BOSTON — Small particles trapped in the ocean's continental shelf may hold a key to redrawing the world's energy map.
Drilling some 250 miles off the Carolinas, scientists have directly measured these particles, known as methane hydrates, for the first time. They estimate that the region may hold enough methane - 35 billion tons - to meet natural-gas demand in the United States for the next century.
Worldwide, researchers add, similar deposits could lock up at least twice as much carbon as all the known land-based deposits of coal, oil, and gas combined.
Energy experts say that its not yet clear if these methane hydrates lend themselves to commercial development. They point to a number of significant engineering hurdles that would need to be overcome.
Even so, the prospect is generating a small flurry of international activity, particularly among countries that have few energy resources. While the US explores its site off the Carolinas, Japan is making plans to drill
for offshore hydrates in 1999. South Korea and India reportedly are eyeing their hydrate stores.
Global estimates of offshore deposits are dicey, notes Charles Paull, professor of geology at the University of North Carolina in Chapel Hill and coleader of the drilling expedition, which an international collaboration.
"You're sampling an incredibly small part of the earth's surface and then inferring a total amount," he says. "The real issue is: Are these reservoirs really big?"
Based on the latest research, reported in the current issue of the journal Nature, "The answer, resoundingly, is yes," he says.
Although scientists have made hydrates in the lab for decades, they were unknown in the environment until 1970. Researchers taking core samples from the seafloor were surprised to find that back in the lab, some of their samples fizzed and popped on the tabletop as the icy hydrates warmed and became methane gas.
They had taken their samples from the continental margins, where the plateau-like continental shelf tumbles to the deep-sea floor in a series of ridges and canyons. Organic material from the biologically rich margin settles to the seabed, where it mixes with the sediment. Bacteria feed on the organic material, giving off methane as waste.
At depths of nearly two miles, the temperature and pressure are sufficient to bind the methane with water as tiny crystals between grains of sediment.
Because hydrate deposits give off a unique echo in response to sonar or to seismic waves generated by underwater explosions, the deposits began to appear frequently as marine scientists used these techniques to study the sea-floor's dramatic contours. "They polka-dot the continental margins of the world in a continuous fashion," Dr. Paull says. Two months ago, a methane hydrate deposit was identified off the coast of Costa Rica, he notes.
By 1980, some researchers calculated the continental margins held 1,000 to 10,000 billion tons. "That sparked the interest," Paull recalls. "If deposits were anywhere near this size, they had to be important."
As more hydrate deposits were mapped, estimates of how much methane they held wandered all over the waterfront. Gerald Dickens, a marine geologist at the University of Michigan in Ann Arbor and the lead author of the article in Nature detailing the latest results, explains that drilling is the only way to establish how extensive a field is. Using a special coring device, he and his colleagues were able to retrieve and measure methane amounts.
Scientists are also keenly interested in the role these hydrates may play in stabilizing the seabed. Hydrates can be released naturally if the water temperature at their depth warms, or if the water pressure eases. Researchers suspect that when this happens, methane is unlocked from its icy form and escapes into the ocean, touching off underwater landslides, slumps, or sometimes even more catastrophic events.
William Dillon, a marine geologist at the US Geological Survey field office in Woods Hole, Mass., points to a 726 square-kilometer area of the Blake Ridge that "looks like it blew out some 10,000 to 15,000 years ago."
Pinning down these events would help determine whether the methane hydrates can safely be recovered, he says. If extracting hydrates destabilizes the seabed, the resulting slumps or landslides could damage drilling equipment and lead to large-scale, uncontrolled releases of methane into the ocean and perhaps the atmosphere.
They also could yield clues about the role the oceans' methane may have played in past climate change. Methane is 10 times more potent as a greenhouse gas than carbon dioxide.
The ocean is estimated to hold up to 3,000 times more methane than the atmosphere. Some researchers, including Dr. Dickens, think they see methane's signals in segments of core samples whose ages coincide with past sudden, rapid increases in temperatures worldwide.