Scientists have discovered methane gas bubbling from the seafloor in an unexpected place: off the East Coast of the United States where the continental shelf meets the deeper Atlantic Ocean.
The methane is emanating from at least 570 locations, called seeps, from near Cape Hatteras, N.C., to the Georges Bank southeast of Nantucket, Mass. While the seepage is widespread, the researchers estimated that the amount of gas was tiny compared with the amount released from all sources each year.
In a paper published online Sunday in the journal Nature Geoscience, the scientists, including Adam D. Skarke of Mississippi State University and Carolyn D. Ruppel of the U.S. Geological Survey, reported evidence that the seepage had been going on for at least 1,000 years.
They said the depths of the seeps suggested that in most cases the gas did not reach the atmosphere but rather dissolved in the ocean, where it could affect the acidity of the water, at least locally.
But methane is a potent, if relatively short-lived, greenhouse gas, so the discovery should aid the study of an issue of concern to climate scientists: the potential for the release of huge stores of methane on land and under the seas as warming of the atmosphere and oceans continues.
“It highlights a really key area where we can test some of the more radical hypotheses about climate change,” said John D. Kessler, a professor at the University of Rochester who was not involved in the research.
Methane seeps occur in many places, but usually in areas that are tectonically active, such as off the West Coast of the United States, or connect to deep petroleum basins, as in the Gulf of Mexico. The Atlantic margin, as the region where the shelf meets the deeper oceanic crust is known, is tectonically quiet, and most of the seeps are not thought to be linked to oil and gas deposits.
“This is a large amount of methane seepage in an area we didn’t expect,” Skarke said. “That raises new questions for us.”
Ruppel said that at about 40 of the seeps – those in water depths exceeding 3,300 feet – the methane may be migrating up through the sediments from deeper reservoirs of the gas. Further studies would be needed to confirm this, she said.
If the gas is found to be originating from reservoirs, then oil companies could potentially be interested in determining whether the reservoirs can be tapped.
But Ruppel said that most of the seeps had been found in depths of about 800 to 2,000 feet, where the methane, which is produced by microbes, is most likely trapped in sediments near the seafloor, within cagelike molecules of ice called hydrates. Natural variability in water temperatures, caused by ocean circulation and other factors, may be warming these hydrates just enough to release the gas.
Hydrates at such relatively shallow depths “are exquisitely sensitive to small changes in temperature,” she said. “You don’t have to change things very much to get the methane to come out.”
Kessler, author of an article reviewing the findings in the same journal, said that because the Atlantic margin was unaffected by tectonic activity or other factors, it should prove to be a convenient location to conduct long-term studies of links between climate change and methane releases.
“How will those release rates accelerate as bottom temperature warms, or how will they decelerate if there are some cooling events?” Kessler said. “We don’t really have all of the answers. But this is a great place to try to find them.”