Megabergs left scars in Arctic.

Oceanographers have discovered in the Arctic seafloor long grooves carved by ancient, giant icebergs far larger than any seen today. The evidence of these megabergs challenges previous ideas about what kind of ice covered the Arctic during the last ice age.

Peter R. Vogt and Kathleen Crane of the Naval Research Laboratory in Washington, D.C., and Eirik Sundvor of the University of Bergen in Norway detected the iceberg plow marks while mapping the seafloor northwest of Spitsbergen using a side-scan sonar on the Norwegian ship R/V Hakon Mosby. The depth of these scars indicates that some bergs extended 700 meters below the ocean surface.

"These are, to our knowledge, the first iceberg plow marks mapped in the Arctic Ocean proper and perhaps include the deepest iceberg drafts so far documented anywhere," the researchers write in the May GEOLOGY. Only the Antarctic today produces icebergs even approaching this size. The deepest iceberg keels ever measured reached 330 m below waterline, although researchers have hypothesized that some Antarctic icebergs today have keels 400 m deep.

Vogt and his coworkers discovered the plow marks along the flanks of a submerged feature called the Yermak Plateau. Because the top of the plateau shows few gouges, the oceanographers propose that it was beveled smooth by a thick ice shelf during the most recent ice age.

Climate researchers have long thought that the Arctic held only thin sea ice during the last glacial epoch. But the new results raise the possibility that a floating ice sheet some 400 to 600 m thick could have covered much of the ocean, say the researchers. If so, it may have connected the great glacial sheets covering North America and northern Europe.