RESULTS

Concentration and Isotopic Composition of Methane

Pore-water methane concentrations measured for Hole 995B (Table 1) are plotted in Figure 1, along with pore-water sulfate concentrations from Holes 994B and 995B obtained shipboard (Paull, Matsumoto, Wallace, et al., 1996). The methane concentration profile displays the concave-up curve shape typical of marine sediments, though on a depth scale 10-100 times greater than is generally observed in coastal sediments. The upward concavity is consistent with methane consumption in a depth interval near 21-22 mbsf. Inverse numerical advection-diffusion-reaction modeling (Berner, 1980) of the profile in this region (described by Borowski et al., Chap. 9, this volume) predicts a peak methane oxidation rate of about 13 nM·d^-1 (Fig. 2). The zone of apparent methane oxidation is coincident with the region in which sulfate and methane co-occur, suggesting the process is closely coupled to sulfate reduction. This is consistent with an apparent link between the two processes observed in a number of coastal marine sediments (Alperin and Reeburgh, 1984; Devol, 1983; Hoehler et al., 1994; Iversen and Jørgensen, 1985; Reeburgh, 1980).

The behavior of methane stable carbon isotopes in sediments from Hole 995B (Table 1; Fig. 3) also suggests that AMO is occurring near the sulfate depletion depth. Methane is most depleted in ¹³C (lightest) at 21.45 mbsf. Above this depth, the methane becomes increasingly enriched in ¹³C (heavier), suggesting preferential consumption of ¹²CH₄ as methane diffuses up towards the sediment-water interface. This trend is consistent with kinetic isotope fractionation during methane oxidation (Alperin et al., 1988; Oremland and DesMarais, 1983; Whiticar and Faber, 1986).

Theoretically, the trend towards heavier methane ¹³C should begin at the base of the methane-oxidizing zone. Hence, the isotopic evidence suggests that methane oxidation occurs down to a depth of 21.45 mbsf, while the model predicts an AMO zone that extends about 60 cm deeper. This discrepancy may result from "smoothing" the cubic spline fit used in the model (Borowski et al., Chap. 9, this volume). It is interesting to note in this context, however, that measured sulfate reduction rates are consistent with an AMO zone that closely matches that predicted by the model (as described below and in Fig. 4).

Sulfate Reduction Rates

Tracer-based sulfate reduction rates measured in sediments from Holes 994B and 995B (Table 2; Fig. 4) have two clear features.

First, sulfate reduction occurs at clearly discernible rates in the upper portion of the sediment column. These rates decrease with depth, reaching near-baseline levels below 10 mbsf (Fig. 4A). This is consistent with organic matter-fueled sulfate reduction, which slows as the sediments become increasingly depleted in utilizable organic matter (with increasing depth).

Second, a subsurface maximum in sulfate reduction rates is evident in the apparent zone of methane oxidation. The pore-water chemistry in Holes 994B and 995B is quite comparable, as evidenced by similar sulfate concentration profiles (Fig. 1). Hence, sulfate reduction rates in sediments from Hole 994B may be compared to the modeled methane oxidation rates in Hole 995B with relative confidence. The penetration of sulfate in Hole 994B may be deeper than in Hole 995B by a few tens of centimeters (Fig. 1), so it is possible that other features of the pore-water chemistry (e.g., methane oxidation) are likewise offset. In sediments from Hole 995B, the subsurface peak in sulfate reduction occurred at 21.13 mbsf, whereas the model predicts a peak in methane oxidation at 21.22 mbsf (Fig. 4B). In Hole 994B, this sulfate reducing zone is offset by ~0.2 m (deeper in the sediment column) relative to the model-predicted zone in Hole 995B. Interestingly, the subsurface sulfate reducing zone in Hole 994B spans a depth (~2 m) that is quite similar to the model-predicted AMO zone in Hole 995B (1.85 m).

The highest sulfate reduction rate within the secondary maximum in Hole 995B is approximately twice the maximum model-predicted rate (Fig. 4B). The integrated sulfate reduction rate over the entire 2-m span of the subsurface maximum (pooling the data from Holes 994B and 995B) is quite comparable to the integrated model-derived methane oxidation rate, at approximately 1.3 and 1.1 nmol·cm^-2·d^-1, respectively. Given the uncertainty in the tracer-based sulfate reduction rates, the close agreement between these values may be largely fortuitous, but it is probably not unreasonable to suggest that they agree within a factor of 2-3.

The integrated sulfate reduction rate at the top of the core is substantially higher than in the subsurface maximum. A minimum estimate, based on the few available data points, is ~40 nmol·cm^-2·d^-1, roughly 30-fold higher than the quantity of sulfate apparently used to oxidize methane.

Tracer-Based Methane Oxidation Rates

The samples incubated with ¹⁴CH₄ did not produce ¹⁴CO₂ at significant levels. This is likely due to a combination of very low in situ AMO rates (model-predicted rates are 1-2 orders of magnitude lower than rates previously measured in coastal sediments) and an inadequate tracer activity. The detection limit for methane oxidation rates can be calculated given the activity of the tracer, the procedural blank for the overall rate measurement, and the in situ methane concentration. This limit was above the model-predicted rates for Hole 995B (Fig. 2A). Hence, the lack of measurable methane oxidation rates should not be taken to indicate a lack of activity; rather, this can be viewed as setting an upper limit on methane oxidation rates that corresponds to the detection limit for the process.