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Studying the Link Between Arctic Methane Seeps and Degassing Methane Hydrates
In spring and summer 2009, scientists with the U.S. Geological Survey (USGS) Gas Hydrates Project studied a methane seep in a lake near Alaska's Arctic coast, in part to determine whether the methane is coming from destabilization of gas hydrate—an ice-like crystalline solid formed from a mixture of water and natural gas, most commonly methane. Gas hydrates are stable at moderate pressures and low temperatures and are widespread in:
Globally, gas hydrate sequesters huge amounts of methane, which is known to be a far more potent greenhouse gas than CO2. Climate perturbations could destabilize gas hydrate deposits and potentially release substantial amounts of methane to the atmosphere.
The USGS Gas Hydrates Project's climate-related research focuses on assessing the contribution of degassing methane hydrates to contemporary atmospheric methane concentrations and evaluating the vulnerability of gas-hydrate deposits to Holocene, present, and future climate perturbations. For at least the past 10,000 years, terrestrial and shallow offshore Arctic regions have undergone particularly rapid climate change. We are therefore targeting permafrost-associated gas hydrates to advance research on climate-hydrate interactions.
With sponsorship from the U.S. Department of Energy (DOE) National Methane Hydrates R&D Program and in collaboration with the University of Alaska, Fairbanks (UAF), our 2009 program has focused on Lake Qalluuraq, a thermokarst lake located in continuous permafrost approximately 60 mi south of Barrow, Alaska, near the Inupiat village of Atqasuk. Thermokarst lakes form in shallow depressions and are filled by meltwater from thawing permafrost. Lake Qalluuraq lies close to the thinnest part of the Alaskan North Slope gas-hydrate stability field, as inferred by USGS scientist Timothy Collett partly on the basis of deep borehole temperature data acquired by USGS geothermal investigator Art Lachenbruch. Ebullition (rapid release of gas) at a seep in Lake Qalluuraq is daily emitting nearly 140 m3 of methane to the atmosphere, according to flux measurements completed by UAF researcher K. Walter Anthony. Seep methane sources in this area may include conventional deep-seated thermogenic gas (such as that associated with oil deposits), coalbed methane, dissociating gas hydrates near the top of the gas-hydrate stability zone, or microbial methane originating from decay of organic matter in the lake's thaw bulb (zone of thawed ground below the lake). As part of this DOE-sponsored project, we are working with Andrew Hunt (USGS, Denver, Colorado) to develop a chemical fingerprinting method that could distinguish hydrate-derived gases from other gas sources contributing to seeps.
A major component of the fieldwork was designed to document how geochemical and microbial cycles within the lake sediments and water column are influenced by profound seasonal changes in the lake's redox state: The lake is anoxic (lacking dissolved oxygen) in winter and fully saturated with oxygen in summer. In spring 2009, USGS geochemist John Pohlman and colleagues from UAF conducted operations from the lake's ice cover and retrieved short (less than 1 m long) percussion cores of lake-bottom sediments along a transect from the center of the seep to an area unaffected by seep activity. The sites were re-sampled during ice-free conditions in summer 2009. The USGS is responsible for conducting analyses of pore-water chemistry, gas composition, and sedimentary microbial biomarkers. The data will be used in conjunction with complementary analyses performed by UAF to delineate gas sources and the sedimentary biogeochemical cycles that influence contemporary and historical methane emissions from thermokarst lakes. Links between sedimentary cycles and the overlying water column are being addressed by Monica Heintz (University of California, Santa Barbara), who is measuring the rates at which microbes in the water column oxidize methane; microbial oxidation is the most critical sink for methane emitted into water bodies.
During the summer 2009 fieldwork, the USGS also carried out extensive geophysical characterization of Lake Qalluuraq to constrain thaw-bulb thickness, the locations of lakebed pockmarks and other seep-related features, the distribution of gas-charged sediments, and the deeper structure of sediments beneath the ebullition site. The remoteness of Lake Qalluuraq and its shallow water depths (2 m maximum) posed challenges, but Carolyn Ruppel and Charles Worley successfully acquired Chirp seismic-reflection data with an Edgetech 424 fish towed at the lake's surface and operated at 4 to 24 kHz. They also rigged a Mala Geoscience 50-MHz ground-penetrating-radar remote-terrain antenna for water towing. The acoustic energy emitted by the Chirp system penetrates the lakebed and reflects off boundaries according to their acoustic properties, ultimately producing an image that captures sediment and gas distribution. (For more information about Chirp and other seismic-reflection systems, see WHSC Seismic Profiling systems). Ground-penetrating radar (GPR) employs electromagnetic waves to image sediment boundaries below the lakebed on the basis of their dielectric properties. Seismic and GPR imagery can provide complementary information about sediment stratigraphy and about gas distribution and migration patterns.
Geophysical surveys also included continuous resistivity profiling, which detects vertical and horizontal variations in electrical conductivity. Frozen sediments generally have much higher resistivity than unfrozen sediments. Thus, resistivity profiles can be used to constrain the thickness of the thawed permafrost beneath the lake. Additional surveys used a simple fishfinder equipped with an 83-kHz transducer to locate water-column methane plumes and with a sidescan-sonar to image lake-bottom pockmarks.
Reconnaissance Chirp seismic-reflection data were also acquired at lakes in the nearby Teshekpuk Lake area of the North Slope to support research being conducted by geographer Benjamin Jones of the Alaska Science Center (see "Erosion Doubles Along Part of Alaska's Arctic Coast—Cultural and Historical Sites Lost," Sound Waves, May 2009).
The next phase of the project involves October 2009 aerial photographic surveys to identify lake-based seeps between Teshekpuk Lake on the west and Milne Point on the east. Photographer Dann Blackwood will participate in these surveys, which are timed to capture imagery of just-frozen lake surfaces and as-yet unfrozen seeps within the lakes using an approach pioneered by K. Walter Anthony. The Milne Point area is of particular interest for this study, owing to the availability of data on the chemical composition of deep methane-hydrate samples recovered during 2007 drilling by DOE and British Petroleum, with significant involvement from the USGS Gas Hydrates Project. Starting in spring 2010, the coring scientists in our group will sample sediments from newly identified lake-based seeps in the Prudhoe Bay area. They also plan to revisit Lake Qalluuraq in order to document interannual variations in methane dynamics.
Additional details about the 2009 fieldwork are available in the article "Permafrost Gas Hydrates and Climate Change: Lake-Based Seep Studies on the Alaskan North Slope" in the summer 2009 issue of the National Methane Hydrates R&D Program newsletter, Fire in the Ice.
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