A LARGE METHANE PLUME EAST OF BEAR ISLAND (BARENTS SEA) - IMPLICATIONS FOR THE MARINE METHANE CYCLE

A pockmark field extending over 35 km(2) at 74 degrees 54'N, 27 degree s 34'E, described by Solheim and Elverhoi (1993), was re-surveyed and found to be covered with more than 30 steep-sided craters between 300 and 700 m in diameter and up to 28 m deep. The craters are thought to have been formed by an explosive gas eruption. Anomalously high concen trations of methane in the shelf waters around the craters suggest tha t a strong methane source near this area is still active today. Methan e enrichment more than 10 km away from the crater field indicates the large dimensions of a plume and the amount of gas released from source s below the seafloor of the Barents Sea shelf. From the characteristic vertical decrease of methane towards the sea surface, it is concluded that biota are extensively using this energy pool and reducing the me thane concentration within the water column by about 98% between 300 m depth and the sea surface. Degassing to the atmosphere is minimal bas ed on the shape of the methane concentration gradient. Nevertheless, t he net flux of methane from this area of the Barents Sea is about 2.9 x 10(4) g CH4 km(-2) yr(-1) and thus in the upper range of the present ly estimated global marine methane release. This flux is a minimum est imate and is likely to increase seasonally when rough weather leads to more effective vertical mixing during autumn and winter. The amount o f methane consumed in the water column, however, is about 50 times gre ater and hence should significantly contribute to the marine carbon in ventory.