Seafloor pockmarks are common indicators for vertical fluid flow and frequently associated with methane discharge through the gas-hydrate stability zone (GHSZ). The present-day flux through these degassing systems is presumably at a low level on most rifted continental margins. A pockmark-field on the NW-Svalbard passive margin is located on young ocean crust (< 20 Ma) and shows evidence of ongoing, episodic degassing. New geophysical data from the Vestnesa Ridge (similar to 79 degrees N), a mounded and elongated sediment drift in the eastern Fram Strait, reveal a gas-hydrate, free-gas and venting system that is exceptionally more dynamic than documented elsewhere along the northeastern North Atlantic margin. The prominent bottom-simulating reflection (BSR), about 200 mbsf, separates anomalously high P-wave velocities in the GHSZ from a remarkable underlying low-velocity zone, indicating the presence of gas hydrate and gas in the pore space. Inversion of P-wave velocity data using the differential effective medium theory yields a two-dimensional concentration model of methane hydrate and free gas. The model predicts saturations of up to 11% in the hydrate reservoir, which due to the seafloor topography forms a large anticlinal permeability-barrier. Below, in the low-velocity zone (i.e., 1350-1500 m/s), up to 3% of free gas is predicted across the apex of the Vestnesa Ridge and in the immediate vicinity of extensional faults. A conservative estimate indicates that 225 kg/m(2) of pure methane is stored in hydrate and gas in the upper 230 m of the sedimentary column. An elongated pockmark-field, consisting of >100 individual pockmarks up to 600 m wide, systematically aligns the apex of the Vestnesa Ridge. Active, vigorous degassing from the topography-controlled pressure-valve system was evident from a 750-m-high and similar to 150-m-wide gas flare observed in the water column during a cruise with R/V Jan Mayen in October 2008. The gas flare documents dynamic degassing through the corresponding chimney, which penetrates the entire GHSZ and into the underlying free gas zone. Cruises in 2006 and 2007 did not detect active gas venting above the pockmark-field. Accordingly, vigorous degassing may operate in an episodic mode, where hydrothermal circulation systems through young ocean crust may play a significant role. (C) 2009 Elsevier B.V. All rights reserved.
