New research suggests a large amount of methane was released in the Arctic Ocean during a period of warming 110 million years ago and the methane "burp" points to the possibility of a similar release in today's warming conditions.
The discovery happened in the remote High Arctic, on Ellef Ringnes Island, about 500 kilometres north of Resolute, Nunavut. During the Cretaceous period, 55 million years before the dinosaurs disappeared, the island was deep underwater.
Suddenly, from under the mud, bubbles of methane began to emerge as frozen deposits began to thaw.
The bubbles left traces in the form of over 130 mounds that persist on the island today, complete with fossils of life that formed around the methane seeps.
"There must have been some brief, rapid release into the ocean," explained Steve Grasby, a Natural Resources Canada geochemist who visited the island between 2009 and 2011.
"Because we don't see them in the older rocks and we don't see them in the younger rocks. So something must have happened in the Earth's history at that time to release a bunch of methane into the sea," he said.
The research was published in the Geological Society of America Bulletin.
Evidence of ancient climate change
The thawing of the so-called methane hydrates coincides with a period of warming following a volcanic eruption, which released a cloud of carbon dioxide into the atmosphere.
"That was when Earth transitioned from a cold climate to a warm climate," Grasby said.
"So we see this sudden and short term release of methane is coincident with this period of global climate warming."
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Today there are still deposits of methane hydrates buried under the sea. There have been concerns that their thawing could cause runaway climate change, since methane is a powerful, though short-lived, greenhouse gas that could further warm the climate, melting ever more hydrates.
But ocean chemist David Archer from the University of Chicago does not believe the hydrates present such a danger.
"It's not totally clear that the bubbles are going to get out [into the atmosphere]," he said. "It's got a whole lot of mud to fight their way through where they're chemically vulnerable."
A recent review by the United States Geological Survey has a similar judgment, saying "most" of the methane never reaches the atmosphere.
Grasby says his research points to a period in the Earth's geologic history when warming was releasing the gas from its frozen state, but, like today, it's hard to say what happens next.