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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
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3: Upper continental slope environments
(about 300-500 metres water depth)
Ruppel (2011) calculates that about 3.5 per cent of the world’s
gas hydrates exist in this environment (Zone 3 in Fig. 3.10).
At this feather-edge of stability, bottom-water warming could
destabilize the entire thickness of gas hydrates in the shallow
subsurface. Reagan and Moridis (2007) estimate gas hydrates
in these shallow systems extending nearly 50 metres into the
sediment could dissociate within 100 years. If gas hydrates
have not already begun dissociating along the Arctic slope,
the process could begin within the next century (Biastoch
et al.
2011, Marin-Moreno
et al.
2013) and progress rapidly to lower
latitudes (Reagan
et al.
2011). However, as noted by Ruppel
(2011), methane gas released from sediments at these water
depths would likely be oxidized prior to reaching the atmos-
phere (see also Text Box 3.1 for a discussion of evidence of
climate-related hydrate dissociation in this zone).
4: Deepwater marine environments (greater
than 500 metres water depth)
These gas hydrate deposits (Zone 4 of Fig. 3.10) probably ac-
count for about 95 per cent of all gas hydrates on Earth. In spite
ACTIVE LAYER:
aerobic oxidation
Thermokarst
lake
1- Thick onshore
permafrost
2- Shallow
Arctic shelf
3- Upper edge
4- Deepwater
5- Sea oor
mound/seep
of stability
ATMOSPHERE:
Photolytic CH
4
oxidation
in around 10 years
Permafrost
Gas hydrate stability zone
Methane population sources:
Young microbial gas
Old microbial gas
Thermogenic gas
Released from gas hydrates (microbial or thermogenic)
Gas hydrate
stability zone
Seep
Upper edge
of stability
500m
OCEAN: aerobic (microbial)
CH
4
oxidation
NEAR SEA FLOOR
SEDIMENTS:
Anaerobic
CH
4
oxidation
Figure 3.10:
Pathways for methane release from sediment. This schematic cross-section of a high-latitude ocean margin, broken into five
distinct zones, contains four potential methane sources. Methane is released from sediment along much of the cross-section, but over the
next few hundred years, methane released from gas hydrates, in particular, is likely to reach the atmosphere only from Zone 2, the shallow
Arctic shelf (From Ruppel (2011)).