Degradation of Subsea Permafrost and Associated Gas Hydrates Offshore of Alaska in Response to Climate Change
Much of the shallow Arctic seafloor just off Alaska's North Slope was formerly subaerial tundra that was flooded during rapid sea-level rise beginning 15,000 years ago. In August 2010, U.S. Geological Survey (USGS) scientists collected data from part of this area—Harrison Bay in the U.S. Beaufort Sea—to study the degradation of subsea permafrost and associated gas hydrate in response to climate change. The survey was the first by the U.S. Department of the Interior since 1980 to target shallow-water areas of the U.S. Beaufort Sea and also the first systematic USGS survey of Alaskan subsea permafrost.
Gas hydrate—an icelike combination of water and certain gases, but most commonly methane—represents a highly concentrated form of gas and is stable only within a specific range of temperatures and pressures beneath the seafloor and in and beneath permafrost. Globally, gas hydrate sequesters large volumes of methane and thus may have potential as a future energy resource. On the other hand, methane is 20 times more potent than CO2 as a greenhouse gas, and a large release of methane from dissociating gas hydrates could exacerbate global warming.
On the central Alaskan North Slope and in the nearshore environment, gas hydrate could occur in some strata within a zone ranging from 400 to more than 1,000 meters in thickness, starting at depths as shallow as 200 to 250 m. An estimated 3x1013 m3 of methane (measured at standard pressure and temperature) may be sequestered within gas hydrates in sands in permafrost areas. The deep conventional gas reservoirs that supply some of the source gas for gas hydrates currently sequestered in sedimentary deposits on the Alaskan North Slope and in shallow offshore regions also contain significant CO2. This CO2 could be reintroduced to the atmosphere through climate-induced gas-hydrate dissociation.
The August 2010 study focused on the Harrison Bay area of the U.S. Beaufort Sea inner shelf (the part of the continental shelf closest to shore) and was conducted from the research vessel (R/V) 1273 of the Bureau of Ocean Energy, Management, Regulation and Enforcement (formerly the Minerals Management Service). The scientists collected approximately 185 km of high-resolution geophysical imagery of the water column, the seafloor, and the upper 100 m or more of the sediments beneath the seafloor. This research is one component of a multiyear effort by the USGS Gas Hydrates Project to study the impact of late Pleistocene to contemporary climate change on circum-Arctic gas-hydrate deposits, both offshore and onshore. (See related Sound Waves article "Studying the Link Between Arctic Methane Seeps and Degassing Methane Hydrates.")
Arctic regions are undergoing rapid contemporary warming, but the overall warming trend in the Arctic began at the end of the Last Glacial Maximum (approx 19,000 years ago), and sea level began rising sharply about 15,000 years ago, with the onset of major melting in the southern Hemisphere. Much of the shallow Arctic shelf at water depths between approximately 10 and 100 m was formerly subaerial tundra that was flooded during rapid sea-level rise before 3,000 years ago. At water depths less than 10 m, such as those in most of the Harrison Bay survey area, the inundation may be more recent. The thick permafrost underlying the flooded tundra experiences thawing because the temperature of the overlying water is higher than the average surface temperature when the tundra was subaerial. Such thawing processes have been recognized in other parts of the circum-Arctic but have never been studied on a regional basis in the U.S. Arctic Ocean. The thawing of subsea permafrost leads to enhanced methane emissions due to microbial degradation of newly released organic carbon that had previously been trapped in permafrost, and possibly due to methane hydrate degassing. (For more information, see recent studies by Natalia Shakhova and others in the Journal of Geophysical Research, http://dx.doi.org/10.1029/2009JC005602, and Science, http://dx.doi.org/10.1126/science.1182221.)
Before the August 2010 cruise, the USGS Gas Hydrates Project re-evaluated seismic-refraction data acquired on the shallow (less than 25-m water depth) Beaufort Sea shelf by Western Geophysical in the late 1970s (see map). The "pre-stack" unprocessed field data are available to the USGS through a licensing agreement; the "post-stack" processed data can be freely accessed through the National Archive of Marine Seismic Surveys. Harrison Bay was chosen as the initial focus area for the re-analysis because an older study by the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL Report 82-24) had used the same data to map subsea permafrost there. The new analysis challenges the older interpretation of permafrost distribution in western Harrison Bay and reveals deepening of the top of permafrost to more than 300 m beneath the seafloor in the eastern part of Harrison Bay. Because permafrost onshore near Harrison Bay may reach thicknesses of 400 m, we infer that much of the permafrost now in the bay has probably thawed within less than 10 km offshore. The data pattern interpreted as permafrost on seismic-refraction records abruptly disappears within central Harrison Bay at a location mapped as the apparent seaward extent of the subsea permafrost. Numerical models predict gas-hydrate dissociation at this location.
The August 2010 cruise used low-energy sound sources (such as the Edgetech 424 Chirp and mini-sparker) and fishfinder sonar to (a) track the top of subsea permafrost from the shallowest nearshore waters of Harrison Bay to several kilometers offshore, (b) map the distribution of gas in the shallow subseafloor, and (c) image seafloor features associated with ice scouring and possible methane emissions. The 200-kHz fishfinder mode was used to target possible methane plumes in the water column, and data obtained during a single sonobuoy deployment will provide constraints on sound velocities in subseafloor layers, required for interpretation of the seismic data. The low-energy seismic sources chosen for the 2010 cruise were intended to provide data of overlapping vertical resolution and depth of penetration, not to reproduce the results of the earlier Western Geophysical surveys. Important criteria in choosing instrumentation for the 2010 cruise were portability, compact size, low power demand, and manageable regulatory and permitting requirements, particularly with respect to the Marine Mammal Protection Act and the Endangered Species Act.
Shown below are sample data obtained in August 2010. Sound energy from the mini-sparker source penetrated more than 100 m below the seafloor in some places, and the data reveal the presence of three prominent seaward-dipping reflectors in the example shown here (labeled "B"). The Chirp data vary in quality depending on the lithology and sea state; the section reproduced here (labeled "A") shows thinly laminated sediments draping gas-charged sediments approximately 3 m below the seafloor. This gas may originate through microbial degradation of organic matter in sediments that formerly hosted permafrost, with the possible addition of some methane that has migrated from deeper in the sedimentary section. Further processing will be required to generate images of the seafloor from the fishfinder sonar; the map view labeled "C" shows one example of a seafloor feature of unknown origin.
To date, an estimated 5 percent of the Western Geophysical seismic data for the U.S. Beaufort Sea has been re-analyzed. Weather and time limitations permitted us to reoccupy only about 50 percent of the legacy seismic lines in Harrison Bay during the summer 2010 cruise. In October, National Research Council/Department of Energy Methane Hydrates postdoctoral fellow Laura Brothers joined the USGS in Woods Hole, Massachusetts, to commence re-analysis of the most promising of the remaining legacy refraction data. In summer 2011, reconnaissance surveys will likely target degradation of subsea permafrost and associated gas hydrate both westward and eastward (toward Prudhoe Bay) from Harrison Bay. During that research, we will also be able to continuously measure methane concentrations in seawater to map methane hotspots. Eventually, the USGS Gas Hydrates project intends to extend climate-hydrate studies farther offshore to the Beaufort Sea continental slope, where the upper edge of the deepwater gas-hydrate system could be dissociating and possibly exacerbating slope failures as Arctic Ocean temperatures and sea-ice conditions change in response to global warming.
Partial support for this research was provided by interagency agreements DE-FE0002911 and DE-AI26-05NT42496 between the USGS and the U.S. Department of Energy's National Methane Hydrates R&D Program. We thank the Bureau of Ocean Energy, Management, Regulation and Enforcement and C. Coon for arranging our use of the R/V 1273; G. Lawley from Kinnetics Laboratories, Inc., for serving as captain for the cruise; and the National Marine Fisheries Service and the U.S. Fish and Wildlife Service for assistance with permitting and compliance. This research was made possible through the advice and support of the Native Village of Nuiqsut, native observer E. Nugapigak, and several North Slope Borough officials, particularly T. Hepa and W. Williams. T. Collett (USGS, Denver, Colorado) and B. Jones (USGS, Anchorage, Alaska) were generous in sharing knowledge and advice that contributed to the project's success.
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