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Thick ice scraped rock bottom in Arctic

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10:58am, March 21, 2001
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Scuffs, scrapes, and gouges found atop undersea plateaus and ridges in the Arctic Ocean suggest that kilometer-thick ice shelves covered much of the ocean there during some previous ice ages. The unexpected features show up in sonar images collected by a U.S. nuclear submarine in 1999.

Parallel grooves mark the submerged Chukchi Plateau, north of Alaska, at depths as great as 700 meters. These features, spaced between 50 and 200 m apart, extend in some cases for more than 15 kilometers, says Leonid Polyak, a paleo-oceanographer at Ohio State University in Columbus. He and his colleagues describe their findings in the March 22 Nature.

Glaciers cause similar features on land, Polyak notes. The ice stream that scraped the undersea plateau probably flowed into the ocean from an ice sheet that straddled western Canada and eastern Alaska, he adds. Other features perpendicular to the grooves likely mark the retreat of the ancient ice sheet's grounding line, the offshore boundary where the ice sheet lifts off the seafloor.

Polyak's team also found a 50-km-wide gouge across the Lomonosov Ridge, the top of which is currently 1 km below the Arctic Ocean's surface. The shape of the scrape and the direction of its grooves indicate that it was scoured by an ice shelf flowing from the northern shores of Asia, more than 1,000 km away, Polyak says. The amount of eroded debris spilling down the ridge's southeastern flank implies that the ancient ice trimmed about 50 m off the top of the ridge.

"I think [Polyak's] finding will surprise a lot of people," says Robert Spielhagen, a paleoclimatologist at the Research Center for Marine Geosciences in Kiel, Germany. He notes that there's currently no evidence of a large ice sheet in northern Asia that would have spawned one or more large ice shelves during the last ice age.

Although the sea level was lower during recent ice ages, it probably didn't drop more than 150 m below current levels, Polyak notes. That means the ice shelf that scraped the top off the Lomonosov Ridge still had to be about 1 km thick, including the height of the ice that extended above sea level, he adds. In the Arctic Ocean today, winter ice reaches down only about 50 m.

It's unclear when the ice gouged out these features. Polyak believes that the scrapes were caused by ice sheets during several different ice ages.

Spielhagen says that some of the features that Polyak's team describes--including the wide scuff atop the Lomonosov Ridge--could have been caused by extremely large icebergs rather than ice shelves. Either way, he says, 1-km-thick ice shelves must have floated atop the Arctic Ocean at some time in the past.

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