Westbrook, G. K., Minshull, T., Berndt, Christian , James, R. , Paelike, H., Rohling, E., Chabert, A., Fisher, R., Green, D., Huehnerbach, V., Lanoiselle, M., Thatcher, K., Burchell, A., Piotrowski, A., Aquilina, A., Crocker, A., Bolton, C. and Osbourne, A. (2008) Active gas Venting at the Landward Limit of Hydrate Stability Offshore Svalbard. [Paper] In: AGU Fall Meeting 2008. , 15.-19.12.2008, San Francisco, USA .

Abstract

Most climate models predict rapid warming in the Arctic during the next few decades. This warming is expected to lead to warming of the seabed and destabilisation of submarine methane hydrates in the Arctic, with possible additional release of methane into the atmosphere. The part of the submarine methane hydrate system that is most sensitive to such warming is the region where the base of the hydrate stability field intersects the seabed. We report initial results from a multidisciplinary cruise in August-September 2008 from such a region on the western margin of Svalbard, where gas hydrate-related bottom-simulating reflectors (BSRs) are widespread and numerous seabed pockmarks are present. We acquired high- resolution ocean-bottom seismometer, multi-channel seismic reflection, subbottom profiler and sidescan sonar data, swath bathymetry, sediment cores, water column samples, and surface air samples. An extensive field of gas flares in the water column was imaged with 38 kHz sonar where the seabed is less than 370 m deep, just shallower than where the base of the hydrate stability field is predicted to intersect the seabed. Temperature and salinity measurements and water sampling indicate that the ocean water in these flares is well mixed and has enhanced concentrations of methane. The BSR is present at water depths of 700 m and greater, but cannot be traced into the region of gas flares. However, widespread high-amplitude, reversed- polarity seismic reflectors beneath the hydrate stability field in 500-800 m water depth indicate the presence of free gas at depth. A zone of acoustic scattering was observed beneath the region of gas flares, extending down slope to the region of high-amplitude reflectors. Such a zone is consistent with the presence of pathways for gas migration. Several discrete fluid-escape structures were imaged that appear to penetrate through the hydrate stability field to the seabed. Our observations suggest localised gas venting through the hydrate stability field has occurred in postglacial times and ongoing gas venting is occurring at its landward limit.

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