60 / 78
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).