Gas venting and hydrate deposits in the Okhotsk Sea

Active gas vents and methane hydrate deposits were found in two areas of the Okhotsk Sea in October, 1991 onboard the R. V. Antropov, a Russian research vessel operated by Dalmorgeolo­gia from the Far-East Region of Russia. Gas vents were identified in water depths of 700 to 800 m using a 20 khz fish finding echo sounder. Sediment and hydrate samples were recovered using a 3 m long gravity corer. Some cores contained 30 to 40% by volume methane hydrate in lenses, layers and veins. A small fraction of the venting gas is captured in a methane hydrate phase, while a majority of the vented gas is released and dispersed in the overlying seawater. Small gas bubbles were observed at the sea surface, indicating that some of the venting gas reaches the atmosphere directly. Preliminary hydrocarbon gas and methane isotopic composition suggest that the venting gas is biogenic in origin. Based on the flux of organic carbon reaching the seafloor and pore water ammonia results, recent biogenic processes are not sufficient to support the production of the venting methane. It is concluded that biogenic methane from subsurface sediment layers is migrating upwards along preferential pathways associated with faulting.