And through the ground came a bubbling greenhouse gas. Researchers have found 570 plumes of methane leaking from the seabed off the eastern seaboard of the United States, a surprisingly large number of seeps in a relatively calm part of the ocean. The seeps suggest that the contribution of methane to climate change has been underestimated in some models. And since most seeps are at depths where small temperature changes could release methane, it’s possible that climate change itself is playing a role in activating some of them.
Most seeps are thought to be fueled by methane stored in hydrates, crystalline networks of water ice that form at low temperatures and high pressure. The harvesting of methane from seabed hydrates has already generated commercial interest; Japan and the United States have embarked on pilot extraction projects. But hydrates are also important for climatologists: this immense reservoir would contain 10 times more carbon than the atmosphere. The gas, if it reaches the atmosphere, is much more powerful than carbon dioxide as a heat trap. Even in the most likely event that aerobic microbes devoured the methane while it was still in the ocean, it would be converted to carbon dioxide, leading to acidification of the oceans. Some scientists have blamed the uncontrolled releases of methane hydrate in the catastrophic extinctions of marine life at the Permian-Triassic border 252 million years ago.
The current study, published online today in Geosciences of nature, is based on data collected during a survey from 2011 to 2013 by the research vessel Okeanos Explorer. Equipped with multibeam sonar along its hull, the vessel not only mapped the seabed along a strip off the coast of North Carolina to Massachusetts, but also recorded reflections in the column of water. Methane gas bubbles have established themselves 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 quickly falls into the abyssal plain of the ocean at a depth of 5,000 meters.
âSo far, everyone has looked at small points. This is the first time that someone has systematically mapped an entire margin,â says Christian Berndt, marine geophysicist at GEOMAR in Kiel, Germany, who did not participate in the study. This was also a surprise as the seeps are usually found above known methane reservoirs or over regions of active tectonic activity. The continental margin was thought to be virtually free of seeps, until scientists studied the sonar data. âThey discovered that there was a lot more methane coming out than previously suspected,â Berndt said.
For a handful of seeps, researchers were able to take pictures with a remote-controlled submersible. They found carbonate rocks associated with the seeps that would have taken several thousand years to form. But some seeps are shallow – and are at the critical depth where hydrates break down – so they could be sensitive to rising ocean temperatures on much shorter timescales, explains Carolyn Ruppel, co-author of the novel. study and gas hydrate project leader at the US Geological Survey in Woods Hole, Massachusetts. “There is reason to believe that some of the current seepage was triggered by changes in oceanographic conditions,” she said.
Proving that climate change is directly responsible could be difficult, says Berndt. In January, he and his colleagues published a study in Science on methane seeps into the Arctic Ocean off the coast of Svalbard Island, where temperature changes occur more rapidly. Berndt found evidence that seeps had existed for at least 3,000 years and saw no evidence that ocean sediments had warmed – and released methane – over decades associated with climate change. At the very least, he says, the Atlantic Ocean study shows that ocean and climate modelers should start incorporating methane inputs from many other types of underwater soils around the world. âWe have this additional source here,â he says. âWe didn’t pay much attention to it.
Jens Greinert, who heads GEOMAR’s offshore monitoring unit, minimizes the effect of new seeps on the atmosphere or ocean chemistry, as the scale of the releases is overshadowed by human-made inputs, such as than livestock or even other marine sites. . âThose little bubbling up here and there won’t have a memorable impact,â said Greinert. He’s more interested in what will happen as the world heats up. âIt only gets interesting if you have a catastrophic exit,â he says.