By Douglas Main
On the seafloor just off of the U.S. East Coast lies a barely known world, explorations of which bring continual surprises. As recently as the mid-2000s, practically zero methane seeps — spots on the seafloor where gas leaks from the Earth's crust — were thought to exist off the East Coast; while one had been reported more than a decade ago, it was thought to be one of a kind.
But in the past two years, additional studies have revealed a host of new areas of seafloor rich in seeps, said Laura Brothers, a research geologist at the U.S. Geological Survey. And surrounding these seafloor vents, scientists have found a variety of unique life forms, like mussels and crabs, that survive via symbiotic relationships with methane-eating bacteria, Brothers told LiveScience's OurAmazingPlanet. New technologies have allowed scientists to keep locating new seeps, including one that may be the largest in the world. The findings have changed geologists' understanding of the processes taking place beneath the seafloor.
"These newly discovered (seafloor) communities show that there is much more seafloor methane venting then we previously thought, and suggests that there are many more seeps out there that we don't know about," Brothers said.
A study co-authored by Brothers, published online last month in the journal Geology, found several large communities of mussels and other animals at two spots off of the Carolinas where methane is seeping from the seafloor, Brothers said. Although one of these spots had already been discovered, the amount of life the researchers found covered an area about six times larger than previously thought, Brothers said. And the mussels cover a total area equivalent to about 40 football fields.[Photos: Unique Life Found at East Coast Gas Seep]
"We found these in an area we thought we knew well," she said.
The largest seep in the Atlantic
An even larger, previously unknown vent was found off the coast of Virginia, in research by Steve Ross, a scientist at the University of North Carolina at Wilmington, and Sandra Brooke, a scientist at Florida State University. Discovered near the Norfolk submarine canyon, the vent is the largest in the Atlantic, and possibly in all of the world's oceans, Ross told LiveScience.
North America's continental shelf, the underwater edge of the continent that borders the Atlantic Ocean basin, is littered with underwater canyons etched by rivers thousands of years ago when the region was above sea level. These canyons remain little explored, Ross said. But he is helping to change that through his work aboard the Okeanos Explorer, a ship owned by the National Oceanic and Atmospheric Administration (NOAA), which for the last three years has been working to explore these submarine canyons.
Scientists locate the seeps by producing images of methane gas bubbles (and where they originate) using multi-beam sonar, which calculates the amount of time and distance it takes for sound waves to travel from the ship to the bubbles and back. The same technique also produces detailed imagery of the seafloor. Remote-operated vehicles can then be dispatched to bring back photos of the ocean bottom, Ross said. All of these techniques are being used to document the gas seeps, he said.
Many more to be discovered
Further imaging of the seafloor by the Okeanos Explorer last fall revealed another three gas seeps southeast of Nantucket, Mass., at a maximum depth of 1 mile (1.6 kilometers) below the ocean surface. Along with several new findings that haven't been published yet, these newfound seeps add up to a total of about eight regions venting methane off of the East Coast, Ross said.
However, Carolyn Ruppel, a researcher at the USGS, advised against declaring a specific number of seeps found, since this total is constantly increasing and since the determination of what constitutes an individual "seep" can be vague. "We expect many more to be discovered," Ruppel said.
Much of this methane appears to be coming from methane gas hydrates, a crystallized form of methane stored in frigid sediment under the relatively high pressures of the deep ocean, Brothers said. Previously, it wasn't thought that a significant amount of this methane would be released from these deposits, which give up their methane only when weakened by lower pressures or rising temperatures. It's unclear why some of these hydrates are producing methane gas, but it's not a huge amount and unlikely to be enough to currently attract commercial interest, Brothers said.
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