Washington, Aug 25 (IANS): The sea floor at the US East coast could be adding as much as 90 tonnes of the planet-warming methane gas to the atmosphere or overlying waters each year, a team of researchers has revealed.

Plumes of bubbles streaming from hundreds of newly discovered sea-floor seeps between North Carolina and Massachusetts are likely to contain methane, researchers found.

An estimated two-thirds of the emissions emanate from sediments at depths where methane-rich ices may be decomposing due to warming waters along the ocean bottom, the researchers say.

"Within a distance of about 950 km, we found about 570 bubble plumes - an astounding number," said study co-author Adam Skarke, a geologist at Mississippi State University in Starkville.

Although some of the plumes extended hundreds of metres above the ocean floor, "the bubbles emanating from deep-water sources typically dissolved into the sea long before they could breach the surface," Skarke explained.

Sampling the bubbles, along with the waters in and around the plumes, will help scientists estimate the effects of the methane emissions on the climate.

"I am not that surprised that people have not seen these things before. These features are quite narrow, sometimes just a few metres across, and the ocean's a big place," reacted Tim Minshull, an oceanographer at University of Southampton.

The study, published in the journal in Nature Geoscience, suggests that some of the seeps have been active for a millennium or more.