Deep sea methane emissions occurring for millions of years, says study

Gigatonnes of naturally formed methane gas trapped under the Arctic ocean floor has been leaking for as long as 2.7 million years, says new research with major implications for global warming.

The movement of tectonic plates has been cited as the prime reason for the methane leakage.

With a warming potential 20 times stronger than carbon dioxide in the long-term, methane is a greenhouse gas that can create havoc on the earth's climate.

The study done by a scientist at Centre for Arctic Gas Hydrate, Environment and Climate identified two major events of gas emission in the past - 1.8 million years ago and 200,000 years ago.

Looking at Vestnesa Ridge in Fram Strait, a thousand meters under the Arctic Ocean surface offshore West-Svalbard, the study picked 800-metre-high gas flares rising from the seabed.

"Half of Vestnesa Ridge is showing very active seepage of methane. The other half is not. But there are obvious pockmarks on the inactive half, cavities and dents in the ocean floor that we recognized as old seepage features. So we were wondering what activates, or deactivates, the seepage in this area," said Andreia Plaza Faverola, primary author of the study.

A seismic instrument P-Cable recorded the sediments beneath these pockmarks enabling visualisation of the deep sediments in 3D.

The paper suggests that the movement of tectonic plates influences the gas release from below the stable hydrates.

Vestnesa is on a passive margin between two oceanic ridges that are slowly spreading. These spreading ridges resulted in separation of Svalbard from Greenland and opening of the Fram Strait.

The spreading in turn influences the passive margin of West-Svalbard, and even a small mechanical collapse in the sediment can trigger seepage.

The deep sea methane is stored as gas hydrates, chunks of frozen gas and water. Being deep under cold waters and a lot of pressure keeps the hydrates stable and not vulnerable to global temperature changes.

However, below the stable hydrates there is gas that is not frozen. This gas could escape under high pressure or increasing inflow of gas from deeper sediments.

The hydrates would still remain stable in this scenario, says the study published in Geophysical Research Letters.

Global warming potential

Hydrates hold as much as 10 times more carbon than the atmosphere. A University of Rochester had suggested in a recent study that direct transfer of even a small fraction of this methane to the atmosphere can have catastrophic effects on Earth's climate.

Permafrost is another source of methane, made of perennially frozen ground occurring in about 24% of the exposed land surface in the Northern Hemisphere and the Antarctica.

Permafrost contains around 1,700 gigatonnes of carbon in the form of frozen organic matter, almost twice as much carbon as currently in the atmosphere, notes UNEP.

Permafrost-associated gas hydrates differ significantly from deepwater gas hydrates in several ways. They are relatively uncommon compared to nearly ubiquitous deepwater gas hydrates, according to the US Geological survey.

A permafrost seal believed to keep the gas locked underneath in the Arctic region was found leaking profoundly recently.

Around 60% of the methane in the atmosphere comes from emissions from human activities.