Scientists eye link between warmer ocean and methane bubbles

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Global warming could trigger more methane releases from the ocean floor. Photo courtesy NASA’s Blue Marble project.

Frozen gas may escape and intensify ocean acidification along the Pacific Northwest Coast

Staff Report

Warmer deep-ocean temperatures off the coast of the Pacific Northwest may be triggering methane releases, scientists said after studying an unusually high number of bubble plumes in the region.

The gas bubbles may be originating from melting deposits of methane “ice” that have been frozen and dormant for thousands of years, according to University of Washington researchers.

“We see an unusually high number of bubble plumes at the depth where methane hydrate would decompose if seawater has warmed,” lead author H. Paul Johnson said in a press release. “So 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,” said Johnson, a UW professor of oceanography.

“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,” Johnson said. “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 is a powerful heat-trapping gas that has been a factor in historic climate shifts. In the study, the scientists were looking specifically at frozen methane deposits that may be affected by warmer ocean temperatures. The methane could also be coming from organic sediments, they said, explaining that a chemical analysis of the gas will help reveal its origins.

Most of the methane released from the ocean floor may never find its way to the atmosphere, as marine microbes convert the gas into carbon dioxide. That can lower the oxygen content of the water and produceĀ  more-acidic conditions in the deeper offshore water, which eventually wells up along the coast and surges into coastal waterways, where acidification is already a problem for shellfish.

“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 said.

Another potential consequence, he said, is the destabilization of seafloor slopes where frozen methane acts as the glue that holds the steep sediment slopes in place.

In a press release, the researchers explained that methane deposits are abundant on the continental margin of the Pacific Northwest coast. Previous studies have shown that the ocean in the region is warming at a depth that could affect those pockets of frozen methane, where they’ve been trapped by dense, cold water in a crystal lattice structure.

A 2014 study calculated that, with present ocean warming, such hydrate decomposition could release roughly 0.1 million metric tons of methane per year into the sediments off the Washington coast, about the same amount of methane from the 2010 Deepwater Horizon blowout.

Methane gas is slowly released at almost all depths along the Washington and Oregon coastal margin. But the plumes are significantly more common at the critical depth of 500 meters, where hydrate would decompose due to seawater warming.

“The results are consistent with the hypothesis that modern bottom-water warming is causing the limit of methane hydrate stability to move downslope, but it’s not proof that the hydrate is dissociating,” said co-author Evan Solomon, a UW associate professor of oceanography.

The study is set to appear in the journal Geochemistry, Geophysics, Geosystems. The research was funded by the National Science Foundation and the U.S. Department of Energy.

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