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FROZEN HEAT
40
The presence of gas hydrates generally strengthens the host
sediment. When gas hydrates break down into water and
free methane gas, however, what are the consequences for
sediment stability?
When gas hydrate dissociates, the released gas and water occupy
a greater volume than they do in the solid hydrate structure.
This expansion means gas hydrate dissociation in sediment can
increase pressure in the pore space (McIver 1982; Kayen and
Lee 1991; Xu and Germanovich 2006), weakening the sediment
by pushing sediment grains apart. It has been suggested that
dissociated gas hydrate could form a fluid- and gas-rich glide
plane, upon which the overlying sediment might be able to slide
(see Fig. TB-2.2) (McIver 1982). The slide-triggering, gas-hydrate
dissociation might itself be brought on by a pressure decrease
due to an earthquake (Bugge
et al.
1987), a drop in sea level
(Maslin
et al.
2004), or a temperature increase due to rising
bottom-water temperatures (Dickens
et al.
1995).
Gas hydrates have been tied to submarine slides the world
over, including the colossal Storegga slide offshore Norway
(Bugge
et al.
1987), along the western Atlantic Margin (Booth
et al.
1993; Lee 2009), offshore Brunei (Gee
et al.
2007), and
on many other continental slopes around the world. While gas
Box 2.2
Can gas hydrate breakdown trigger submarine slides?
hydrates may have played a role in some isolated slides (Lopez
et
al.
2010), definitive proof of gas hydrate dissociation substantially
contributing to major submarine slides remains elusive, even for
the heavily-studied Storegga slide (Mienert, 2008). There are two
drawbacks to the gas-hydrate triggering mechanism theory:
1. Because sediments are generally permeable (meaning fluid can
flow through them to some extent), gas hydrate dissociation
may simply push fluid and gas away from the dissociation site
without generating any significant pressure increase. Bouriak
et al.
(2000) suggest that, for the Storegga slide, gas hydrate
dissociation would only have increased the pore pressure by 0.2
per cent, not enough to trigger a slide.
2. The distribution of gas hydrates seldom coincides with the initial
slide failure location or the glide plane along which the sediment
subsequently slides. The Storegga slide, for example, began at the
toe of the slide (Kvalstad
et al.
2005). Gas hydrates were likely
to be dissociating in much shallower water, landward toward the
slide headwall (Mienert
et al.
2005). Moreover, the non-uniform
distribution of gas hydrates does not coincide with the slide
surface, so gas hydrate dissociation did not provide a glide plane
for the Storegga slide (Bryn
et al.
2005; Kvalstad
et al.
2005).
An alternative gas hydrate breakdown mechanism, in which the
topmost gas hydrates dissolve in response to sea-floor warming, has
Methane is a dynamic component not just of the sub-sea-floor
environment, but the global environment as a whole. The role
of gas hydrate in the movement of methane through the global
carbon cycle can be visualized by characterizing the gas hy-
drates as a methane “capacitor” (Dickens 2001; Dickens 2003;
Dickens 2011). Like a capacitor in an electrical network, gas
hydrates can become charged with methane over time and also
discharge, releasing a significant quantity of methane.
2.3
A GAS HYDRATE CAPACITOR IN
THE GLOBAL CARBON CYCLE?
Under steady-state conditions, methane slowly enters the
gas hydrate stability zone through organic carbon degrada-
tion, methane production, and methane migration. Meth-
ane slowly leaves this volume through gas hydrate dissocia-
tion, gas hydrate dissolution, AOM, venting, and burial. If
methane inputs to gas hydrate exceed methane outputs,
gas hydrate volumes grow as long as pore water is available;
otherwise, they shrink. Gas hydrates, therefore, can act as a