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Natural Methane Seepage Is Widespread on the U.S. Atlantic Ocean Margin
Natural methane leakage from the seafloor is far more widespread on the U.S. Atlantic margin than previously thought, according to an August 2014 study published in Nature Geoscience (“Widespread methane leakage from the sea floor on the northern US Atlantic margin”) by researchers from Mississippi State University, the U.S. Geological Survey (USGS), and other institutions.
Methane plumes identified in the water column between Cape Hatteras, North Carolina, and Georges Bank, Massachusetts, are emanating from at least 570 seafloor cold seeps on the outermost continental shelf and the continental slope (see map). Taken together, these areas, which lie between the coastline and the deep ocean, constitute the continental margin.
Prior to 2012, only three seep areas had been identified beyond the edge of the continental shelf, which occurs at approximately 180 meters (590 feet) water depth between Florida and Maine on the U.S. Atlantic seafloor. Two of the three previously known seep areas occur on the southeastern U.S. continental margin above salt diapirs, which are columns of buoyant salt that rise through the sediments. The salt diapirs were first described decades ago by Bill Dillon, now a USGS scientist emeritus. A study led by USGS research geologist Laura Brothers and reported in 2013 in Geology (“Evidence for extensive methane venting on the southeastern U.S. Atlantic margin”) was the first to use modern methods to map methane plumes from these areas. The third previously known seep area is in Baltimore Canyon, where USGS research ecologist Amanda Demopoulos participated in research cruises in 2012 and 2013 (“Benthic community structure at newly investigated hydrocarbon seeps on the continental slope of the western North Atlantic”).
Cold seeps are areas where gases and fluids leak into the overlying water from seafloor sediments. They are designated as “cold” to distinguish them from hydrothermal vents, which are sites where new oceanic crust is being formed and hot fluids are being emitted at the seafloor. Cold seeps can occur in a much broader range of environments than hydrothermal vents.
“Widespread seepage had not been expected on the Atlantic margin. It is not near a plate tectonic boundary like the U.S. Pacific coast, nor associated with a petroleum basin like the northern Gulf of Mexico,” said Adam Skarke, the study’s lead author and an assistant professor at Mississippi State University.
The gas being emitted by the seeps has not yet been sampled and analyzed, but researchers believe that most of the leaking methane is produced by microbial processes in shallow sediments. This interpretation is based primarily on the locations of the seeps and knowledge of the underlying geology. Microbial methane is different from thermal methane, which is found in deep-seated reservoirs and often tapped as a natural gas resource.
Most of the newly discovered methane seeps lie at depths close to or upslope from the shallowest conditions at which deepwater marine gas hydrate can exist on the continental slope. Gas hydrate is a naturally occurring, ice-like combination of methane and water that forms at temperature and pressure conditions commonly found in waters deeper than approximately 500 meters (1,640 feet). (Learn more about gas hydrate.)
“Warming of ocean temperatures on seasonal, decadal, or much longer time scales can cause gas hydrate to release its methane, which may then be emitted at seep sites,” said Carolyn Ruppel, study co-author and chief of the USGS Gas Hydrates Project. “Such continental slope seeps have previously been recognized in the Arctic, but not at mid-latitudes. So this is a first.”
Most seeps described in the new study are too deep for the methane to directly reach the atmosphere (see “Gas Hydrates and Warming—Why a Methane Catastrophe is Unlikely,” Sound Waves, May/June 2012, ), but the methane that remains in the water column can be oxidized to carbon dioxide. This in turn increases the acidity of ocean waters and reduces oxygen levels; both of these effects are deleterious to marine organisms (for example, see “USGS Arctic Ocean Research: A Polar Acidification Study,” Sound Waves, August 2011).
A key question about the newly discovered methane seeps is whether they emanate from any of the nearly 5,500 pockmarks recently mapped on the northern U.S. Atlantic margin by USGS research geophysicist Daniel Brothers, one of the new study’s coauthors, and his USGS colleagues (“Seabed fluid expulsion along the upper slope and outer shelf of the U.S. Atlantic continental margin”). A pockmark is a seafloor depression that can reach 10s of meters across and several meters deep. Pockmarks are believed to form when gas is emitted from the seafloor. The new study carefully compares the margin’s pockmark and seep distributions, both of which have been mapped at high resolution. The conclusion is that the pockmarks are not the sites of active gas emission and may therefore have been produced during an older episode of gas leakage.
The new seep study provides particular insight into processes in Hudson Canyon, an undersea gorge that represents the offshore extension of the Hudson River. Under the leadership of Brad Butman, the USGS has been involved in a number of studies of Hudson Canyon and its extension onto the continental shelf (for example, see “Sea Floor Topography and Backscatter Intensity of the Hudson Canyon Region Offshore of New York and New Jersey”). The late Peter Rona, a long-time Rutgers University marine geologist, had for years conducted surveys in the canyon seeking seafloor cold seeps that might explain methane values observed in the water column. The new seep study for the first time identified at least 50 water-column plumes in Hudson Canyon. Many of these originate in the canyon’s floor at a depth corresponding to the upslope pressure-temperature limit for gas hydrate stability.
Shallow-water seeps that may be related to offshore groundwater discharge were detected at the edge of the shelf at various places along the margin and in the upper part of Hudson Canyon. Methane emitted at these seeps could directly reach the atmosphere, contributing to increased concentrations of this potent greenhouse gas. More extensive shallow-water surveys than described in this study will be required to document the extent of such seeps and those distributed across the continental shelf.
Some of the methane seeps were discovered in 2012. In summer 2013, a Brown University undergraduate and National Oceanic and Atmospheric Administration (NOAA) Hollings Scholar Mali’o Kodis worked with Skarke to analyze about 94,000 square kilometers (36,000 square miles) of water-column imaging data to map the methane plumes. The data had been collected during research cruises on the vessel Okeanos Explorer between 2011 and 2013. (Read about one of these expeditions in “Exploring Undersea Terrain Off the U.S. Atlantic Coast,” Sound Waves, November/December 2013.) The Okeanos Explorer and the Deep Discoverer remotely operated vehicle, which has photographed the seafloor at some of the methane seeps, are managed by NOAA’s Office of Ocean Exploration and Research.
“This study continues the tradition of advancing U.S. marine science research through partnerships between federal agencies and the involvement of academic researchers,” said John Haines, coordinator of the USGS Coastal and Marine Geology Program. ”NOAA’s Ocean Exploration program acquired state-of-the-art data at the scale of the entire margin, while academic and USGS scientists teamed up to interpret these data in the context of a research problem of global significance.”
The full citation for the recent report is:
USGS Gas Hydrates Project
The USGS has a globally recognized research group studying natural gas hydrates in deepwater and permafrost settings worldwide (http://woodshole.er.usgs.gov/project-pages/hydrates/). USGS researchers focus on the potential of gas hydrates as an energy resource, the impact of climate change on gas hydrates, and seafloor stability issues.
For more information about the newly discovered Atlantic margin methane seeps, visit the USGS Gas Hydrates Project’s website.
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