Stable isotope tracing of anaerobic methane oxidation in the gassy sediments of Eckernforde Bay, German Baltic Sea

Methane concentrations in the pore waters of Eckernforde Bay in the German Baltic Sea generally reach gas bubble saturation values within the upper meter of the sediment column. The depth at which saturation occurs is controlled by a balance between rates of methane production, consumption (oxidation), and transport. The relative importance of anaerobic methane oxidation (AMO) in controlling dissolved and gas bubble methane distributions in the bay's sediments is indirectly revealed through methane concentration versus depth profiles, depth variations in the stable C and H isotope composition of methane, and the C isotope composition of total dissolved inorganic carbon (Sigma CO2). Direct radiotracer measurements indicate that AMO rates of over 15 mM yr(-1) are focused at the base of the sulfate reduction zone. Diagenetic equations that describe the depth distributions of the delta(13)C and delta D values of methane reproduce isotopic shifts observed throughout the methane oxidation zone and are best fit with kinetic isotope fractionation factors of 1.012 +/- 0.001 and 1.120 +/- 0.020 respectively.