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article imageMethane gas plumes erupting along Atlantic Coast

By Karen Graham     Aug 25, 2014 in Science
A two-year sweeping survey of the U.S. Atlantic Coast by the National Oceanic and Atmospheric Administration (NOAA) has led to an astounding discovery. From 2011 to 2013, scientists documented 570 methane gas seeps along the continental slope break.
The discovery of the gas bubbling up from the sea floor was reported in the August 24 issue of the journal Nature Geoscience. Researchers say that although the scientists have yet to test the gas, it is undoubtedly methane. "We don't know of any explanation that fits as well as methane," said lead study author Adam Skarke, a geologist at Mississippi State University.
In the study, researchers found that between Cape Hatteras, North Carolina and Massachusetts' Georges Bank, there are 570 methane gas seeps, an impressive number, given that only three such seeps were found in a previous study of the same area.The majority of the seeps were found in eight different areas along the continental slope break, an area where the sea floor's topography slopes down to the Atlantic Basin.
The NOAA ship Okeanos Explorer used sound waves to detect the methane bubbles and in mapping the sea floor. The method is called multibeam-sonar, a means of calculating the time and distance it takes for sound waves to travel from the ship down to the sea floor and back. This technique also can detect the difference between gas bubbles and seawater.
A close-up of methane hydrate observed at a depth of 3 460 feet (1 055 meters) off the U.S. Atlantic...
A close-up of methane hydrate observed at a depth of 3,460 feet (1,055 meters) off the U.S. Atlantic Coast.
Researchers say that at least two-thirds of the seeps were from depths where the methane-rich ices in the sea floor sediment may be decomposing because of warming ocean temperatures. The Atlantic Coast has long been considered a "passive" area where methane seepage was not expected to be present. "I usually describe passive margins as cold, old and boring," said study co-author Carolyn Ruppel, chief of the U.S. Geological Survey gas hydrates project in Woods Hole, Massachusetts.
Missing from the Atlantic Coast margins are layers of salt, such as those seen in the Gulf of Mexico, and are responsible for the Gulf's oil and gas deposits. The researcher team surmises that if the Atlantic Coast has been able to hide hundreds of Methane pits, then it is possible there could be as many as 30,000 more bubbling pits just waiting to be discovered. Skarke told Live Science, "There is no evidence whatsoever that there are conventional deep-seated oil and gas reservoirs underneath the Atlantic margin."
While scientists take the discovery of these new methane seeps as an opportunity to study how methane hydrates respond to climate change, they also say the East Coast methane seeps will not contribute much to greenhouse gases in the atmosphere because most of the gas will dissolve in the ocean before reaching the surface. The total amount of gas is also much smaller than sources found on land, like cattle or gas drilling. David Valentine, a geochemist at the University of California, Santa Barbara, while not a member of the research team, said, "It's probably on the order of a feedlot of methane."
"Sampling the bubbles, along with the waters in and around the plumes, will help scientists to estimate the effects of the methane emissions," says Skarke. When methane gas reacts with dissolved oxygen, it diminishes it, a process that creates carbon dioxide that has been blamed in the acidification of ocean waters..
More about Methane gas plumes, Atlantic coast, Okeanos Explorer, NOAA, multibeam sonar
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