by Liz Kalaugher, environment research web, April 22, 2008
On 28th and 29th February 2008, a 400 sq. km area of the Wilkins Ice Shelf on the Antarctic Peninsula broke up. That adds to the total of seven ice shelves that have disappeared in the West Antarctic between 1995 and 2002, a phenomenon believed to be due to increased temperatures.
According to Angelika Humbert of Darmstadt University of Technology and Matthias Braun of Bonn University, both in Germany, the area of loss in February is less spectacular than the effect on the whole ice shelf. The fracture created a large number of intersecting rifts in around 5000 sq. km of the northern part of the ice shelf.
"The failure zones from the break-up make the shelf vulnerable," said Braun.
Over the past two years, Humbert and Braun have examined 22 years' worth of European Space Agency remote sensing data for the region. They believe their study of the break-up has given them new insights into some of the mechanisms behind ice shelf failures.
Rifts and failure zones first started to develop on the Wilkins Ice Shelf in 1993/94, according to the researchers. In February 2008 a rift formed very quickly between the central part of Wilkins Ice shelf and the stabilizing Charcot Island -- on a scale of just minutes. The connection lost 40% of its total size.
The pair have mapped visible structures on the ice shelf, such as flow lines, rifts and grounded areas. They say that the Wilkins shelf is unusual in having lots of small ice rises -- areas that protrude above the surface where the ice shelf meets the seabed below. The rises, which on Wilkins generally have a diameter less than 1 km, divide the ice flow and cause small rifts, acting as nuclei for the development of failure zones. During the February breakup, these rifts typically extended to roughly 20 km either side of the rises. A number of new rifts also formed.
"The rift zones are connected now and that makes a part of the ice shelf vulnerable to breakup," said the researchers. This is the first time they have seen such a creation of new ice rifts and joining of existing rifts -- they believe it may be unique. The result is a more fragile ice shelf.
Humbert and Braun reckon that unequal buoyancy forces arising from differences in ice thickness may have caused the accumulation of bending stress, leading to this rift behavior. They are currently looking at further observations to find out more and say that more satellite measurements and field surveys of ice shelves are needed to help examine the causes of ice shelf disintegration.
The Wilkins Ice shelf is confined by four islands -- Alexander, Latady, Charcot, and Rothschild, giving it a fairly unique structure. The northeastern part of the ice shelf is around 50-150 m thick while the western and southern regions are thicker, with ice 170-270 m thick.
The shelf is unusual in that it receives only a small contribution from glaciers, gaining most of its mass from snow build up. Wilkins loses most of its mass by basal melting, as well as experiencing discontinuous fast breakup events rather than the slower, more standard iceberg calving that occurs elsewhere. The shelf is also comparably warm, with a temperature of -9 °C at the surface.
Humbert and Braun reported their work at the European Geosciences Union General Assembly in Vienna, Austria. They have submitted papers to Geophysical Research Letters and to The Cryosphere.
About the author
Liz Kalaugher is editor of environmentalresearchweb.
Link to article: http://environmentalresearchweb.org/cws/article/research/33873