Natural methane leakage from the seafloor is far more widespread on the US Atlantic margin than previously thought, according to a study out of Mississippi State University, the US Geological Survey, and others.
None of the seeps mapped was known to researchers before 2012.
Methane plumes identified in the water column between Cape Hatteras, North Carolina and Georges Bank, Massachusetts, come from at least 570 seafloor cold seeps on the outer continental shelf and the continental slope.
Prior to this study, only three seep areas had been identified beyond the edge of the continental shelf, which occurs at about 180m (590ft) water depth between offshore Florida and Maine.
“Widespread seepage had not been expected on the Atlantic margin. It is not near a plate tectonic boundary like the US Pacific coast, nor associated with a petroleum basin like the northern Gulf of Mexico,” said Adam Skarke, the study’s lead author and a professor at Mississippi State University.
The gas being emitted by the seeps has not yet been sampled, 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 not the type found in deep-seated reservoirs and often tapped as a natural gas resource.
Most newly discovered seeps lie at depths close to the shallowest conditions at which deepwater marine gas hydrate can exist on the US slope.
“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, chief of the USGS Gas Hydrates Project.
“Such continental slope seeps have previously been recognised in the Arctic, but not at mid-latitudes. So this is a first.”