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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
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Gas plume
Hydrate-bearing
sediment
Layer of dissociated
gas hydrate
Slide
Schematic of a submarine slide triggered
by gas hydrate dissociation
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)).
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).
sink, sequestering methane as the hydrate forms, or act as a
source, releasing methane as the hydrate breaks down.
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
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).