rates (suggested by the model and measured sulfate reduction rates) are 2-3 orders of magnitude lower than in most coastal settings. Effectively, methane oxidation in Blake Ridge sediments is "stretched out" relative to nearshore environments.
The "stretching out" is characteristic of the entire process of organic matter remineralization, as illustrated by a simple comparison. In organic-rich sediments from nearshore Cape Lookout Bight, North Carolina (which has among the highest rates of overall remineralization reported for marine sediments), sulfate reduction rates as high as 2000 µM·d-1 (Crill and Martens, 1987) lead to sulfate depletion within 10 cm of the sediment-water interface (Martens and Klump, 1980). Sulfate-coupled methane oxidation at this site is limited to a zone of 5-10 cm (Hoehler et al., 1994). In the more oligotrophic Blake Ridge sediments, measured sulfate reduction rates near the sediment-water interface are four orders of magnitude lower than in Cape Lookout Bight, permitting penetration of sulfate to 20 mbsf (deeper by two orders of magnitude). The AMO in Blake Ridge sediments occurs at rates 2 orders of magnitude lower than in Cape Lookout and over a 20-fold broader zone.
The presence of measurable sulfate reduction in the upper portion of the sediment column at Site 994B brings to light an interesting discrepancy. Borowski et al. (1996) note that pore-water sulfate concentrations in the sediments of this region typically exhibit a linear decrease with depth and suggest that this indicates a lack of organic matter-driven sulfate reduction. Similar geochemistry was recently observed in sediments of the upwelling region off western Africa (Niewöhner et al., 1998). To reconcile these observations with the measured sulfate reduction rates, it is critical to note that linear sulfate concentration profiles can only demonstrate a lack of net sulfate reduction. It has been repeatedly shown that sulfides resulting from sulfate reduction can be reoxidized to sulfate in situ by ferric and manganic oxides (Aller and Rude, 1988; Fossing and Jørgensen, 1990; Sørensen and Jørgensen, 1987). If such a process occurred in the Blake Ridge sediments, any evidence of organic matter-driven sulfate reduction would be lost from the pore-water sulfate concentration profiles (because, in effect, no net reduction of sulfate has occurred if the sulfides are reoxidized).
In such a case, the consumption of sulfate via methane oxidation at depth might represent a much smaller fraction of the total sulfate reduction than suggested by pore-water concentration gradients alone. Borowski et al. (Chap. 9, this volume) calculate methane and sulfate fluxes for Hole 995B based on pore-water concentration gradients. These calculations indicate that ~35% of the downward sulfate flux can be accounted for by upwardly diffusing methane. Measured sulfate reduction rates ascribe a lesser importance to methane oxidation: the depth-integrated sulfate reduction rate in the subsurface, AMO-associated zone (which is approximately equal to the integrated methane oxidation rate) is only about 2%-5% of the depth-integrated rate in the upper 10 m of the sediment column.
It is clearly reasonable to expect at least some organic matter-driven sulfate reduction in these sediments. Biogenic production of methane in the region immediately underlying the apparent AMO zone is indicated both by the diagenetic model (Fig. 2A) and by the extremely light (13C-depleted) isotopic signature of methane in the depth interval surrounding the sulfate-methane transition depth (Fig. 3). The biogeochemical processes (i.e., fermentation of organic matter) that drive biogenic methane production will preferentially fuel sulfate reduction when sulfate is present (Lovley et al., 1982). Hence, the evidence of in situ biogenic methane formation in the sediments of Hole 995B indirectly suggests the occurrence of sulfate reduction in the upper 20 m of the sediment column.
A small contribution of AMO to absolute sulfate reduction rates does not necessarily imply a small contribution to net sulfate reduction rates. If the suggested metal-based oxidizing capacity of the sediments is exhausted at a depth shallower than the zone of methane oxidation, then AMO could in theory represent the entire contribution to net consumption of sulfate. In such a case, the upward flux of methane would strongly influence the downward sulfate flux and corresponding sulfate concentration gradient, as proposed by Borowski et al. (1996).