Evidence of methane venting from decomposing permafrost and gas hydrate on the shelf and slope of the Canadian Beaufort Sea
The seafloor under the Arctic Shelf is arguably the part of the Earth that is undergoing the most dramatic change due to global warming (e.g., Paull et al., 2007). In the southern Beaufort Sea, off the north coast of Canada, the shelf area was terrestrially exposed during much of the Quaternary period when sea level was ~120m lower than present. As a consequence, many areas are underlain by >600m of ice-bonded permafrost that conditions the geothermal regime such that the base of the methane hydrate stability can be >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. The thermal disturbance caused by transgression is still influencing the upper one kilometer of subsurface sediments. 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, gaseous gas will form and may generate excess pore water pressure. The fate of this gas and in particular whether it is escaping from the seafloor is at issue.
In the fall of 2010 a collaborative research cruise was conducted to the Beaufort Sea on the Canadian Coast Guard Ice Breaker Sir Wilfred Laurier to investigate sites where methane venting possibly associated with decomposing gas hydrate and/or permafrost was inferred to occur. During this expedition a small ROV was used to document the nature of these seafloor gas vents. Here are videos clips that illustrate two different styles of methane venting which were observed on this expedition (Paull et al., 2010).
The video clip A (below) shows methane venting at a site in the middle of the Arctic shelf. In this image, bubble releases are migrating along the seafloor, apparently following the tip of a propagating crack in the seafloor. A cloud of sediment is created by the vigorous gas venting. Such vigorous venting is commonly observed at discrete geomorphic features known as Pingo-Like-Features (Paull et al., 2007).
The video clip B (below) shows methane bubbles intermittently emanating from several small holes that are rimmed with white bacterial mats. This style of venting of methane is occurring over a large area along the edge of the continental shelf.
The methane gas emanating from both areas have stable isotopic compositions that indicate they are microbial in origin, are radiocarbon dead and thus sourced from geologic deposits. The methane vents at the Pingo-Like-Features are believed to be sourced from the top of the gas hydrate stability field, while the gas emanating along the shelf edge can be from either permafrost or gas hydrate decomposition. Gas venting from both these shelf edge environments is consistent with heating associated with the last transgression.
This research has been advanced in collaboration with scientists from the Geological Survey of Canada, Fisheries and Oceans Canada and MBARI. Future research cruises will include an expanded research network with participation by the Korean Polar Research Institute.
Paull, C.K., Ussler, W. III, Dallimore, S., Blasco, S., Lorenson, T., Melling, H., McLaughlin, F., and Nixon, F.M., 2007, Origin of pingo-like features on the Beaufort Sea shelf and their possible relationship to decomposing methane gas hydrates, Geophysical Research Letters, 34, L01603, doi:10.1029/2006GL027977.
Paull, C.K., Dallimore, S., Hughes-Clarke, J., Blasco, S. Lundsten, E., Ussler, W. III, Graves, D., Sherman, A., Conway, K., Melling, H., Vagle, S., and Collett, T., 2011, Tracking the decomposition of permafrost and gas hydrate under the shelf and slope of the Beaufort Sea, 7th International Conference on Gas Hydrate, 12 p.
Video clip A - Vigorous Methane Vent from Mid-Shelf Features in the Arctic
Video clip B - Arctic Shelf Edge Methane Venting