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The percentages assigned to each sector below assume that 99% of global gas hydrate is within the deepwater marine realm (Mc Iver 1981, Collett et al. Future refinements of the global ratio of marine to permafrost-associated gas hydrates will require adjustment of the assigned percentages.

Warming Arctic temperatures tracked in deep boreholes since the 1960s provide no evidence for climate perturbations reaching as deep as 200 m (Judge & Majorowicz 1992, Lachenbruch & Marshall 1986) in normal (e.g., not beneath lakes) continuous permafrost.

Some researchers have argued that gas hydrates formed during previous periods of ice/water loading may persist today at subsurface depths as shallow as 20 m in areas of continuous permafrost (Chuvilin et al. Although their existence is controversial, such shallow gas hydrates would clearly be highly susceptible to dissociation in response to climate warming.

Example: In the Mackenzie Delta, where permafrost is several hundreds of meters thick, Bowen et al.

(2008) document seep gas composition similar to that of gas sequestered in nearby gas hydrates.

In marine sediments, the released CH to be injected directly into the water column or, in some cases, partially consumed by aerobic microbes (Niemann et al. Methane emitted at the seafloor only rarely survives the trip through the water column to reach the atmosphere. United States Geological Survey, Open File Report 94-694 (1994).

At seafloor depths greater than ~100 m, O Methane hydrates occur in five geographic settings (or sectors) that must be individually evaluated to determine their susceptibility to warming climate (Figure 1). Gas hydrates: Entrance to a methane age or climate threat?

There is no proof that active gas hydrate dissociation, as opposed to leakage of compositionally-similar free gas, supplies these seeps or other CH ebullition sites that occur throughout the Arctic region (e.g., Walter et al. Such sites should continue to be evaluated for evidence of a potential link to climate-driven gas hydrate dissociation. "The anaerobic oxidation of methane: New insights in microbial ecology and biochemistry," in Ocean Margin Systems, eds.

Sediments on shallow marine continental shelves that fringe the Arctic Ocean are often underlain by permafrost and associated gas hydrates that formed in Pleistocene time, when these regions were subaerial and exposed to much colder annual temperatures.

Changing CH4 solubility in pore waters and endothermic heat of gas hydrate dissociation are not considered. Stable isotope evidence for methane seeps in Neoproterozoic postglacial cap carbonates.

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