How an ancient methane 'blow
The Arctic sea floor was once like a pressure cooker, with methane gas building, building, building beneath an ice sheet for thousands of years. But when the ice eventually melted, the lid suddenly burst off, unleashing enormous amounts of methane and forming deep craters.
Some 12,000 years later, the craters are still profusely leaking methane — a potent greenhouse gas and a major contributor to global warming. While scientists have known about pockmarks since the 1990s, a new study shows the craters cover a much larger area than previously thought.
SEE ALSO:What we do in the next 5 years will determine the fate of the melting ArcticResearchers identified more than 600 methane flares in and around the Arctic craters, which are steadily releasing gas into the water column, according to a paper published Thursday in the journal Science.
But today's leaks are "nothing compared to the blow-outs of the greenhouse gas that followed the deglaciation," said Karin Andreassen, the study's lead author and a professor at Norway's CAGE Center for Arctic Gas Hydrate, Environment, and Climate.
Several hundred of craters are seen in this area.Credit: K. Andreassen/CAGE"The amounts of methane that were released [millennia ago] must have been quite impressive," she said in a press release.
It's not clear whether the deep-sea explosion actually pumped methane into atmosphere, where it would contribute to climate change, or if it all remained in the ocean. Today's gas leaks follow a similarly uncertain path. Methane can be oxidized to carbon dioxide, which increases ocean acidity, or turned into carbonate rock, though tiny bits can still escape into the air.
Andreassen and her CAGE colleagues used detailed imaging technology to map about 100 craters in the Barents Sea, near the Arctic archipelago Svalbard. The craters ranged from 984 feet to 3,280 feet wide, though hundreds of smaller craters are also scattered across the study area.
During the last Ice Age, this swath of the Arctic was covered by a thick ice sheet, as large as West Antarctica is today. The ice loaded what is now the ocean floor with heavy weight, creating a lid to what became the methane pressure cooker.
The study area is in Bjørnøyrenna, close to the Arctic archipelago Svalbard.Credit: K. Andreassen/CAGEBeneath the Arctic sea floor, vast amounts of methane are trapped as ice-like, solid mixtures of gas and water called hydrates, which are stored in the sediment and constantly fed by gas from below. Hydrates are most stable under high pressure and cold — conditions found on the underside of a glacier.
But as the ice sheet rapidly retreated some 12,000 years ago, the hydrates piled up in mounds. Eventually, they began to melt and expand, increasing pressure in the area.
"The principle is the same as in a pressure cooker: If you do not control the release of the pressure, it will continue to build up until there is a disaster in your kitchen," Andreassen said. The hydrate mounds quickly collapsed, creating craters and spewing methane into the water column.
The new research might hold clues as to what could happen to the vast reserves of hydrocarbons that are currently trapped beneath the load of the West Antarctic and Greenland ice sheets.
The study provides a "good past analogue" for what could happen with future methane releases as today's ice sheets retreat, Andreassen said.
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