By Sid Perkins, Science News
During the last couple of decades, scientists poring over satellite images have noticed several large icebergs breaking up as they wafted along a particular stretch of the Antarctic coast. Now, thanks to data gathered in part by an instrument-laden iceberg, the researchers know why: The ice masses were crashing into a previously unreported submarine ridge.
Most icebergs that calve from Antarctica’s Ross Ice Shelf are carried by shore-hugging currents past Cape Adare, a section of coast that lies south of New Zealand. That remote stretch of ocean is covered with sea ice about 10 months of the year, says Seelye Martin, an oceanographer at the University of Washington in Seattle. But in October 2005, iceberg B15A — the largest remnant of a 300-meter-thick, Connecticut-sized berg that split off the ice shelf in March 2000 — cleared a path as it bulldozed through the area.
Such massive icebergs break free of their ice shelves only once every decade or so and can last for years if collisions don’t break them into smaller pieces. Before sailing past Cape Adare, B15A had spent much of its life at sea stuck in shallow water (SN: 1/15/05, p. 45).
Scientists had installed several instruments on B15A early in 2001 — including a seismometer, a compass, and GPS equipment — so they could monitor the megaberg as it moved north, melted and broke apart (SN: 5/12/01, p. 298). Those instruments indicate that in late October 2005, B15A briefly came to a stop, rotated a few degrees counterclockwise, and began cracking up, a splintering also detected by seismometers on shore and by Earth-orbiting satellites.
Sonar surveys in the same region during a rare ice-free interval revealed a 9-kilometer-long, previously unrecognized undersea ridge, Martin and his colleagues report online June 18 in Journal of Geophysical Research–Solid Earth. That shoal, whose peaks lie at a depth of about 215 meters, sits about 25 kilometers off Cape Adare, smack dab in the path of the coast-hugging currents. “This is a big navigational hazard for icebergs,” Martin notes. The ridge lies too deep, though, to be a danger to ships that might navigate these mostly ice-bound waters, and many small icebergs have passed over the shoal without incident.