By Gail Gallessich

Methane, a potent greenhouse gas, is emitted in great quantities as bubbles from seeps on the ocean floor near Santa Barbara. About half of these bubbles dissolve into the ocean, but the fate of this dissolved methane has been uncertain. Researchers at UC Santa Barbara have discovered that only 1 percent of this dissolved methane escapes into the air––good news for the Earth’s atmosphere.

Coal Oil Point (COP), one of the world’s largest and best-studied seep regions, runs along the northern margin of the Santa Barbara Channel. Thousands of seep fields exist on the ocean bottom around the world, according to David Valentine, associate professor of Earth science. Valentine and other members of a seeps group studied the plume of methane bubbles from the seeps at COP.

The amount of methane released from COP seeps is around 2 million cubic feet a day, according to Valentine. About 100 barrels of oil a day oozes out of this area as well. This is the first time that dissolved gases from the plume have been studied. The results were recently published as the cover story of Geophysical Research Letters.

When averaged over a century, methane is 23 times more powerful in warming the Earth than carbon dioxide.

“We found that the ocean has an amazing capacity to take up methane that is released into it––even when it is released into shallow water,” said Valentine. “Huge amounts of gas are coming up here, creating a giant gas plume. Until now, no one had measured the gas that dissolves and moves away in the plume.”

He hypothesized that the methane is oxidized by microbial activity in the ocean, relieving the ocean of its methane “burden.” “We showed that the currents control the fate of the gas, and supply it to bacteria in a way that allows them to destroy the methane,” said Valentine.

To arrive at this hypothesis, he and lead author Susan Mau, a postdoctoral fellow in Valentine’s lab, tracked the plume down current from the seeps at 79 surface stations. They found that the methane spread over 70 square kilometers in a 280-square-kilometer study area.

The authors sampled the water by boat on a monthly basis. They found methane concentrations that corresponded with changes in surface currents. They also found that higher winds released more methane into the atmosphere. They discovered that on average about 1 percent of the dissolved methane escapes over the long term into the atmosphere in the area they studied. This led to the hypothesis that most of the methane is transported below the ocean’s surface, then oxidized by microbial activity.

Valentine said that while the seeps at COP are among the largest in the world, they can be found just about anywhere.