Methane key to Arctic mystery mounds.

Methane key to Arctic mystery mounds

Explanations in science often must await the right time. Since 1974, geologists have puzzled over mounds of marine fossils found on some bleak Arctic islands. But seemingly unrelated discoveries in the last five years in other parts of the world have paved the way for a crew of Canadian scientists to decipher the ancient mounds.

The Arctic deposits are roughly circular and stand up to 8 meters high, shaped sometimes like a wide hill and other times in pillar forms. Researchers found them first on Ellef Ringnes Island and more recently on Prince Patrick Island, both along the rim of the Arctic Ocean. Within the carbonate rock of the mounds are thousands of fossilized mussels, worm parts, fish teeth and other evidence of undersea oases. When these animals lived, the islands were located at the bottom of the sea, covered by at least 400 meters of water.

Benoit Beauchamp of the Geological Survey of Canada in Calgary says he and his colleagues were able to explain the mounds only when they became aware of recent work in the Gulf of Mexico and other spots. In these areas, scientists have discovered dense communities of mussels and worms clustered around cracks where natural gas (methane) and oil bubble to the surface. Bacteria living on these hydrocarbons and on hydrogen sulfide provide food for the more complex creatures.

The Arctic mounds are the first proof that these hydrocarbon communities existed in the distant past, according to a report in the April 7 SCIENCE by Beauchamp, J. Christopher Harrison and Walter W. Nassichuk of the Survey along with H. Roy Krouse of the University of Calgary and Leslie S. Eliuk of Shell Canada.

Evidence for a methane seep comes from analysis of carbon isotope ratios in the carbonate rock around the fossils. Beauchamp's group found the carbonate had extremely low ratios of carbon-13 to carbon-12 in comparison to normal seawater. Methane, which is depleted in carbon-13, must have provided the source for the carbon in the rock, say the researchers.

Formed by the decay of ocean organisms buried under sediments, methane and other hydrocarbons become trapped in porous layers of rock under the ground. Fractures in the crust provide a plumbing system that allows the light hydrocarbons to seep upward. The Arctic mounds sit along known faults that would have tapped methane deposits in the crust, according to the geologists.

In this area of the Arctic, the geologic setting is quite similar to regions off the Louisiana coast that hold significant oil and methane deposits, says Beauchamp. The discovery of ancient seeps, he says, "has some economic implications. It might tell us that hycrocarbons are still present below the surface." The mounds may also help reveal how these seep communities evolve.

James Brooks, who has worked on hydrocarbon seeps in the Gulf of Mexico, says the Arctic deposits call to question whether geologists have found any fossil hydrocarbon seeps before. "They could have been identified as shallow-water deposits when they were actually deep-water cold-seep deposits, which would change the geologic interpretation considerably," says Brooks, a geochemist at Texas A&M University in College Station.

The methane vents are called cold seeps to distinguish them from another kind of seafloor oasis where jets of superhot water support lavish communities of tubeworms, shrimp and other animals. In the past several years, geologists have found fossil examples of these hot-vent communities from tens of millions of years ago.