FROZEN HEAT | Volume 1

5. Gas hydrate mounds on the sea floor In the presence of seeps, gas hydrate mounds can occur in Zones 2 to 5 (see Fig. 3.10). Whether methane frommounds and seeps is being transferred to the atmosphere is a cur- rent topic of debate (Solomon et al. 2009; Hu et al. 2012). The direct exposure of gas hydrate mounds to sea water means they are constantly dissolving, and their breakdown increases with increasing temperature. As with the upper- continental-slope gas hydrates, methane released from mounds will be subject to dissolution and oxidation in the water column. If gas hydrate mounds break apart or dis- lodge from the sediment surface, however, the gas hydrate can rise through the water column and allow methane to reach the mixed layer near the sea surface and enter the atmosphere (Brewer et al. 2002; Paull et al. 2003). As noted by Ruppel (2011), however, mounds represent only a trace fraction of the global gas hydrate reservoir.

of their abundance, they would contribute little to no meth- ane to the ocean, even over a 3 000-year period after impos- ing a 1.25°C bottom-water temperature increase over present conditions (Ruppel 2011). A heat pulse entering the sediment would require millennia to reach the vulnerable gas hydrates at the base of the gas hydrate stability zone. In addition, the methane would likely remain trapped below the GHSZ or be converted back into gas hydrate as it migrated up through the sediment. An exception to this recycling model could occur if over pressuring associated with methane gas release gener- ated highly permeable pathways that facilitated the transit of the gas through the overlying gas hydrate stability zone. Once released from the sea floor at these depths, methane would likely be consumed in the water column prior to reaching the atmosphere (McGinnis et al. 2006). However, as discussed in Chapter 2, bubbles released at these depths could formhydrate shells that would limit the rate at which methane in the bubble dissolved and allow methane to reach shallower depths.

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