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The abundance and distribution of dissolved CH4 were determined from 1987-1990 in Lake Fryxell, Antarctica, an amictic, permanently ice-covered lake in which solute movement is controlled by diffusion. CH4 concentrations were < 1 muM in the upper oxic waters, but increased below the oxycline to 936 muM at 18 m. Sediment CH4 was 1100 mumol (l sed)-1 in the 0-5 cm zone. Upward flux from the sediment was the source of the CH4, NH4+, and DOC in the water column; CH4 was 27% of the DOC + CH4 carbon at 18 m. Incubations with surficial sediments indicated that (HCO3-)-C-14 reduction was 0.4 mumol (l sed)-1 day-1 or 4x the rate of acetate fermentation to CH4. There was no measurable CH4 production in the water column. However, depth profiles of CH4, NH4+, and DIC normalized to bottom water concentrations demonstrated that a significant CH4 sink was evident in the anoxic, sulfate-containing zone of the water column (10-18 m). The (deltaCH4)-C-13 in this zone decreased from -72 parts per thousand at 18 m to -76 parts per thousand at 12 m, indicating that the consumption mechanism did not result in an isotopic enrichment of (CH4)-C-13. In contrast, (deltaCH4)-C-13 increased to -55 parts per thousand at 9 m due to aerobic oxidation, though this was a minor aspect of the CH4 cycle. The water column CH4 profile was modeled by coupling diffusive flux with a first order consumption term; the best-fit rate constant for anaerobic CH4 consumption was 0.012 yr-1. On a total carbon basis, CH4 consumption in the anoxic water column exerted a major effect on the flux of carbonaceous material from the underlying sediments and serves to exemplify the importance of CH4 to carbon cycling in Lake Fryxell.