© 2014 Peter Free
Citation — to study
A. Skarke, C. Ruppel, M. Kodis, D. Brothers, and E. Lobecker, Widespread methane leakage from the sea floor on the northern US Atlantic margin, Nature Geoscience, DOI:10.1038/ngeo2232 (advance online publication, 24 August 2014)
Citation — to press release
US Geological Survey, Natural Methane Seepage on U.S. Atlantic Ocean Margin Widespread, United States Department of the Interior (25 August 2014)
From the abstract:
Here we use multibeam water-column backscatter data that cover 94,000 km2 of sea floor to identify about 570 gas plumes at water depths between 50 and 1,700 m between Cape Hatteras and Georges Bank on the northern US Atlantic passive margin.
About 440 seeps originate at water depths that bracket the updip limit for methane hydrate stability.
Contemporary upper-slope seepage there may be triggered by ongoing warming of intermediate waters, but authigenic [originating in place] carbonates observed imply that emissions have continued for more than 1,000 years at some seeps.
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.
© 2014 A. Skarke, C. Ruppel, M. Kodis, D. Brothers, and E. Lobecker, Widespread methane leakage from the sea floor on the northern US Atlantic margin, Nature Geoscience, DOI:10.1038/ngeo2232 (advance online publication, 24 August 2014) (paragraph split)
Method — multibeam water column backscatter
From the National Oceanic and Atmospheric Administration:
Multibeam sonar, an echo sounding technology commonly used to map the seafloor, can also be used to map and detect gaseous seeps in the water column, according to scientists testing the technology on board NOAA Ship Okeanos Explorer last week in the Gulf of Mexico.
Unlike other types of sonar, multibeam technology is able to survey a wide area of the seafloor and water column.
“This capability will help increase our knowledge of the marine environment, including the distribution of natural sources of methane input into the ocean and the identification of communities of life that are often associated with methane gas seeps,” said Thomas Weber, Ph.D. . . .
Since multibeam sonar obtains information from a wide fan-shape of beams, it maps a wider area more quickly and efficiently.
The multibeam sonar on Okeanos Explorer is one of the few that is specially configured to collect water column data to characterize gaseous seeps in wide areas of the deep-ocean’s water column in high resolution.
© 2014 Fred Gorell, NOAA and partners demonstrate success of multibeam sonar to detect and map deep-sea gas seeps, National Oceanic and Atmospheric Administration (25 September 2011) (extracts)
Study findings in more detail
From the US Geological Survey:
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 outer continental shelf and the continental slope.
Taken together, these areas, which lie between the coastline and the deep ocean, constitute the continental margin.
Prior to this study, 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.
Cold seeps are areas where gases and fluids leak into the overlying water from the 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 . . . .
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 [approximately 440 — see abstract] of the newly discovered methane seeps lie at depths close to 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, and forms at temperature and pressure conditions commonly found in waters deeper than approximately 500 meters (1640 feet).
Most seeps described in the new study are too deep for the methane to directly reach the atmosphere, 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.
© 2014 US Geological Survey, Natural Methane Seepage on U.S. Atlantic Ocean Margin Widespread, United States Department of the Interior (25 August 2014) (extracts)
The gas has not been sampled. The team is assuming microbial origin. Educated guesses address whether the gas is making it into the atmosphere. One might (I assume) even be able to quarrel with the accuracy of multibeam water column backscatter in this context.
The moral? — There may be substantially more methane off the American Atlantic coast than previously realized
But it will take further investigation to prove its extent, volume, origin, and which portion of that is directly affecting the atmosphere.
Then one will want to estimate how global warming is affecting the seepage rate and its greenhouse influence.