Dating past methane release from the sea floor

Collecting gas samples from the floor of the Barents Sea.

It is widely appreciated that methane is a potent greenhouse gas, and that stores of frozen methane (gas hydrates) in sediments below the sea floor could potentially impact climate if released to the oceans and atmosphere. BGS and Geological Survey of Norway (NGU) researchers have been dating past methane emissions using carbonate layers (or crusts) from the Barents Sea, north of Norway, in order to determine and constrain the rate of release. The new study has been published in Nature Communications.

Collecting crust samples from the floor of the Barents Sea.

Diana Sahy, research scientist at the BGS, says:

"The scale of this impact depends on the rate at which methane is released; emissions over a short time span of a few decades could have a significant effect on climate. But the same amount of methane, released slowly over several thousand years, would have a much lower impact on greenhouse gas concentrations."

Collecting crust samples from the floor of the Barents Sea.

Antoine Crémière, post-doc researcher at NGU, explains:

"We have used carbonate crusts that form just below the sea floor as a direct result of the oxidation of methane moving upwards though the sediment layers from below. The chemistry of the crusts tells us that the fluid that formed them was rich in methane, and the dating tells us when this methane release happened."

Data from the Barents Sea tell us there have been ongoing methane emissions over a period of several thousand years following the last glaciation. The theory is that the pressure of the ice sheet stabilised the methane as frozen gas hydrate below the sea floor, however, when the ice sheet was removed the methane became unstable and changed to a free gas.

"Dating shows that emissions began when the frozen gas hydrates melted as the ice disappeared, but continued for several thousand years," explains researcher Aivo Lepland of NGU, head of the project. "This may very well help explain why we do not see a signal for such events in the past in the climate record: the release was too slow to significantly affect the concentration of methane in the atmosphere."

The new results have been developed by an international team of scientists at the Geological Survey of Norway (NGU), the Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) at UiT Norges Arctic University of Tromsø, the BGS, Tallinn University of Technology and the oil company Lundin Petroleum.