Active gas venting through hydrate-bearing sediments on the Vestnesa Ridge, offshore W-Svalbard

Gas hydrate systems offshore western Svalbard are extensive and include the whole Vestnesa Ridge, an elongated sediment drift north of the Molloy Transform and just east of the Molloy Ridge, one of the shortest segments of the slow spreading North Atlantic Ridge system. The crest of the Vestnesa Ridge at water depth between 1200–1300 m is pierced with fluid flow features. Seafloor pockmarks vary in size up to 700 m in diameter. Pockmarks are generally larger at the eastern start of the Vestnesa Ridge than at its western termination. A recent cruise with R/V Jan Mayen discovered methane flares in the water column above the pockmark field at the onset of the Vestnesa Ridge. Over a period of two days at least 3 pockmarks were continuously active and methane flares in the water column reached a height of approximately 800 m. It is still unclear what has triggered the increase in gas expulsion from seafloor sediments. High-resolution 3D seismic data acquired in 2007 and 2010 show significant differences of the subseafloor expression of these fluid leakage systems. At the western end of the Vestnesa Ridge, sub-seafloor fluid flow features resemble well-described chimney structures. However, the seismic expression of the active fluid flow features is much broader, much more chaotic, dome-shaped and is not very similar to a typical chimney structure. The Vestnesa Ridge gas-hydrate and free-gas system occurs within few kilometers of a mid-oceanic ridge and transform fault, which makes this gas-hydrate system unique on Earth. The close proximity to the spreading center and its hydrothermal circulation system affects the dynamics of the gas-hydrate and free-gas system. The high heat flow together with the high tectonic activity of this region, a thick sedimentary cover, a shallow maturation window and an accelerated rate of biogenic and thermogenic gas production cause substantial disturbance to the free-gas system leading to high variability in gas supply, gas migration and gas hydrate build up and dissociation.