FROZEN HEAT | Volume 1

1- Thick onshore permafrost

2- Shallow

3- Upper edge

4- Deepwater

5- Sea oor

mound/seep

Arctic shelf

of stability

Thermokarst lake

ACTIVE LAYER: aerobic oxidation

ATMOSPHERE: Photolytic CH 4 in around 10 years

oxidation

Permafrost

Upper edge of stability

500m

Gas hydrate stability zone

OCEAN: aerobic (microbial) CH 4 oxidation

NEAR SEA FLOOR SEDIMENTS: Anaerobic CH 4 oxidation

Methane population sources: Young microbial gas

Old microbial gas

Seep

Gas hydrate stability zone

Thermogenic gas

Released from gas hydrates (microbial or thermogenic)

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)).

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

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 69