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Scientists say frozen methane deposits off the coast of the US Pacific Northwest might be melting

  • Researchers found methane plumes to be significantly more common at a critical depth where it was projected frozen deposits would start to melt due to rising ocean temperatures.
  • Current environmental issues in Washington and Oregon that are already impacting local wildlife and fisheries could be exacerbated by the release of more methane.
  • Methane is a greenhouse gas 25 times more potent than CO2, but much of the methane in the plumes is consumed by marine microbes that turn it into CO2, contributing to ocean acidification.

New research suggests methane deposits that have been frozen in the seabed off the coasts of Oregon and Washington for thousands of years might have begun to melt and bubble up through the warming ocean waters.

Methane gas is slowly released from the ocean’s floor at almost all depths along the Pacific Northwest’s coastal margin, but researchers at the University of Washington say they’ve found that plumes of the potent greenhouse gas are significantly more common at a depth of 500 meters (0.3 miles) — precisely where it was expected that frozen methane, known as methane hydrate, would start to melt due to rising ocean temperatures.

A team of researchers at the University of Washington studied 168 methane bubble plumes that had been observed over the past decade and determined that 14 of them were located at that crucial transition depth, far more plumes per unit area than any other part of the region’s seafloor.

The results of the UW researchers’ study have been accepted for publication in the American Geophysical Union journal Geochemistry, Geophysics, Geosystems.

Sonar image of bubbles rising from the seafloor off the Washington coast. The base of the column is 515 meters deep (about 1/3 of a mile) and the top of the plume is at 1/10 of a mile (180 meters) deep. Credit: Brendan Philip/University of Washington

“What we’re seeing is possible confirmation of what we predicted from the water temperatures: Methane hydrate appears to be decomposing and releasing a lot of gas,” H. Paul Johnson, a UW professor of oceanography and lead author of the report, said in a statement.

“If you look systematically, the location on the margin where you’re getting the largest number of methane plumes per square meter, it is right at that critical depth of 500 meters.”

Methane deposits are abundant on the Pacific Northwest’s continental margin, the area of the ocean’s floor where the thick continental crust meets the thinner oceanic crust. The frigid temperatures and high pressure at the bottom of the ocean on the continental margin causes methane gas deposits in seafloor sediments to freeze into methane hydrate, an ice-like solid.

When the ocean warms, methane hydrate crystals dissociate and the methane gas leaks into the surrounding sediment. Some of that gas escapes and, if it bubbles all the way to the surface, enters the atmosphere, where it acts as a greenhouse gas 25 times more potent than CO2, according to the US Environmental Protection Agency.

But most of the methane gets consumed on the way up by marine microbes that convert the methane into carbon dioxide, contributing to acidic, low-oxygen conditions in the ocean water, which eventually well up along the coastline and into coastal waterways, according to the UW researchers.

“Current environmental changes in Washington and Oregon are already impacting local biology and fisheries, and these changes would be amplified by the further release of methane,” Johnson noted.

Methane bubbles flow in a small stream out of the sediment on an area of seafloor offshore Virginia north of Washington Canyon. Photo by NOAA.

The frozen methane also acts as something of a binding agent that holds steep sediment slopes in place, he said, meaning that as more methane hydrate dissociates, the seafloor becomes less stable.

Johnson and team write in the study that it cannot be said for certain that the abnormally high number of methane plumes they observed at the critical 500 meter depth was really from the dissociation of frozen deposits.

Though naturally occurring methane plumes are not uncommon off the coasts of Washington and Oregon, however, Johnson said “it is not likely to be just emitted from the sediments; this appears to be coming from the decomposition of methane that has been frozen for thousands of years.”

Methane hydrate destabilization is one of climate scientists’ biggest fears, and is therefore a growing area of research. Other recent studies have found global warming-related methane emissions in the Arctic permafrost and off the Atlantic coast.