FROZEN HEAT | Volume 1

On the U.S. Beaufort shelf/slope area, recent measurements suggest there is no measurable difference in the surface wa- ter’s methane content across the zone where methane hy- drate may currently be dissociating, meaning that while the surface waters are methane rich, widespread gas bubbling in the water column that can be attributed to hydrate dissocia- tion is not observed (Pohlman et al. 2012).

The Laptev Sea and the surrounding Siberian shelf areas are also quite rich in methane (Shakhova et al. 2010b) and bub- ble plumes have been observed, but there are many methane sources in that system and it is not yet known the extent to which dissociating hydrates are contributing to the observed methane concentrations (Text Box 3.2).

Evolution of a pingo-like feature (PLF)

Vent gas

Pingo-like Feature

Moat

Gas hydrate bearing sediments

Gas hydrate bearing sediments

Figure 3.9: Evolution of a pingo-like feature (PLF). As the subsurface warms, the top of the gas hydrate stability zone moves downward (yellow arrows in the left panel). Warming results in gas hydrate dissociation in a gradually thickening zone (brown), releasing gaseous methane into the sediments (yellow bubbles). Bubble formation associated with this phase change creates overpressured conditions. The right-hand panel shows how material may flow (red arrows) both laterally and vertically in response to overpressure. Displaced sediments rise upward to form the PLF and allow the gas to vent. As the pressure is dissipated through both the transfer of solids and degassing, subsidence in the area immediately surrounding the PLF (black arrows) creates the moat.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 65