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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L01603, doi:10.1029/2006GL027977, 2007
Origin of pingo-like features on the Beaufort Sea shelf and their possible relationship to decomposing methane gas hydrates
Charles K. Paull and William Ussler III
Monterey Bay Aquarium Research Institute,
Moss Landing, California, USA
Scott R. Dallimore
Natural Resources Canada,
Sidney, British Columbia, Canada
Steve M. Blasco
Natural Resources Canada,
Dartmouth, Nova Scotia, Canada
Thomas D. Lorenson
U.S. Geological Survey,
Menlo Park, California, USA
Humfrey Melling
Fisheries and Oceans Canada,
Sidney, British Columbia, Canada
Barbara E. Medioli and F. Mark Nixon
Natural Resources Canada,
Ottawa, Ontario, Canada
Fiona A. McLaughlin
Fisheries and Oceans Canada,
Sidney, British Columbia, Canada
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<4> Little is known about the fate of methane released from decomposing gas hydrate under the Arctic Shelf . Our research was carried out in the offshore areas adjacent to the Tuktoyaktuk Peninsula , an area where many distinct ovoid and concentric-shaped positive relief features, referred to as pingo-like-features or PLFs have been identified on the sea floor. Our supposition was that the PLFs might be formed from methane released from decomposing gas hydrates at depth.
<5> Early researchers studying the submarine geology of the Beaufort Shelf used the name PLF to describe bathymetric features similar to terrestrial pingos observed along the coastal plain in this area . Terrestrial pingos are conical, ice-cored hills or mounds commonly 10–40 m in height and 100 m or more in diameter. The coastal plain adjacent to the Beaufort Sea contains over 1350 pingos, most of which are thought to have formed within a closed system as permafrost aggraded into previously thawed lake basins. The positive relief of most terrestrial pingos is almost entirely attributed to expansion associated with ground ice formation . Submarine PLFs are similar in size to the largest terrestrial pingos, rising as shallow as 18 m below the sea surface. Some, but not all, PLFs emanate from within roughly circular, 1–2 km diameter, 10- to 20-m deep bathymetric depressions, referred to here as moats.
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22] Upon warming caused by transgression, dissociation of intra-permafrost gas hydrate would first occur at the top of the methane hydrate stability field at temperatures substantially less than zero degrees Celsius. In the environment where the gas hydrate is dissociating, decomposing gas hydrate, free gas, and freshwater ice co-exist. For liquid water to occur immediately above the gas hydrate stability zone, substantial quantities of salt or other physical-chemical inhibitors are required. The occurrence of freshwater ice in the PLFs argues against the existence of brines in these sediments.
<23> Industry coring has confirmed that at Admirals Finger PLF, high ground ice contents extend to at least 40 m below the surface. With 30% volumetric ice fraction, the freezing of ground water within a gasified sediment fabric can account for approximately 12 m of heave at the sea floor. Because the relief of many PLFs is more than 12 m, additional material movement is needed to satisfy mass balance and the age of the material.
<24> The occurrence of freshwater ice within the shallow sediments sampled within the Beaufort Shelf is unique to the PLFs and not the surrounding sediments. This implies a subsurface source for the freshwater. Decomposition of gas hydrate within the permafrost would leave behind freshwater ice, because bottom water temperature data and thermal models indicate ground temperatures remain below the freezing point for freshwater for hundreds of meters below the seafloor. Thus, freshwater ice may be carried upwards in the extruded sediments. How water migrates into the gas voids and optically clear ice forms in the subsurface remains unclear.
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