Deep beneath the ocean, off the coast of Washington, Oregon, and northern California, scientists have discovered roughly 500 bubbling vents spewing methane, a potent greenhouse gas.
Underwater methane vents were first discovered in the 1980s and have since been identified all over the world. “In the right geologic setting, they are everywhere. And the Pacific northwest of the U.S. is such a setting,” said Andrew Thurber, an associate professor at Oregon State University who has worked on other methane discovery efforts.
Given that methane is 84 times as potent as carbon dioxide in the first decades after its release, underwater vents could have an outsize impact on climate change. “Methane vents are a potentially important, but unaccounted for, source of methane in global greenhouse gas budgets,” said Nicole Raineault, director of science operations at Ocean Exploration Trust, the nonprofit that made the discovery.
The vents, also known as seeps, were discovered along an area known as the Cascadia Margin during June and September expeditions of the research vessel EV Nautilus. The discovery, which was presented at the National Ocean Exploration Forum in New York on October 20, comes two years after some 570 similar methane vents were found off the eastern seaboard.
“It appears that the entire coast off Washington, Oregon, and California is a giant methane seep,” said Robert Ballard, founder of Ocean Exploration Trust, in a statement.
The researchers located the bubble streams with a sonar system and in some locations observed them directly with video cameras on remotely operated vehicles, Raineault said.
She said that methane takes different forms depending on temperature and pressure—at a depth of 500 meters or below along the Cascadia Margin, it occurs in the form of methane hydrate, a water-methane ice. If this ice melts, the methane trapped within is released as a gas that bubbles to the surface. Most of the bubbling methane streams were observed at around 500 meters of depth, the upper limit for methane stability in that area, Raineault said.
“It doesn’t take very many degrees of ocean warming to basically melt [the methane hydrate],” said Lisa Levin, director of the Center for Marine Biodiversity and Conservation at the Scripps Institution of Oceanography. “The thought is that warming of the oceans is leading to increased [methane release].”
But the feedback loop—where methane from undersea vents bubbles to the surface and acts as a greenhouse gas, causing further ocean warming that in turn melts methane hydrate and releases more methane gas—is still hypothetical, Thurber said.
“The vast majority of methane released from these reserves is consumed by archaea [single-celled microorganisms] and bacteria initially in the sediment and then in the water column as the methane is released,” Thurber said. He estimated that some 90% of methane released is consumed by these organisms and so does not reach the water’s surface and escape into the atmosphere.
For undersea methane reservoirs to affect global warming, Thurber said the gas would need to be released either shallow enough that there is not time for it to be consumed before it reaches the surface or in such large quantities that it overwhelms microbial communities that consume it. New methane seeps also could affect global warming if they occur in an area where the microbes that eat the methane are not yet present, he said.
While methane from the vents may not reach the atmosphere in large amounts, its consumption in the water could affect oxygen levels in the ocean. Some of the bacteria that consume methane use oxygen to break it down, and Levin said that one concern is whether methane oxidation in the water column is “using up the oxygen” in the areas where vents are found.
“Since we have a problem of ocean deoxygenation associated with climate change and warming, it could be that methane [release] is exacerbating the problem,” she said.
Advances in the past 15 years have improved our understanding of the methane cycle in the marine environment, Thurber said, giving us “a pretty good handle” on the basics of how methane is consumed in marine sediment and the water overlying it.
“We have a fundamental understanding,” he said, “but in no way a complete understanding of the fate of methane from methane seeps.”
Photo credit: EV Nautilus