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Changes in thickness of the GHSZ caused by temperature increase

A very different scenario is possible in the shallower settings typical of the upper continental slope. Here, even modest warming can destabilize gas hydrates (Fig. 3.6, right panel). Hydrate dissociation would start at the top of the deposit, and the entire gas hydrate inventory in this setting could theo- retically be transformed into water and methane. If the meth- ane release rates were high enough, methane could escape the sediment’s methane biofilter and be released into the water column (See also Volume 1, Chapter 2). Gas hydrate outcroppings at the upper-slope sea floor might react instan- taneously to sea-floor warming, while gas hydrates situated at greater water and sediment depth would dissociate only after a prolonged heating period of one hundred to several hundred years (Reagan and Moridis 2007; Garg et al. 2008; Reagan and Moridis 2008; Ruppel 2011). The effect on gas hydrate stability of the predicted warming of the Arctic sea floor was estimated by Biastoch et al. (2011). According to their model, the GHSZ thickness will be signifi- cantly reduced at several continental-slope areas due to global warming (Figs. 3.5 and 3.7). The authors proposed that about 10 14 Gt of methane carbon might be released from dissociating gas hydrates deposited in Arctic slopes at greater than 60 °N over the next 100 years, considering the slow penetration of heat into the sediments (Fig. 3.6, right panel). Their gas hy- drate concentration estimates are based on the work of Klauda and Sandler (2005), which are at the high end of the published estimates. If released completely to the atmosphere, even this upper-estimate methane release would be too small to signifi- cantly enhance global warming in a 100-year time span. None- theless, this quantity of methane has the ability to enhance ocean acidification and oxygen depletion along the continental slope (see Volume 1 Chapter 2, Text Box 2.1). It should also be noted that the estimated amount of methane likely to be released remains uncertain, since the methane release rate de- pends on the largely unconstrained distribution and inventory of methane gas hydrates in shallow Arctic slope sediments. 3.5.3 Field evidence for ongoing marine gas-hydrate dissociation Methane release as free-gas venting at the sediment-water interface is observed in many deep-water environments around the world. Some of these active gas seeps are from

Laptev Sea

Arctic Ocean

European Nordic Sea

Metres

-20 -30

-10

0

10 20 30

Source: redrawnfrom Biastoch, A., etal ., Rising Arctic Ocean temperatures cause gas hydrate destabilization and ocean acidi cation 400 metres isobath

environments where pressure and temperature settings are conducive to gas hydrate formation (Ginsburg et al. 1993; MacDonald et al. 1994; Suess et al. 1999; Van Dover et al. 2003; Tomaru et al. 2007). In many cases, it appears this phenomenon is not related to gas hydrate dissociation, but is the result of complex porous-media processes that allow some free gas to pass through the gas hydrate stability zone without forming gas hydrates (Liu and Flemings 2006). Po- tential links between climate change and sea floor methane release due to dissociating marine gas hydrates have been found along the shallow-water-limit of hydrate stability along the upper continental slope, however (Westbrook et al. 2009; Mienert et al. 2010; Berndt et al. 2014; see also Text Box 3.1).

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