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[References: p. 2290-2292]

The Arctic Ocean, especially the East Siberian Arctic Shelf (ESAS), has been proposed as a significant source of methane that might play an increasingly important role in the future. However, the underlying processes of formation, removal and transport associated with such emissions are to date strongly debated. CH4 concentration and triple isotope composition were analyzed on gas extracted from sediment and water sampled at numerous locations on the shallow ESAS from 2007 to 2013. We find high concentrations (up to 500 µM) of CH4 in the pore water of the partially thawed subsea permafrost of this region. For all sediment cores, both hydrogen and carbon isotope data reveal the predominant occurrence of CH4 that is not of thermogenic origin as it has long been thought, but resultant from microbial CH4 formation. At some locations, meltwater from buried meteoric ice and/or old organic matter preserved in the subsea permafrost were used as substrates. Radiocarbon data demonstrate that the CH4 present in the ESAS sediment is of Pleistocene age or older, but a small contribution of highly 14C-enriched CH4, from unknown origin, prohibits precise age determination for one sediment core and in the water column. Our sediment data suggest that at locations where bubble plumes have been observed, CH4 can escape anaerobic oxidation in the surface sediment.