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  Geotimes - June 2007 - Antarctic ice may be grinding to a halt

Antarctic ice may be grinding to a halt

Some of Antarctica’s ice sheets may not be in as much danger as once thought. New research indicates that a wedge of sediment beneath the outer edge of a major ice stream may be stabilizing it from retreating as the climate warms. This discovery may change the whole picture of Antarctic ice dynamics, say researchers who discovered the wedge.

In December 2004, researchers traveled to the Whillans Ice Stream on the West Antarctic Ice Sheet to track its movement. Using GPS sensors, they located the point at which the ice drops off its base and begins floating in seawater, or the stream’s grounding line. Much to the researchers’ surprise and delight, says team member Sridhar Anandakrishnan, a glaciologist at Pennsylvania State University, they discovered that a thick sedimentary wedge had formed beneath the ice stream leading up to the grounding line. The wedge indicates that sediments have been rapidly building beneath the ice stream, Anandakrishnan and colleagues wrote in a pair of papers in Science on March 30. And that sedimentation, they say, will provide the stream with “substantial stability” against a sea-level rise of up to 10 meters (33 feet).

This is the first documentation of such stabilization of a modern ice stream, says John Anderson, an oceanographer at Rice University, who wrote an accompanying commentary in the same issue of Science. Researchers predicted that such sedimentary wedges exist, he says, because numerous relict wedges dating as far back as 25,000 years ago exist on the floor of the Ross Sea. Furthermore, such wedges also exist at the base of fast-moving modern tidewater glaciers, Anandakrishnan adds. The new wedge matches closely in size, shape and setting with many of the relict wedges in the Ross Sea, he says.

Not until satellite and radar technologies improved, however, were researchers able to map topographic changes on the ice sheet to the centimeter-level and thus gain direct evidence of such sedimentary wedges in the Antarctic. The team found that about 13 kilometers (8 miles) before the grounding line, the ice stream begins to form an upward ramp, which then drops off just before the grounding line. The ice has advanced just beyond the crest of its wedge, says Richard Alley, a paleoclimatologist at Pennsylvania State University, lead author of one of the Science papers.

Right now, Alley says, the wedge appears to be holding the ice stream stable. And it will likely stay that way until the dynamics change, Alley says, such as sea level rising more than 10 meters (33 feet) and above the grounding line or warmer temperatures melting the top off the ice streams. Most likely to affect the ice stream’s stability, however, would be “warmer ocean waters melting beneath the ice shelf, removing its buttressing effect,” which would cause the ice to calve off in icebergs, he says. When the grounding line area destabilizes, he adds, ice will likely retreat very quickly until it reaches another steady state.

“Our first knee-jerk reaction to this news is ‘Wow, this is nice. The ice sheet is a little more stable than we thought,’” Alley says. But “unfortunately, there’s a big asterisk.” Ice flow models do not include stabilizers such as a sediment wedge. If ice-flow models were “tuned” to match the current ice sheet or the history of the ice sheet, and the models do not include the effects of sediment wedges, then the models are “likely underestimating other sources of instability,” he says.

“The question that needs to be answered is ‘can the wedges stop a glacial retreat that is already happening, or do they simply stabilize a glacier for awhile before it starts retreating?’” says Christian Schoof a glaciologist at the University of British Columbia. If the ice streams are already retreating at a fairly steady pace, he says, they may not have time to build a stabilizing sediment wedge.

Another unknown, Schoof says, is whether these sediment wedges exist at the grounding lines of all ice streams or just one or two. The location and number of the relict wedges differs from one ice stream to another, “so the indication is that each ice stream may have retreated somewhat independently,” Anderson says. The relict wedges indicate that the retreat of the ice streams — some 700 to 1,000 kilometers (435 to 620 miles) over the last 25,000 years — has been “punctuated by pauses when the grounding line stabilized long enough to form these wedges,” he says. That may or may not be happening presently.

Megan Sever

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