Methane-belching earthquakes could throw off calculations of how much of the potent greenhouse gas is entering the atmosphere every year, say scientists who have found evidence of massive releases from the floor of the Arabian Sea.
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The methane in the seafloor was safely sequestered in the form of a gas hydrate - a water-methane ice mix - until an 8.1 magnitude earthquake in 1945 fractured the seafloor sediments making it possible for the hydrates to escape. The hydrates turned into gas and steadily bubbled into the ocean, and possibly into the atmosphere, over the following decades, according to a study in the July 28 issue of the journal Nature Geoscience.
"Our seismic reflection data suggest that co-seismic shaking fractured gas-hydrate-bearing sediments, creating pathways for the free gas to migrate...into the water column," writes David Fischer of the University of Bremen, and his colleagues. They conservatively estimate that 326 million moles of methane (about 5.2 million kilograms) have bubbled out of the seafloor since the earthquake.
The team thinks the 1945 quake started the release - which they found still underway on the seafloor today - by estimating the amount of time it took for colonies of methane-dependent organisms to grow in the area. The colonies at the gas hydrate site were very small compared to areas where hydrocarbons have been seeping for much longer. In the latter the colonies are huge.
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Why all this matters is that gas hydrates are very common in the most seismically active regions of the world. So it seems rather likely, the researchers say, that earthquakes could be a trigger for releasing them from their graves.
IMAGE: A sample of methane-rich gas hydrate is pulled from a coring tool that grabbed it from the Arctic seafloor, 2,550 meters under the waves. Credit: Helen Gibbons, USGS / ECS Project