December 9, 2012
Antarctic Ice Shelf Holding Together Thanks To Suture Zones
redOrbit Staff & Wire Reports - Your Universe Online
The largest remaining Antarctic ice shelf contains several cracks and crevasses that could make it prone to collapse, but also areas in which different types of frozen water blend together to create areas of bendable ice that help hold it together, according to a new study presented late last week.
Experts at the University of Colorado, Boulder's Cooperative Institute for Research in the Environmental Sciences (CIRES) discovered the resilient areas known as suture zones while studying the Larsen C Ice Shelf in the Antarctic Peninsula over the past four Southern Hemisphere summers.
According to a December 7 statement released by the university, the CIRES team found gaping crevasses penetrating upwards from the bottom of the ice shelf, known as basal crevasses, made it "more susceptible to collapse," but the suture zones help hold it together as the Larsen C Ice Shelf tries to avoid the fate of the Larsen A and Larsen B Ice Shelves, which collapsed suddenly in January 1995 and February 2002, respectively.
Researchers believe the collapses of those two ice shelves were at least partially caused by a 4.5 degree Fahrenheit increase in temperature in the Antarctic Peninsula since the middle of the last century -- a warming level six times that of the global average, according to the researchers. The temperature hike led to increased meltwater production, which led to more liquid to collect atop the ice shelves.
That water then drained into the surface crevasses, acting like a wedge, cracking them into separate icebergs and quickening the disintegration process, the university said. However, because of the sudden way in which the Larsen A and B shelves met their demise, scientists were unable to study their structures to see whether or not they had played a role in the process -- which was the reason why doctoral student Daniel McGrath and the CIRES team made sure they were able to analyze the Larsen C ice shelf before it meets a similar fate.
"The scientists used ground penetrating radar to map out the basal crevasses, which turn out to be massive," officials from the university explained. "The yawning cracks can run for several miles in length and can penetrate upwards for more than 750 feet. While the basal crevasses have been a part of Larsen C for hundreds of years, the interaction between these features and a warming climate will likely make the shelf more susceptible to future disintegration.
"The research team also studied the impact of suture zones in the ice shelf," they added. "Larsen C is fed by 12 distinct glaciers, which dump a steady flow of thick ice into the shelf. But the promontories of land between the glacial outlets, where ice does not flow into the shelf, allow for the creation of ribbon-like suture zones, which knit the glacial inflows together and which turn out to be important to the ice shelf's resilience."
A combination of different ice types combine to make the suture zones malleable, the researchers said. When the bottoms of the dozen glaciers begin to melt, it causes more buoyant freshwater to combine with surrounding seawater. The freshwater rises upward to thinner areas of ice between the glacial inflows, where it refreezes and creates suture zones comprised of ice that is more flexible than that surrounding it.
"It turns out that the resilient characteristics of the suture zones keep cracks, including the basal crevasses, from spreading across the ice shelf, even where the suture zone ice makes up a comparatively small amount of the total thickness of the shelf," the university said. "The CIRES team found that at the shelf front, where the ice meets the open sea, suture zone ice constitutes only 20 percent of the total thickness of the shelf but was still able to limit the spread of rifts through the ice."
McGrath, who presented his findings at the annual meeting of the American Geophysical Union (AGU) in San Francisco on Thursday, called the Larsen C Ice Shelf "the perfect natural laboratory" and said his team wanted to analyze its structure "in order to get a better understanding of the processes that affect ice shelf stability." He added the basal crevasses were "a pretty small part of the total ice thickness" but that it had "this really important role of holding the ice shelf together."