A major part of the upward methane flux in anoxic marine sediments appears to be consumed in a subsurface zone of anaerobic methane oxidation. Slope changes in the depth distributions of methane, sulfate, and total carbon dioxide concentration indicate that the downward flux of sulfate and the upward flux of methane approach zero in this zone and that the upward flux of carbon dixoide increases. A minimum in the stable carbon isotope ratio of carbon dioxide (^dgr¹³CO₂) coincides with this zone; the minimum appears to be due to local injection of isotopically light, methane-derived carbon dioxide. Differences in methane distributions from freshwater and marine sediments suggest that sulfate reducers are responsible for anaerobic methane oxidation in m rine systems. Recent "quasi in-situ" tracer experiments using ¹⁴CH₄ confirm that methane is anaerobically oxidized. Depth distributions of methane oxidation rates in Chesapeake Bay and Skan Bay sediments show a maximum whose location and magnitude are in accord with model predictions.

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