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Imaging Methane Seeps and Plumes on the U.S. Atlantic Margin
Scientists from the U.S. Geological Survey (USGS) Gas Hydrates Project surveyed methane seeps and plumes on the northern part of the U.S. Atlantic margin aboard the research vessel (R/V) Endeavor in April 2015. The researchers collected high-resolution seismic data (cross-sectional views of sediment layers and other features beneath the seafloor) along ship tracks totaling nearly 580 kilometers (360 miles), in addition to continuous imagery of methane plumes in the water column above seafloor cold seeps. They also measured the flux of methane and carbon dioxide (CO2) from the ocean to the atmosphere.
In 2014, a Nature Geoscience paper coauthored by USGS scientists Carolyn Ruppel and Daniel Brothers described hundreds of newly identified methane seeps in the area later visited by the 2015 R/V Endeavor cruise. The goal of the 2015 cruise was to study the connection of these seeps to methane gas and methane hydrate in the underlying sediments. Methane hydrate is a frozen form of gas and water (“methane ice”) stable at the pressure and temperature conditions that prevail on the U.S. Atlantic margin in sediments below at least 505 to 550 meters (1,650–1,800 feet) of water. The Bureau of Ocean Energy Management (BOEM) estimates that as much methane is trapped in gas hydrates in sediments on the U.S. Atlantic margin as in the northern Gulf of Mexico, which is known as a world-class petroleum basin.
Many of the seeps first described in 2014 are at upper continental slope depths (200–550 meters, or 650–1,800 feet). The prevailing hypothesis is that these seeps may be leaking methane that is released when gas hydrate breaks down due to warming ocean temperatures (see “Methane Hydrates and Contemporary Climate Change”). A 2014 study led by Daniel Brothers mapped more than 5,000 pockmarks—shallow seafloor depressions thought to be mostly related to past methane expulsion events—on the upper slope and outer continental shelf (depths as much as 180 meters, or 590 feet) on the northern U.S. Atlantic margin. That study also showed that seismic data from the New York segment of the margin might be consistent with ongoing breakdown of gas hydrate in response to decades-long ocean warming.
On the April 2015 R/V Endeavor cruise, researchers collected high-resolution 72-channel multichannel seismic (MCS) data of unprecedented quality for this USGS instrumentation. A sparker towed behind the ship provided the source of the seismic (sound) energy. A sparker creates electrical pulses that emit sound that penetrates the seafloor and is reflected from boundaries between layers that have contrasting physical properties. The reflected sound is recorded by a 450-meter (about 1,500 foot)-long streamer of 72 hydrophones (or receivers). A sparker is less powerful than an airgun source, but allows scientists to image features as much as several hundred meters (approximately 1,000 feet or more) below the seafloor at a resolution of less than a meter.
The seismic data acquisition was supported by Thomas O’Brien, Emile Bergeron, Charles Worley, and Alex Nichols from the USGS Woods Hole Coastal and Marine Science Center in Woods Hole, Massachusetts. Processing of the seismic data was completed shipboard under the direction of USGS scientists Jared Kluesner and William Danforth, with assistance from University of California-Santa Cruz graduate student Joel Edwards. Also participating in the cruise was Massachusetts Institute of Technology (MIT) graduate student Xiaojing (Ruby) Fu, who does numerical modeling with MIT Associate Professor Ruben Juanes in support of an MIT-USGS-University of New Hampshire (UNH) study funded by the U.S. Department of Energy (DOE) to examine the fate of methane plumes in the water column (“The National Methane Hydrates R&D Program: DOE/NETL Methane Hydrate Projects”). Carolyn Ruppel was the Chief Scientist on the cruise, which was supported by a USGS-DOE Interagency Agreement.
Michael Casso and Thomas (Wally) Brooks (USGS) led shipboard geochemical measurements, acquiring data to constrain the flux of methane from the ocean to the atmosphere. Over the past 5 years, USGS scientists John Pohlman, Emile Bergeron, and Casso have combined two cavity ring-down spectrometers with other USGS-designed peripherals to permit continuous measurement of methane and CO2 concentrations in near-surface waters and in the air right above the sea surface (“Real-Time Mapping of Seawater and Atmospheric Methane Concentrations Offshore of Alaska's North Slope”). When these data are combined with other information, the sea-air flux of methane, a potent greenhouse gas, can be determined. The instrumentation has been deployed by the USGS Gas Hydrates Project in the U.S. Arctic (see page 7 of “Iġnik Sikumi Gas Hydrate Field Trial Completed,” PDF, 2.7 MB) and on the Svalbard margin (“Atmospheric methane emissions coupled to a CO2 sink at an Arctic shelf seep area offshore NW Svalbard: Introducing the ‘Seep-Fertilization Hypothesis’,” PDF, 49 KB). The R/V Endeavor cruise added new data over the northern Atlantic margin methane seep province at water depths of approximately100 to 1,400 meters (330–4,600 feet). The data provide an important test of the hypothesis that methane seeps at greater water depths contribute little or no methane directly to the atmosphere, regardless of the amount of methane they emit.
The cruise also marked the first deployment of a water-column imaging system recently acquired by the USGS Gas Hydrates Project. The discovery of U.S. Atlantic margin methane seeps described in the 2014 paper relied on imaging methane plumes in the water column using a multibeam sonar system that is more typically applied to map seafloor bathymetry. The instruments best suited to mapping water-column methane plumes are those that are normally used to locate fish. The Gas Hydrates Project has previously adapted a simple recreational fishfinder for imaging methane plumes in freshwater lakes (“Studying the Link Between Arctic Methane Seeps and Degassing Methane Hydrates”). For the R/V Endeavor cruise, the USGS used a special 38-kHz transducer (transmitter) and fisheries transceiver (receiver) to continuously map deepwater methane plumes, as previously done by collaborator Thomas Weber (UNH). Michael Jech, Joseph Godlewski, and their coworkers at the National Oceanic and Atmospheric Administration (NOAA) Northeast Fisheries Science Center in Woods Hole assisted the USGS with system calibration prior to the ship’s departure. The data collected during the cruise showed that some previously mapped seeps were not emitting methane when the ship crossed over and also revealed new methane seeps, particularly at the edge of the continental shelf.
The data from the R/V Endeavor cruise were used for planning deep submergence vehicle (DSV) Alvin dives at some of the seeps on a cruise led by Cindy Van Dover (Duke University) in July 2015. The new data may also inform studies of seeps near Baltimore Canyon and Norfolk Canyon by USGS ecologist Amanda Demopoulos (“Benthic Community Structure at Newly Investigated Hydrocarbon Seeps on the Continental Slope of the Western North Atlantic”) and USGS oceanographer Nancy Prouty (“Biologic Indicators of Seabed Methane Venting Along the US Mid-Atlantic Margin”). The new results are also being used to choose sites for piston coring, heat flow, and biogeochemical studies to be conducted in September 2015 as part of a DOE-funded cruise led by the USGS Gas Hydrates Project, with collaboration from Frederick Colwell (Oregon State University), Tina Treude (University of California, Los Angeles), and Matt Hornbach (Southern Methodist University). Since the newly acquired data constrain gas hydrate distributions in an area not previously surveyed with modern imaging techniques, the data are also expected to prove useful if future studies of gas hydrate as a potential energy resource shift to the U.S. Atlantic margin.
Learn more about research by the USGS Gas Hydrates Project.
in this issue:
Imaging Methane Seeps and Plumes on the U.S. Atlantic Margin
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