FROZEN HEAT | Volume 1

2.3 A GAS HYDRATE CAPACITOR IN THE GLOBAL CARBON CYCLE?

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.

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

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).

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

An alternative gas hydrate breakdown mechanism, in which the topmost gas hydrates dissolve in response to sea-floor warming, has

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