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FROZEN HEAT
62
Over 200 methane gas plumes have been observed coming
from the seabed offshore West Svalbard (Fig. TB-3.1.1). There
are plumes on the continental shelf in relatively shallow water
near the shelf edge, on the continental slope in water shallower
than the ~400 m water depth required for methane hydrates to
be stable, and a few plumes have even been observed in deeper
water (Westbrook
et al.
2009; Sarkar
et al.
2012; Rajan
et al.
2012).
From a climate-change perspective, it is of interest to know if 1)
methane in the plumes reaches the atmosphere to directly increase
atmospheric greenhouse gas levels, and 2) if recent climate change
is responsible for creating the methane plumes. It does not appear
methane in these plumes is reaching the atmosphere (Fisher
et
al.
2011), but there is ongoing debate about whether the plumes
are caused by gas hydrates that are dissociating in response to
bottom-water warming over the past ~30 years (Thatcher
et al.
2013; Sarkar
et al.
2012; Rajan
et al.
2012; Berndt
et al.
2014).
Box 3.1
Methane bubble plumes from the sea floor off West Svalbard
Much of the methane feeding into this region appears to be
migrating toward the sea floor from below the gas hydrate stability
zone, further down the continental slope than where most of the
methane flares are found (Hustoft
et al.
2009, Thatcher
et al.
2013;
Sarkar
et al.
2012; Rajan
et al.
2012). As methane migrates upward
through sediment, it can occasionally find flow conduits through
which it can escape vertically through the gas hydrate stability zone
(GHSZ) (Liu and Flemings 2006), but in this region that type of
flare is quite rare (Sarkar
et al.
2012).
More generally, methane either becomes incorporated into the gas
hydrate or migrates up the continental slope through permeable
sediment layers and conduits, some of which reach all the way to
the shelf (Thatcher
et al.
2013; Sarkar
et al.
2012; Rajan
et al.
2012)
(Fig. TB-3.1.2). Of particular interest, however, are the plumes located
just upslope from the current limit of gas hydrate stability (Fig. TB-
Figure TB-3.1.1:
Methane-rich
plumes in the water column on the
West Svalbard continental margin.
A: Location of survey area west of
Svalbard; bathymetry (Jakobsson
et
al.
2008). B: Positionsof acoustically
imaged plumes are depicted
by “pins” superimposed on a
perspective view of the bathymetry of
part of the area of plume occurrence.
The 396-metre isobath contour is the
expected landward limit of the GHSZ.
C: Part of the record from an acoustic
survey showing examples of observed
plumes. All plumes show a deflection
towards the north caused by the West
Svalbard Current. The sea floor, at
around 240-metre depth, is shown by
the strong (red) response (adapted
from Westbrook
et al.
2009).