Arctic hydrate dissociation as a consequence of climate change: determining the vulnerable methane reservoir and gas escape mechanisms

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Along the western margin of Spitsbergen, where the northern extension of Gulf Stream system conveys warm Atlantic water into the Arctic Ocean, hundreds of plumes of bubbles of methane gas were discovered in 2008, rising from the seabed at a depth close to that of the landward limit of the methane hydrate stability zone. Methane hydrate is a solid with the appearance of ice, in which water forms a cage-like structure enclosing molecules of methane. Methane hydrate is stable under conditions of low temperature and high pressure such as those found in regions of permafrost or under the ocean in water deeper than 300-600 metres, depending on the water temperature. Over the past thirty years, the ocean's temperature at the seabed has increased by 1 degree C, causing the zone in which hydrate is stable to contract down the continental slope, with the apparent consequence that hydrate has broken down and released methane, which has migrated to the seabed and into the ocean. At present, the rate of release of methane is generally too slow to overcome dissolution and oxidation in the ocean to reach the atmosphere, except in very small quantities. However, catastrophic gas venting, which is known to occur elsewhere, could release large amounts of methane over a short period of time. The strength of such venting depends upon the how much gas is stored locally beneath the seabed and the kinds of pathways that bring gas to the seabed. The proposed research seeks to define these pathways and to quantify the amount of gas. A marine research expedition will use a deep-towed, very high-resolution seismic system to image the small-scale structures that convey gas to the seabed and to detect the presence of gas in the sediments beneath the seabed. This will be done in conjunction with an electromagnetic exploration system that uses a deep-towed transmitter and receivers on the seabed to derive the variations in electrical resistivity in the sediments beneath the seabed. Higher-than-normal resistivity is caused by both gas and hydrate, whereas the presence of gas reduces seismic velocity and hydrate increases it. In combination, the two techniques can distinguish the separate amounts of hydrate and gas. The deep-towed seismic system, SYSIF, which uses a piezo-electric chirp source that gives very-high-resolution images and deeper sub-seabed penetration than similar systems mounted on a ship's hull, will be supplemented by the use of ocean-bottom seismometers to provide precise measurements of the variation of seismic velocity with depth, and seismic profiles with small airgun (mini-GI gun) to provide deeper high-resolution seismic imaging. Multibeam sonar will be used to improve definition of the shape of the seabed and high-frequency, fish-finding sonar will image plumes of gas bubbles and define their positions, providing, in many cases, comparisons with the images obtained in 2008 when they were first discovered. Two areas will be investigated, the region of the landward limit of the methane hydrate stability zone, where many bubble plumes occur in water shallower than 400 metres, and, for comparison, a pockmark in the Vestnesa Ridge, at a depth 1200 metres, from which gas is escaping and is underlain by 'chimneys' that convey gas to the seabed through the hydrate stability zone, where the gas would normally form hydrate. Geological and geophysical data, including 96-channel seismic reflection profiles, acquired in both areas during a research cruise in 2008, will complement the new data. The project will provide the sub-seabed context for a seabed observatory (MASOX Monitoring Arctic Seafloor - Ocean Exchange), which will be established in the shallow plume area in summer 2010 by a European scientific consortium to monitor the activity of the plumes and the physical and chemical fluxes through the seabed.