Microbially mediated methane and sulfur cycling in pockmark sediments of the Gdansk Basin, Baltic Sea

In the Russian sector of the Gdansk Basin (Baltic Sea), high organic matter influx fuels microbial processes resulting in the formation of reduced sediments with elevated methane concentrations. Investigated areas of geoacoustic anomalies (similar to 245 km(2)) were found to contain three distinct geomorphologic structures (pockmarks), with a total area of similar to 1 km(2). Methane anomalies recorded in the water above one of these pockmarks were traced as high as 10 m above the bottom. In pockmark sediments, sulfate reduction and anaerobic oxidation of methane (AOM) occurred at high rates of 33 and 50 A mu mol dm(-3) day(-1), respectively. Integrated over 0-180 cm sediment depths, AOM exceeded methanogenesis almost tenfold. High AOM rates resulted from methane influx from deeper sediment layers. The delta(13)C signature of methane carbon (-78.1 to -71.1aEuro degrees) indicates the biogenic origin of pockmark methane. In pockmark sediments, up to 70% of reduced sulfur compounds was possibly produced via AOM.