Methane from more than 500 natural, gas-exuding vents on the seafloor is leaking into the Atlantic Ocean along the coast of the eastern U.S., researchers report in Nature Geoscience this week. If large volumes of this potent greenhouse gas reach the overlying water or the atmosphere, it could add to ocean acidification and climate warming.
Cold seeps are areas where gases and fluids leak into the water from sediments, and they’re found in more places than hydrothermal vents, which is where new crust is formed and hot fluids are emitted. Until now, only three seep areas had been identified beyond the edge of the continental shelf between Florida and Maine.
Now, a team led by Adam Skarke of Mississippi State University used sonar instruments onboard NOAA’s remotely-operated Okeanos Explorer to scan 94,000 square kilometers of the water column along the Atlantic margin -- between the coastline and the deep ocean, at depths of 50 to 1,700 meters -- from Cape Hatteras, North Carolina, to Georges Bank, Massachusetts. They identified at least 570 methane seeps.
Methane often naturally leaks from the seafloor, especially in petroleum basins like the Gulf of Mexico or on tectonically active continental margins like the U.S. Pacific Coast, Skarke explains in a news release. “Widespread seepage had not been expected on the Atlantic margin,” he adds, a technically passive area without any underlying petroleum.
Pictured to the right, methane seeping from near a mound covered with deep-sea mussels at around 1,400 meters water depth just south of Norfolk Canyon, offshore Virginia.
These large volumes of methane are stored in ocean sediments as methane clathrate hydrate -- a naturally-occurring, ice-like crystalline structure. The majority of the seeps (around 440 of them) occur at the shallowest conditions where marine hydrates can stably exist. That means small changes in water temperature could destabilize the “methane ice,” releasing the gas.
“Warming of ocean temperatures on seasonal, decadal or much longer time scales can cause gas hydrate to release its methane, which may then be emitted at seep sites,” says Carolyn Ruppel from the U.S. Geological Survey in a statement. Though some subsets of the seeps have likely been active for over 1,000 years already.
While methane in the shallow-water seeps could directly reach the atmosphere, most of the seeps are still too deep for the methane to directly reach the atmosphere. Instead, gas released in the water column can be oxidized to carbon dioxide, which then increases the acidity of ocean waters and reduces oxygen levels.
The emanating gas hasn’t been sampled yet, though researchers believe most of the leaking methane is produced by microbes digesting organic matter in shallow sediments -- not the type found in deep-seated reservoirs being tapped for natural gas.
Images: NOAA Okeanos Explorer Program, 2013 Northeast U.S. Canyons Expedition