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Fieldwork

Imaging Methane Seeps and Plumes on the U.S. Atlantic Margin



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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.

Map shows newly discovered methane seeps on the northern U.S. Atlantic margin
Above: Large map shows newly discovered methane seeps on the northern U.S. Atlantic margin (white circles) described in a 2014 Nature Geoscience paper. Red box indicates area of inset map. Green dot shows locations of Norfolk Canyon deepwater seeps depicted in close-up perspective view below. Inset: The upper continental slope between Baltimore Canyon on the north and Washington Canyon on the south hosts more than 200 methane seeps and was a particular focus of the R/V Endeavor cruise. Red dots are seeps found with geophysical methods; small yellow dots are upper slope pockmarks mapped by Daniel Brothers. White lines show locations of some of the newly acquired multichannel seismic (MCS) data. Orange circle highlights the seeps overlooking Baltimore Canyon. Amanda Demopoulos and Nancy Prouty have analyzed data obtained during remotely operated vehicle exploration of nearby seeps. [larger version]

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.

Three-dimensional perspective view of deepwater seeps south of Norfolk Canyon
Above: Three-dimensional perspective view of deepwater seeps south of Norfolk Canyon on the northern U.S. Atlantic margin. Image looks westward toward the shelf edge at approximately 150 meters (490 feet) water depth. The bathymetric data were published in USGS Open File Report 2012–1266 led by Brian Andrews. The seafloor seeps that produce methane plumes up to 900 meters (2,950 feet) high in the water column lie at a water depth of 1,500 meters (4,900 feet) and are called the “Norfolk seeps” by researchers. They lie on two lines forming an oblique angle and may originate with methane leaking from fractured Eocene rock. The methane plumes were recorded with a fisheries-type water-column imaging system operated at 38 kHz. The multichannel seismic (MCS) data in the cutaway show coherent reflections to at least 300 meters (985 feet) below the seafloor. Blue features on the seismic image show gas-migration pathways beneath the seafloor determined through specialized analyses. Seismic processing and analysis by Jared Kluesner (USGS); figure prepared by William Danforth (USGS). Inset: Close-up of area enclosed by black box in top image, from a perspective slightly to the southwest of the top view. [larger version]

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.

Researcher moves a monitor that contains electronics used to position the seismic streamer at the correct depth and angle
Above: Alex Nichols (USGS) moves a “bird” that contains electronics used to position the seismic streamer (blue cable on reel on the right) at the correct depth and angle in the water column when the streamer is being towed behind the ship and receiving sound from the sparker source. [larger version]

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.


Related Sound Waves Stories
Natural Methane Seepage Is Widespread on the U.S. Atlantic Ocean Margin
September / October 2014
Seismic-Imaging Research Cruise Investigates Deepwater Gas Hydrate Deposits in the Gulf of Mexico
July / Aug. 2013
Real-Time Mapping of Seawater and Atmospheric Methane Concentrations Offshore of Alaska's North Slope
May / June 2012
Gas Hydrates and Climate Warming—Why a Methane Catastrophe Is Unlikely
May / June 2012
Studying the Link Between Arctic Methane Seeps and Degassing Methane Hydrates
October 2009

Related Websites
Gas Hydrates Primer: What is Gas Hydrate?
USGS
Assessment of In-Place Gas Hydrate Resources of the Lower 48 United States Outer Continental Shelf (PDF, 754 KB)
BOEM
Methane Hydrates and Contemporary Climate Change
Nature.com
Seabed fluid expulsion along the upper slope and outer shelf of the U.S. Atlantic continental margin
AGU
The National Methane Hydrates R&D Program: DOE/NETL Methane Hydrate Projects
National Energy Technology Laboratory
Iġnik Sikumi Gas Hydrate Field Trial Completed (PDF, 2.7 MB)
National Energy Technology Laboratory
Atmospheric methane emissions coupled to a CO2 sink at an Arctic shelf seep area offshore NW Svalbard (PDF, 49 KB)
European Geosciences Union
Benthic Community Structure at Newly Investigated Hydrocarbon Seeps on the Continental Slope of the Western North Atlantic
2014 Ocean Sciences Meeting
Biologic Indicators of Seabed Methane Venting Along the US Mid-Atlantic Margin
AGU Fall Meeting
Gas Hydrates Project: Energy Studies
USGS
Gas Hydrates Project
USGS

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in this issue:

Fieldwork
Scientists Investigate the Virtually Unexplored Mariana Trench

Expedition Explores Deep-Sea Areas near Puerto Rico, U.S. Virgin Islands

Imaging Methane Seeps and Plumes on the U.S. Atlantic Margin

Research
Some Communities May Not Have Time for Tsunami Evacuation

Spotlight on Sandy
Mendenhall Postdoc Joins Estuarine Physical Response Project

Meetings
Spring 2015 Monterey Bay Marine GIS User Group Meeting

Awards
Amanda Demopoulos Receives USGS Leadership Award

Fran Lightsom Receives Leadership and Innovation Award for Data Integration

Jim Hein Receives DOI Distinguished Service Award

Jim Jacobi Receives DOI Distinguished Service Award

Staff
Cheryl Hapke Is New Director of St. Petersburg, Florida, Science Center

Publications New Maps Reveal Seafloor off San Francisco

March–June Publications

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