A methane mound in the Canadian High Arctic. (Stephen Grasby and Geological Society of America Bulletin)

More than 100 million years ago, a sudden period of global warming — possibly brought on by a series of volcanic eruptions — released a massive upwelling of previously frozen methane gas from the bottom of the Arctic Ocean into the water, new research suggests. And the discovery has scientists pondering whether human-caused climate change could cause it to happen again — and what the consequences would be.

Scientists have long debated whether methane venting from the seafloor is able to reach the surface. But even the possibility has some experts concerned. If a sudden surge of methane did manage to escape into the atmosphere today, there could be dire consequences for the global climate. Methane is a potent greenhouse gas, whose climate-warming influence in the atmosphere is up to 30 times as strong as carbon dioxide over short time periods. Some scientists worry that a sudden, large release of methane could trigger a dangerous climate feedback loop, exacerbating global warming and causing even more gas to be released as a result.

“What’s been suggested as happening both in the geologic record and as a concern for the modern is you sort of have a runaway scenario,” said Stephen Grasby, a research scientist with Natural Resources Canada, who contributed to the new research.

The good news is that many scientists believe that most methane leaking from the ocean floor is unlikely to make it into the atmosphere because it will get trapped or dissolve into the water column on the way up. And some research has also suggested that, despite rapid warming in the Arctic, a large-scale release of methane is unlikely to be triggered by modern climate change any time soon.

Still, the new findings provide suggestive evidence that changes in the ancient climate may have had a dramatic effect on the earth’s methane deposits — and they suggest that, at some point, a similar relationship could occur again.

Grasby, along with a team of scientists from institutes in Canada and Europe, discovered evidence for the ancient methane leak during a recent expedition to remote Ellef Ringnes Island in the Canadian Arctic, which they have described in a paper published this month in the Geological Society of America Bulletin. There, they found a cluster of 139 strange, rocky mounds, which they say were formed by a rapid release of large amounts of methane from the ocean floor.

At the time, the mounds would have been submerged hundreds of meters below the sea surface. They’re composed mainly from rocky carbonate minerals, which form when methane seeps out of the seafloor and is chemically changed by microbial communities at the bottom of the ocean. The same process still occurs at methane vents in the ocean today.

Methane leaks, or “seeps,” in the ocean aren’t exactly unusual. Methane naturally forms as organic matter decomposes on the seafloor. Under mild temperature conditions, it tends to gradually bubble up from the sediment at the bottom of the ocean, and it happens slowly enough that much of it ends up dissolving in the water column before it reaches the surface. Scientists have discovered hundreds of these seeps throughout the ocean.

In deep parts of the ocean or in cold regions like the Arctic, though, methane can get trapped in the seafloor, freezing into an ice-like substance called a methane hydrate. An increase in temperature can cause these hydrates to destabilize and release the methane in particularly large amounts.

And this is what appears to have happened a hundred million years ago in the Canadian Arctic. During their expedition, Grasby and the rest of the research team spent weeks trudging from one rocky mound to another across the muddy Arctic tundra, ultimately mapping out 139 mounds spread out over nearly 4,000 square miles of land.

An analysis of carbon isotopes from each site suggested that the mounds were all formed by the release of methane, as opposed to some other fluid, Grasby said. And the fossils collected from each area supported this idea. Methane vents tend to attract specific types of organisms, such as tube worms, which the researchers found at many of the sites they visited.

Further analysis suggested that the mounds had all formed during the same relative period of time.

“They’re all the exact same layer in the geologic record,” Grasby said. “There’s this one horizon where you have all these methane seep mounds that were discovered.”

This point in the geological record — dating back to around 110 million years ago — happens to coincide with a sudden warming period in the ancient earth’s climate, Grasby added. Many scientists believe this warming was likely caused by a series of volcanic eruptions, which poured large quantities of carbon dioxide into the atmosphere.

The researchers believe that the resulting rapid global warming caused frozen methane hydrates, formerly trapped in Arctic sediment, to destabilize and send a massive upwelling of methane gas bubbling out into the ocean — a kind of methane “burp,” Grasby said.

It’s a theory that other scientists, not involved with the new research, find credible as well.

“It is certainly plausible that these seeps could have emitted methane that had been sequestered in marine gas hydrates until warming caused the hydrates to destabilize,” said Carolyn Ruppel, a geophysicist and head of the U.S. Geological Survey’s Gas Hydrates Project, in an email.

The theory raises the idea that human-induced global warming could cause a similar event to happen in the future, Grasby suggested. In the paper, the researchers suggest that the collection of seep mounds at Ellef Ringnes Island may provide “an excellent analogue for the present-day potential of global warming — induced hydrate destabilization.”

In fact, there’s already evidence that rising ocean temperatures may be causing gas hydrates to destabilize in the Arctic and elsewhere, Ruppel pointed out.

“On present-day Earth, gas hydrate breakdown caused by the impingement of warming ocean waters is thought to be widespread on upper continental slopes in the Arctic Ocean … and in temperate latitudes,” Ruppel said. “Contemporary cold seeps recognized in the Arctic Ocean and on the U.S. Atlantic and Pacific margins leak methane that may in part originate with gas hydrate breakdown.”

But as for a catastrophic, rapid release of methane from the ocean into the atmosphere, the odds — for now — are probably slim. While the newly discovered mounds do suggest that large amounts of methane surged from the sea floor over a relatively short period of time, it would be difficult to prove whether the gas made it into the earth’s ancient atmosphere or simply dissolved into the water after that point.

“The recent study does not change our understanding of the contemporary record or our predictions about the fate of deepwater methane hydrate deposits in the future,” Ruppel said. Just a few months ago, Ruppel co-authored a cautionary paper suggesting that the climate-induced breakdown of gas hydrates is unlikely to cause any runaway greenhouse gas effect in the atmosphere.

That doesn’t mean that methane bubbling out into the ocean can’t still cause problems. Methane in the ocean can undergo chemical reactions that deplete the surrounding water of oxygen, produce carbon dioxide and cause the ocean to become more acidic. The carbon dioxide that’s produced may also eventually make its way to the surface and enter the atmosphere.

So keeping an eye on modern-day methane hydrates is still important. And while many scientists do believe that a large-scale release of methane from these hydrates into the atmosphere is unlikely any time soon, there’s still some uncertainty about how the deposits might react to varying levels of future warming.

“It’s certainly an area of concern,” Grasby said.