Estimating gas escape through taliks in relict submarine permafrost and methane hydrate deposits under natural climate variation
Abstract
Permafrost-associated methane hydrate deposits exist at shallow depths within the sediments of the Arctic continental shelves. This icy carbon reservoir is thought to be a relict of cold glacial periods, when sea levels are much lower, and shelf sediments are exposed to freezing air temperatures. During interglacials, rising sea levels flood the shelf, bringing dramatic warming to the permafrost and gas hydrate bearing sediments. Degradation of this shallow-water reservoir has the potential to release large quantities of methane gas directly to the atmosphere. Although relict permafrost-associated gas hydrate deposits likely make up only a small fraction of the global hydrate inventory, they have received a disproportionate amount of attention recently because of their susceptibility to climate change. This study is motivated by several recent field studies which report elevated methane levels in Arctic coastal waters. While these observations are consistent with methane release as a result of decomposing submarine permafrost and gas hydrates, the source of gas cannot easily be distinguished from other possibilities, including the escape of deep thermogenic gas through permeable pathways such as faults, or microbial activity on thawing organic matter within the shelf sediments. In this study, we investigate the response of relict Arctic submarine permafrost and permafrost-associated gas hydrate deposits to warming with a two-dimensional, finite-volume model for two-phase flow of pore fluid and methane gas within Arctic shelf sediments. We track the evolution of temperature, salinity, and pressure fields with prescribed boundary conditions, and account for latent heat of water ice and methane hydrate formation during growth/decay of permafrost or methane hydrate. The permeability structure of the sediments is coupled to changes in permafrost. We assess the role of taliks (unfrozen portions of continuous permafrost) as a pathway for methane gas escape and make predictions of gas flux to the water column as a result of relict permafrost-associated gas hydrate dissociation due to natural climate variations. Several hydrate saturation values (20%, 50%, 80% pore volume within hydrate layers) and talik widths (0.5 km, 1.0 km, 1.5 km, 2.0 km) are explored for model parameters representative of the 20 m isobath at the North American Beaufort and East Siberian Arctic Seas (ESAS). Preliminary results estimate the maximum present-day gas flux at the North American Beaufort is 0.229 kg/yr/m2 (average 0.005 kg/yr/m2), which produces a methane concentration of 75 nM in the overlying water column for a representative ocean current of 4 cm/s. For the ESAS, preliminary results estimate the maximum present-day gas flux is 0.277 kg/yr/m2 (average 0.030 kg/yr/m2), which produces a methane concentration of 452 nM in the overlying water column. A desired outcome of this study is to provide a framework for discussion on the potential magnitude of methane release that might be attributed to relict permafrost-associated hydrate deposits in regions where the submarine permafrost has been compromised.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B33K0605F
- Keywords:
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- 3004 MARINE GEOLOGY AND GEOPHYSICS Gas and hydrate systems;
- 0475 BIOGEOSCIENCES Permafrost;
- cryosphere;
- and high-latitude processes;
- 4820 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL Gases;
- 4219 OCEANOGRAPHY: GENERAL Continental shelf and slope processes