Oceanic methane: Greenhouse leakage

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  • Published: Sep 1, 2014
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Oceanic methane: Greenhouse leakage

Cold seepage

Widespread methane leakage from the sea floor on the northern US Atlantic margin Credit: A. Skarke et al/Nature Geosci

Natural methane leakage from the seafloor is far more widespread on the US Atlantic margin than geological scientists previously thought. Data from multibeam water-column backscatter experiments covering a 94000 square kilometre area reveal approximately 570 gas plumes at depths between 50 and 1700 metres. At these depths methane hydrates, in which this potent greenhouse gas is usually kept trapped under pressure, are unstable potentially allowing huge volumes to enter atmosphere.

Adam Skarke of Mississippi State University in Starkville, Mississippi and colleagues have demonstrated that plumes of methane are bubbling up through the water column between Cape Hatteras, North Carolina and Georges Bank, Massachusetts emerging from at least 570 cold seeps on the sea bed of the outer continental shelf and the continental slope. This region taken as a whole is the continental margin and prior to this research only three such seep areas were known beyond the edge of the continental shelf, at an approximate depth of 180 metres depth between Florida and Maine in the floor of the US Atlantic.

Skarke and colleagues explain that cold seeps are areas where gases and fluids leak into the waters above them from the sediments and sedimentary rock. The word "cold" is used specifically to distinguish them from the perhaps more well-known hydrothermal vents, which sit in regions where new oceanic crust is being formed and spit hot fluids from the seabed. As such, cold seeps can occur in many more diverse submarine environments than hydrothermal vents, the composition of the material ejected is of critical importance to predictions concerning greenhouse gas effects if the main component releases a potent gas, such as methane.

Widespread seepage

“Widespread seepage had not been expected on the Atlantic margin. It is not near a plate tectonic boundary like the Pacific coast, nor associated with a petroleum basin like the northern Gulf of Mexico,” explains Skarke. The gas being emitted by these seeps has not yet been tested, but the researchers suggest that it is most likely to contain large volumes of methane generated by microbial activity in the shallow sediments. This hypothesis is grounded on where the seeps have been observed and the underlying geology. Most of the newly discovered methane seeps lie at depths near to the shallowest at which deepwater marine gas hydrates - ice-like pressurized methane-water materials - can exist on the continental slope, 500 metres down.

Tens of thousands of cold seeps

“Warming of ocean temperatures on seasonal, decadal or much longer timescales can cause gas hydrates to release methane, which may then be emitted at seep sites,” says co-author Carolyn Ruppel. “Such continental slope seeps have previously been recognized in the Arctic, but not at mid-latitudes. So this is a first.”

Fortunately, many of the seeps observed in this current study are at sufficiently low depths for the methane not to emerge directly and reach the Earth's atmosphere. Nevertheless, leaked methane can be oxidized in the water to carbon dioxide, which would become dissolved in the sea forming carbonic acid and reducing the pH, which in turn lowers the oxygen capacity of seawater. The methane emerging from seeps that are not as deep, such as those found at the edge of the shelf and in the upper part of Hudson Canyon could leak methane directly to the atmosphere from the ocean. It is now essential that even more detailed studies of the extent of shallow-water seeps is carried out to assess the potential impact on future climate of such methane.

Estimates of methane leakage from cold seeps were previously given as a mere 8 to 65 million tonnes per year, the revelation that so many other parts of the marine continental margins are also emitting methane suggest that this may a gross underestimate. Moreover, the team suggests that there may be tens of thousands more cold seeps yet to be found.

Related Links

Nature Geosci, 2014, online: "Widespread methane leakage from the sea floor on the northern US Atlantic margin"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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