Methane is leaking from at least 570 locations from a most unexpected place, where the continental shelf meets the deeper Atlantic Ocean, scientists are reporting.
At 40 or so of the seeps found, all in water depths of more than 3,300 feet, the methane may be migrating through the sediments from deeper reservoirs of natural gas, according to U.S. Geological Survey scientist Carolyn Ruppel, who helped to author a paper outlining the findings.
Scientists involved in the paper, published in the journal Nature Geoscience, included Adam Skarke of Mississippi State University.
"This is a large amount of methane seepage in an area we didn't expect," Skarke said. "That raises new questions for us."
Methane seeps are found in many places, but usually they are in areas that are tectonically active, such as off the West Coast of the United States, or when they are connected to deep petroleum basins like the Gulf of Mexico.
The Atlantic margin, as the region is called, is where the continental shelf meets the deeper oceanic crust, and it mostly is tectonically "quiet," according to scientists.
If natural gas were found to be originating from deep pocket reservoirs, it could have reserves potential. Evidence exists the gas seepage has been ongoing for at least 1,000 years.
The seeps to date have been discovered from near Cape Hatteras, NC, to the Georges Bank southeast of Nantucket, MA.
Although the seeps are widespread, only a small amount has been leaking compared to other areas of the world, scientists noted. The Atlantic Ocean is considered the "youngest" on Earth.
The depth of the leaks suggested that the gas did not reach the atmosphere but dissolved in the ocean.
Most of the discovered seeps were found in water depths of 800-2,000 feet, where the methane is produced by microbes and likely has been trapped in sediments near the seafloor within ice molecules, or hydrates, said Ruppel. Natural variability in water temperatures that could be caused by ocean circulation or other factors, could be warming the hydrates enough to release the methane.
Hydrates at relatively shallow depths "are exquisitely sensitive to small changes in temperature," Ruppel said. "You don't have to change things very much to get the methane to come out."
Scientists used sonar to identify the seeps, which indicate the large volumes of methane are stored in methane clathrate hydrate, a crystalline structure similar to ice.
"The U.S. Atlantic margin has not been considered an area of widespread seepage, with only three methane seeps recognized seaward of the shelf break," the scientists said. "Extrapolating the upper-slope seep density on this margin to the global passive margin system, we suggest that tens of thousands of seeps could be discoverable."