Anaerobic methane oxidation rates at the sulfate‐methane transition in marine sediments from Kattegat and Skagerrak (Denmark)1

Concomitant radiotracer measurements were made of in situ rates of sulfate reduction and anaerobic methane oxidation in 2–3‐m‐long sediment cores. Methane accumulated to high concentrations (>1 mM CH4) only below the sulfate zone, at 1 m or deeper in the sediment. Sulfate reduction showed a broad maximum below the sediment surface and a smaller, narrow maximum at the sulfate‐methane transition. Methane oxidation was low (0.002–0.1 nmol CH4 cm−3 d−1) throughout the sulfate zone and showed a sharp maximum at the sulfate‐methane transition, coinciding with the sulfate reduction maximum. Total anaerobic methane oxidation at two stations was 0.83 and 1.16 mmol CH4 m−2 d−1, of which 96% was confined to the sulfate‐methane transition. All the methane that was calculated to diffuse up into the sulfate‐methane transition was oxidized in this zone. The methane oxidation was equivalent to 10% of the electron donor requirement for the total measured sulfate reduction. A third station showed high sulfate concentrations at all depths sampled and the total methane oxidation was only 0.013 mmol m−2 d−1. From direct measurements of rates, concentration gradients, and diffusion coefficients, simple calculations were made of sulfate and methane fluxes and of methane production rates.