Giant seabed slides may have climate link.

British researchers surveying the Mediterranean seafloor have discovered a vast blanket of sand and silt laid down by an underwater landslide larger than any witnessed in historical time. The deposit formed at the peak of the last ice age, 22,000 years ago, a finding that supports the idea that submarine slides and climate influence each other.

The legacy of this ancient landslide lies between France and Algeria at a depth of 2,800 meters, report R. Guy Rothwell and his colleagues at the Southampton Oceanography Centre in the March 26 Nature. The researchers found the layer of deposited sediments, which measures 8 to 10 m thick, by probing the seabed with sound waves and drilling into selected sites. The volume of sediments is enough to bury the island of Manhattan beneath a pile of debris nearly the height of Mount Everest.

Marine geologists label such formations turbidites. These formations develop only during certain submarine landslides, when the mostly solid sliding mass breaks apart, mixes with water, and turns into a turbulent current. Flowing downhill, this roiling slurry can move fast enough to scour the seafloor, a process that adds more sediment to the flow.

Rothwell and his colleagues carbon-dated the turbidite to a time near the end of the ice age, when so much water was locked up in glaciers that global sea levels lay 120 m lower than they do today

Researchers have found many other examples of gigantic turbidites, but they lack such precise information about the timing of those slides, says David J. W. Piper, a marine geologist at the Bedford Institute of Oceanography in Dart mouth, Nova Scotia.

The timing is crucial because some scientists have suggested that the lowering of sea levels during the last ice age helped trigger oceanic landslides. According to this theory, the drop in ocean heights reduced pressure on the seabed and destabilized buried deposits of methane hydrates--icelike molecules that contain solid water and methane gas. As the hydrates broke down, the seafloor weakened and gave way as landslides.

If so, such landslides could have released the greenhouse gas methane into the atmosphere, where it would have warmed the climate, argue Piper and Euan G. Nisbet of the University of London's Royal Holloway College in Egham, England, in an accompanying commentary. In fact, they say, the warming triggered by one large slide could have destabilized methane hydrates in other locations.

"With regard to climate, these could be very important events," says Rothwell.

Researchers have looked for signs of such massive methane releases but have thus far found no direct evidence of them in ancient ice deposits.