And up through the ground came a bubbling greenhouse gas. Researchers have discovered 570 plumes of methane percolating up from the sea floor off the eastern coast of the United States, a surprisingly high number of seeps in a relatively quiescent part of the ocean. The seeps suggest that methane’s contribution to climate change has been underestimated in some models. And because most of the seeps lie at depths where small changes in temperature could be releasing the methane, it is possible that climate change itself could be playing a role in turning some of them on.
Most of the seeps are thought to be fed by methane stored in hydrates, crystal lattices of water ice that form under low temperatures and high pressures. Harvesting methane from hydrates in the sea floor has already aroused commercial interests; both Japan and the United States have embarked on pilot extraction projects. But the hydrates are also significant for climate scientists: This immense reservoir is thought to contain 10 times as much carbon as the atmosphere. The gas, if it reaches the atmosphere, is far more potent than carbon dioxide as a heat trapper. Even in the more likely event that aerobic microbes devour the methane while still in the ocean, it is converted to carbon dioxide, which leads to ocean acidification. Some scientists have implicated runaway methane hydrate releases in the catastrophic extinctions of marine life at the Permian-Triassic boundary, 252 million years ago.
The present study, published online today in Nature Geoscience, is based on data collected in a survey from 2011 to 2013 by the research vessel Okeanos Explorer. Equipped with a multibeam sonar along its hull, the vessel not only mapped the sea floor along a swath off the coast of North Carolina to Massachusetts, but also recorded reflections in the water column. Gas bubbles of methane stood out as a distinctive signature. Most of the seeps were found at depths of 180 to 600 meters along the upper slope of the continental margin. This is the area where the continental shelf rapidly falls to the 5000-meter-deep abyssal plain of the ocean.
“So far everybody has been looking at small spots. This is the first time anyone has systematically mapped an entire margin,” says Christian Berndt, a marine geophysicist at GEOMAR in Kiel, Germany, who was not involved in the study. It was also a surprise because seeps are typically found above known methane reservoirs, or above regions of active tectonic activity. The continental margin was thought to be virtually devoid of seeps—until scientists studied the sonar data. “They found that there was much more methane coming out than was suspected beforehand,” Berndt says.
For a handful of the seeps, the researchers were able to take pictures with a remotely operated submersible. They found carbonate rocks associated with the seeps that would have taken several thousand years to form. But some of the seeps are shallow—and are at the critical depth where hydrates fall apart—so they could be sensitive to rising ocean temperatures on much shorter time scales, says Carolyn Ruppel, a co-author of the new study and chief of the gas hydrates project at the U.S. Geological Survey in Woods Hole, Massachusetts. “There are reasons to believe that some of the present seepage has been triggered by changes in oceanographic conditions,” she says.
Proving that climate change is directly responsible could be difficult, Berndt says. In January, he and colleagues published a study in Science on methane seeps in the Arctic Ocean off the coast of the island of Svalbard, where temperature changes are occurring more rapidly. Berndt found evidence that the seeps there had existed for at least 3000 years and saw no evidence that the ocean sediments had been heating up—and releasing methane—on the decadeslong timescales associated with climate change. At the very least, though, he says, the Atlantic Ocean study shows that ocean and climate modelers should start to incorporate methane inputs from many more types of seafloor terrains around the world. “We have this extra source here,” he says. “Not much attention has been paid to it.”
Jens Greinert, who heads the deep-sea monitoring unit at GEOMAR, downplays the effect of the new seeps on the atmosphere or ocean chemistry because the magnitude of the releases is dwarfed by human-associated inputs, such as livestock, or even other marine sites. “These little bits of bubbling here or there will not make a memorable impact,” Greinert says. He is more interested in what will happen as the world warms. “It becomes interesting only if you have a catastrophic release,” he says.