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[1] The isotopic composition of the dissolved inorganic carbon (DIC) collected at sites of active methane discharge on Hydrate Ridge, Oregon, reveals anaerobic methane oxidation mediated by bacteria, with d 13 C DIC reaching values as low as À48% in the upper 4 cm of the sediment. In spite of the high sulfide levels in the discharging fluids, living specimens of the benthic foraminifera Uvigerina peregrina are abundant in the vents, probably owing to the rich bacterial food source. Although pore water d 13 C DIC is extremely low (À6 to À48%), the d 13 C values of living (Rose Bengal stained) foraminifera shells collected from active methane seeps are not significantly lower than those observed in nonventing pelagic sediments, and are within the range expected from local organic matter decomposition (0 to À4%). The apparent d 13 C disequilibrium between biogenic calcite and DIC suggests that at seep localities, foraminifera calcify mostly during periods when there is little methane discharge or during intermittent episodes of seawater flow into the sediments. The isotopic composition and Mg/ Ca ratios of fossil (unstained) foraminifera recovered at carbonate-rich sites on the northern Hydrate Ridge reveals overprinting of the biogenic record by inorganic calcite with high Mg/Ca and anomalously low d 13 C values. Thus overprinting of the original isotopic composition of foraminifera by overgrowths or recrystallization at or below the sediment surface, rather than primary calcification in contact with 13 C depleted DIC, can explain extreme 13 C depletion observed in fossil foraminifera recovered from sites of active methane discharge.

Methane (CH4) concentrations and oxidation rates were measured throughout the Hudson River Estuary in March and August of 1991. Methane concentrations ranged from 50 to 940 nM and were supersaturated with respect to the atmosphere along the entire length of the river, with generally higher CH4 values in the lower, saline portion of the estuary. A seasonally averaged diffusive flux to the atmosphere from the Hudson River was estimated to be 5.6 mg CH4 m−2 d−1, corresponding to an annual flux of 0.76 × 109 g CH4. The Hudson River Estuary also releases approximately 0.2 × 109 g CH4 annually to nearshore marine waters. Diffusive flux across the air/river interface was the dominant removal mechanism for Hudson River CH4 in March. In August, CH4 oxidation was the dominant CH4 sink in freshwater and brackish (<6‰) sections of the river, removing up to 70% of ambient CH4 per day compared to maximum daily removal rates of 13% in March. Thus methane oxidation can play a major role in limiting releases of CH4 to the atmosphere from rivers and other freshwater environments. Methane oxidation activity decreased rapidly as salinity increased, with less than 2% of ambient CH4 being oxidized per day at salinities greater than 25‰. Addition of NaCl or seawater to freshwater samples resulted in comparable inhibition of methanotrophic activity. Budget calculations showed that flux to the atmosphere and CH4 oxidation in March removed less water column CH4 than was supplied to the Hudson River over its entire length. In August, however, CH4 removal approximately equaled CH4 supply with the result that there was no net accumulation of CH4 over the length of the river.

Zooplankton produced CH, while grazing on marine phytoplankton during laboratory experiments. We consider anaerobic microniches within zooplankton digestive tracts to be the most likely site for such methanogenesis. Methane production appeared to be zooplankton species-specific rather than dependent on the phytoplankton species being grazed. The amount of CH, produced (4-20 nmol copepod-' d-l) was sufficient under experimental conditions to make a significant contribution to the formation and maintenance of oceanic subsurface CH, maxima.The methanogenic substrates monomethylamine and trimethylamine were constituents of the diatom, dinoflagellate, and flagellate phytoplankton species used in the CH, production feeding experiments. Dimethylamine was present in two of the three algal species. Trimethylamine was the most abundant methylamine in all cases and was present in sufficient quantity to account for the CH, production observed during copepod feeding experiments. Methanogens in these experiments would need to convert only 4-12% of the phytoplankton methylamines available to copepods during grazing to produce CH, concentrations observed in the experimental flasks.Dissolved CH4 profiles in the upper oceanic water column are frequently characterized by a subsurface maximum with levels 2-3 times more supersaturated with CH, than are surface waters (e.g. Scranton and Brewer 1977; Brooks et al. 198 1; Conrad and Seiler 198 8). Advection of CH4 from nearshore point sources such as hydrocarbon seeps (Cynar and Yayanos 199 1) and anoxic slope sediments (Brooks et al. 198 1) undoubtedly contributes to the subsurface CH4 maximum in coastal regions. However, Scranton and Brewer (1977) and Conrad and Seiler (198 8) have shown that advection alone is insufficient to maintain such a well-developed feature against large diffusional gradients over the time and distance ' Current address: