Sea-floor methane hydrate and gas wipeouts

Marine seismic observations have been widely used to map the distribution of methane hydrates within sea-floor sediments, and the feature that is most clearly defined in areas of hydrate accumulation is a strong acoustic reflector up to a few hundred metres below the sediment-water interface.  This bottom-simulating reflector (BSR) is interpreted to define the boundary between the methane hydrate phase and the underlying methane gas phase, and, in general, it coincides with the “base of gas hydrate stability” or BGHS.  In many areas, such as the Cascadia margin off Vancouver Island, the BGHS appears to be fairly continuous over distances of kilometres (see Hyndman et al., 2003).

Researchers from the Pacific Geoscience Centre in Sydney, the University of Victoria and the US Naval Research Lab in Mississippi (Wood et al., 2002), have used a deep-towed seismic system to acquire a more detailed picture of methane hydrate distribution in the northern Cascadia area.  This system is able to provide as much as an order of magnitude greater resolution as conventional surface-towed seismic techniques, and it has shown have found that the BGHS boundary in the northern Cascadia area is not as consistently smooth as previously thought.

The data presented by Wood et al. (see figure below) are characterized by numerous vertical columns of poor seismic reflectivity.  Although the exact cause of these so-called “wipeouts” is not clearly understood, Wood et al. interpret that they are related to chimneys of relatively warm gas streaming towards the surface.  In one case the column appears to extend right to surface, and at this location there is evidence of a small crater or pockmark on the sea floor.  It is interpreted that the chimneys affect the stability of the methane hydrate, creating significant perturbations of the BGHS interface.

Deep-towed acoustic geophysics system (DTAGS) data for part of the Northern Cascadia methane hydrate accumulation region.  Vertical exaggeration is about 10x.

Interpretation of part of the Northern Cascadia DTAGS profile (without vertical exaggeration)

One of the implications of the apparent roughness of the BGHS interface, is that it might lead to a decrease in the stability of the methane hydrate deposits in the event of a change in conditions on the sea floor.  The stability of sea-floor methane hydrate is maintained by the high pressure of the ocean water, and the relatively low temperature at the water-sediment interface.  Pecher (2002) shows that an increase in bottom water temperature will lead to greater instability in the case of a rough BGHS interface as compared with a smooth interface.

References

Hyndman et al., 2003, Geophysical studies of marine gas hydrate in Northern Cascadia, http://www.pgc.nrcan.gc.ca/marine/gas_hydrates/gashyd_more.htm

Pecher, I., 2002, Gas hydrates on the brink, Nature, Vol. 420, p. 622-623. (December 2002)

Wood, W., Gettrust, J., Chapman, N., Spence, G. and Hyndman, R., 2002, Decreased stability of methane hydrates in marine sediments owing to surface roughness, Nature, Vol. 420, p. 656-660. (December 2002)

For background information on methane hydrates see: http://www.mala.bc.ca/~earles/m-hydrate-nov99.htm and http://www.mala.bc.ca/~earles/mh-instability-apr00.htm

Steven Earle, Malaspina University College, 2003. Return to Earth Science News