FROZEN HEAT | Volume 1

sink, sequestering methane as the hydrate forms, or act as a source, releasing methane as the hydrate breaks down.

may be acting as methane sources (one potential source be- ing methane released from hydrate in submarine slides, as discussed in Text Box 2.2). The relatively simple gas hydrate capacitor concept is not intended to capture the complete, interconnected system of controls on gas hydrate formation, but it does provide a useful analogy for discussing how gas hydrate volumes change over time in response to natural en- vironmental conditions (see Volume 1, Chapter 3).

Currently, the global methane hydrate capacitor is thought to be relatively stable – with balanced methane inputs and emis- sions from natural gas-hydrate reservoirs. Local variations are possible, with gas hydrates in some areas of the globe acting as methane sinks, while others in different environments

been proposed by Sultan et al. (2004) as a method of weakening the sediment to the point where a slide, such as Storegga, would initially fail near its toe. However, other recognized geologic phenomena are often sufficient to explain large slides, including earthquakes in areas where rapid sediment deposition creates vulnerable slopes (Bryn et al. 2005) and over-steepening of slopes due to localized tectonic

uplift (Hornbach et al. 2007). Although gas hydrate dissociation may not trigger major submarine slides, it might contribute to the slide’s shape. In the Storegga slide scar, for example, the slide headwall coincides with the location at which gas hydrates are thought to have been susceptible to environmental change about 8 200 years ago, when the slide occurred, (Mienert et al. 2005).

Schematic of a submarine slide triggered by gas hydrate dissociation

Gas plume

Figure TB-2.2: Schematic of a submarine slide triggered by gas hydrate dissociation. In theory, gas pressure generated by methane released during gas hydrate dissociation weakens the sediment and provides a glide plane for sediment failure. In practice, gas hydrates are rarely located at sites where slides initially fail, nor are gas hydrates generally distributed uniformly over large enough spatial scales or in the proper orientation to provide effective glide planes for submarine slides. (Figure modified from McIver (1982)).

Slide

Hydrate-bearing sediment

Layer of dissociated gas hydrate

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 41