Geochemical and geophysical evidence of methane release over the East Siberian Arctic Shelf

<jats:p>The East Siberian Arctic Shelf (ESAS), which includes the Laptev Sea, the East Siberian Sea, and the Russian part of the Chukchi Sea, has not been considered to be a methane (CH<jats:sub>4</jats:sub>) source to hydrosphere or atmosphere because subsea permafrost, which underlies most of the ESAS, was believed, first, not to be conducive to methanogenesis and, second, to act as an impermeable lid, preventing CH<jats:sub>4</jats:sub> escape through the seabed. Here recent observational data obtained during summer (2005–2006) and winter (2007) expeditions indicate the ubiquitous presence of elevated dissolved CH<jats:sub>4</jats:sub> and an elevated atmospheric CH<jats:sub>4</jats:sub> mixing ratio. The CH<jats:sub>4</jats:sub> data were also analyzed together with high resolution seismic (HRS) data obtained by means of a “Sonic M‐141” system consisting of a high‐resolution profiler and side‐scan sonar mounted in a towed fish during the Transdrift‐X Expedition (2004) onboard the <jats:italic>R</jats:italic>/<jats:italic>V Yakov Smirnitskiy</jats:italic>. Results show anomalously high concentrations of dissolved CH<jats:sub>4</jats:sub> (up to 5 <jats:italic>μ</jats:italic>M) and an episodically (nongradually) increasing atmospheric mixing ratio of CH<jats:sub>4</jats:sub> (up to 8.2 ppm) in some areas of the ESAS. A most likely source is year‐round CH<jats:sub>4</jats:sub> release through taliks (columns of thawed sediments within permafrost) from seabed CH<jats:sub>4</jats:sub> reservoirs such as shallow hydrates and geological sources. Such releases occur not only within the areas underlain by fault zones but also outside of them. This points to permafrost's failure to further preserve CH<jats:sub>4</jats:sub> deposits in the ESAS. The total amount of carbon preserved within the ESAS as organic matter and ready to release CH<jats:sub>4</jats:sub> from seabed deposits is predicted to be ∼1400 Gt. Release of only a small fraction of this reservoir, which was sealed with impermeable permafrost for thousands of years, would significantly alter the annual CH<jats:sub>4</jats:sub> budget and have global implications, because the shallowness of the ESAS allows the majority of CH<jats:sub>4</jats:sub> to pass through the water column and escape to the atmosphere.</jats:p>