Mid-Atlantic Methane Mystery

Researchers have discovered 570 offshore sites where methane is likely bubbling up from the seafloor 30 to 100 miles offshore from North Carolina to Massachusetts (Nat. Geosci. 2014, DOI: 10.1038/ngeo2232). This finding suggests that methane leakage into the Atlantic is more widespread than many scientists believed. The study’s leaders, however, say these submarine seeps are negligible sources of atmospheric greenhouse gases.

Still, the plumes are not innocuous, says Carolyn D. Ruppel of the U.S. Geological Survey, who uncovered the gas leaks working with researchers from Mississippi State University, Brown University, and government contractor Earth Resources Technology. Methane readily dissolves in ocean water, where microbes oxidize the gas to produce carbon dioxide. This CO₂ goes on to acidify the water and deprive it of oxygen. The team doesn’t yet know how these newfound seeps are affecting underwater ecosystems, but perhaps a more pressing mystery is the methane’s origin.

Methane routinely percolates from sections of seafloor above hydrocarbon reservoirs or atop regions where tectonic activity can liberate gas trapped in sediment. But these scenarios don’t describe the Atlantic margin where the seeps were discovered, says geologist Adam Skarke of Mississippi State.

The team believes most of the gas is escaping from solids called methane clathrates, which keep methane molecules caged within a crystalline lattice of water ice. But roughly two-thirds of the seeps are bubbling at depths where subtle temperature increases could cause the clathrates to release their gas. Warming oceans might therefore drive higher methane emissions.

“The climate system probably is affecting these clathrates,” Ruppel says, but how is unclear. Some of the seeps may have been active for more than 1,000 years, meaning there are many driving forces to consider. “Some are probably anthropogenic; some are more natural.”

Even in a warming world, methane and CO₂ released from the ocean will be negligible compared with human emissions, says David E. Archer, a climate scientist at the University of Chicago who was not involved with the study. “However, the cumulative impact of releasing that carbon from clathrates,” he adds, “could amplify the long-term impact of fossil-fuel release.”