Scientists, who are studying the vast deposits of methane trapped on Arctic Ocean floor, have spotted around 500 vents leaking this powerful greenhouse gas, adding as much as 90 tons of the planet-warming gas to the atmosphere or overlying waters each year.

The research was published Sunday in Nature Geoscience.

According to the researchers, an estimated two-thirds of the emissions originate from sediments at depths where methane-rich ices might be decomposing due to warming waters along the ocean bottom.

The sonar scans spotted the bubble streams, which were taken between September 2011 and August 2013 during oceanographic excursions ranging from Georges Bank off Cape Cod to Cape Hatteras in North Carolina.

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

The team within a distance of 950 kms discovered 570 bubble plumes, which is an astonishing number bearing in mind that scientists had earlier reported only a handful in the region.

Skarke said that though, some of the plumes extended hundreds of meters above the ocean floor, the bubbles originating from deep-water sources dissolved into the sea long before they could breach the surface.

As per Carolyn Ruppel, a geophysicist with US Geological Survey in Woods Hole, Massachusetts, and a co-author on the study, the researchers have not yet collected samples of the bubbles. The bubbles are assumed to contain methane because of their sources.

Some of the shallow-water seeps are likely to be in now-submerged areas that were once ethane-producing wetlands during the most recent ice age, when sea levels were more than 100 meters lower than they are today.

Nevertheless, many of the sources along the continental slope lie at cold depths in which ices have formed at high pressures within sea-floor sediments, which once trapped methane produced by microbes living there. These ices might now be breaking down slowly because of the warming of overlying waters, Skarke said.