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FROZEN HEAT

60

Changes in thickness of the GHSZ

caused by temperature increase

0

-10

10 20 30

-20

-30

Metres

Source: redrawnfrom Biastoch, A.,

etal

., Rising Arctic Ocean

temperatures cause gas hydrate destabilization and ocean acidi cation

400 metres isobath

Arctic Ocean

Laptev

Sea

European

Nordic Sea

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

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).