Degrading permafrost and gas hydrate under the Beaufort Shelf and marine gas hydrate on the adjacent continental slope: Geothermal and porous media setting

The sub-seafloor under the Arctic Shelf is arguably the part of the Earth that is undergoing the most dramatic warming. In the southern Beaufort Sea, the shelf area was terrestrially exposed during much of the Quaternary period when sea level was up to 120m lower than present. As a consequence, many areas are underlain by more than 600m of ice bonded permafrost that conditions the geothermal regime such that the base of the methane hydrate stability can be more than 1000m deep. Marine transgression has imposed a change in mean annual surface temperature from -15°C or lower during periods of terrestrial exposure, to mean annual sea bottom temperatures near 0°C. Decomposition of gas hydrate is inferred to be occurring at the base and the top of the gas hydrate stability zone. As gas hydrate and permafrost intervals degrade, a range of processes occurs that are somewhat unique to this setting. Decomposition of gas hydrate at depth can cause sediment weakening, generate excess pore water pressure, and form free gas. Similarly, thawing permafrost can cause thaw consolidation and liberate trapped gas bubbles within ice bonded permafrost. Understanding the connection between these deep subsurface processes generated by transgression, surficial sediment processes near the seafloor, and gas flux into the ocean and atmosphere is important to assessing geohazard and environmental conditions in this setting. In contrast, conditions for marine gas hydrate formation occur on the adjacent continental slope where water depths are greater than ~270m. In this paper we present a simple two dimensional geothermal model of the shelf during the past 100,000 years to provide a first order assessment of the response of permafrost and gas hydrate intervals to transgression. A complimentary paper by Paull et al. reviews recent geoscience studies on the shelf and slope. Our geothermal modelling utilizes a wealth of knowledge available from the terrestrial sites in the Mackenzie Delta to prescribe the subsurface geology and geothermal properties. We then apply global sea level estimates to designate periods with mean annual surface temperatures associated with terrestrial or marine conditions. Offshore permafrost is observed to warm substantially as a result of transgression but be maintained offshore beneath shelf until it pinches out in approximately 100m water depth. Significant change in the top and the bottom of the gas hydrate stability field is predicted creating a complex porous media environment where a cap of overlying permafrost may act as a permeability barrier or seal above degrading gas hydrate intervals. Industry drilling and seismic data support the occurrence of ice bonded permafrost at depth and intervals of sub-permafrost gas hydrate. Observation of over pressure have been made within the permafrost interval which are consistent with establishing an interconnected system with preferred fluid and gas migration pathways laterally towards the shelf edge where permafrost pinches out laterally and gas hydrates are not stable. Given present bottom water temperatures on the shelf marine gas hydrate is only stable below ~270m water depth.