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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.
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.