Researchers spot hundreds of methane plumes on sea floor around US

Plumes of bubbles streaming from hundreds of newly discovered sea-floor seeps between North Carolina and Massachusetts believed to be containing methane could be adding as much as 90 tonnes of the planet-warming gas to the atmosphere or overlying waters each year, according to research published Sunday in Nature Geoscience.

Scientists had observed the bubble streams but they had not yet sampled the gas within them. They believed however there was an abundance of circumstantial evidence pointing to methane.

An estimated two-thirds of the emissions that originated from sediments at depths were methane-rich ice that might be decomposing due to warming waters along the bottom of the ocean according to researchers. Though the effects of these plumes on climate and ocean chemistry were still unclear, they could well extend well beyond the plumes themselves.

The bubble streams showed up on sonar scans of the sea floor taken between September 2011 and August 2013 during oceanographic expeditions ranging from Cape Hatteras in North Carolina to Georges Bank off Cape Cod.

Altogether, researchers analysed data covering a 94,000-square-kilometre arc that included the edge of the continental shelf and the steep slope just seaward of it, according to co-author Adam Skarke, a geologist at Mississippi State University in Starkville.

Over a 950-kilometres distance, the team came across around 570 bubble plumes.

According to experts, the unexpected discovery indicated there were large volumes of the gas contained in a type of sludgy ice called methane hydrate, BBC reported.

There were concerns that the new seeps could be making a contribution to global warming that had not been noticed.

According to scientists there could be around 30,000 of these hidden methane vents worldwide.

In earlier surveys along the Atlantic seaboard only three seep areas beyond the edge of the US continental shelf had been identified.

The team that had identified the new findings studied what was termed the continental margin, the region of the ocean floor between the coast and the deep ocean.

The 570 seeps lay at varying depths between 50 metres and 1,700 metres.

According to Skarke, it was the first time, this level of seepage outside the Arctic that was not associated with features like oil or gas reservoirs or active tectonic margins had been seen.