Antarctic Team Discovers Mechanism for Massive Ice Shelf Collapse


View of the calving front of the Crane Glacier and open water looking west into the interior drainage system. Water depth here is over 1200 m deep. Photo by Michele Rebesco, 2006.

                           
Special to USF News

ST. PETERSBURG, Fla. (Sept. 11, 2014) - Newly published research on the geologic history of an area of the Antarctic Peninsula where a large floating ice shelf disintegrated in the early 2000’s indicates that the cataclysmic break-up was primarily a result of a rise in air temperature and melting at the surface of the ice, rather than a rapid change in the structure of the glacier.


The new findings support the idea that surface warming of the ice and its after-effects — such as the development of melt ponds and crevasses, or cracks, in the surface of the ice that allow warmer water to infiltrate down into the ice — can cause such a dramatic collapse.


The under-cutting of the bottom of an ice shelf by relatively warm ocean water — which scientists have demonstrated is occurring elsewhere in Antarctica and in Greenland — also causes ice shelves to collapse.


But the new study indicates that most of the changes at the base of the Larsen B ice shelf on the Peninsula, the portion of Antarctica that extends northwards toward South America, had already occurred at the end of the last ice age.


“Now, we can recognize two distinct mechanisms for the destabilization of ice shelves,” said Eugene Domack, a lead author on the new paper and a professor of geological oceanography at the University of South Florida. “This research gives us at least two “devils’ to worry about.”


The findings were published in the Sept. 11 edition of the journal Science.


Scientists are eager to understand how changes in air or ocean temperature may affect the behavior of ice shelves because the shelves act as regulators of the flow of ice from Antarctica.


View looking southwest of the seismic reflection system being towed through the water behind the RVIB N. B. Palmer. The Nordensköld Coast is in the background. Photo by Michele Rebesco, 2006.

The ice sheet contains enough freshwater, that if it were to collapse into the sea and melt completely, global sea level would rise by nearly 200 feet.


While researchers earlier this year published findings that indicate that such a collapse may well have begun in another part of Antarctica, that process is expected to take centuries to complete. The potential response of the ice sheet to air temperature increases can be much faster.


The new work, by an international team of researchers focuses on the collapse of the Larsen B, a slab of ice the size of the state of Rhode Island located on the eastern coast of the Antarctic Peninsula that disintegrated in 2002.


The research was supported by a grant from the Division of Polar Programs in NSF’s Geosciences Directorate: http://www.nsf.gov/awardsearch/showAward?AWD_ID=1430002


The Division manages the U.S. Antarctic Program, which coordinates all U.S. research on the southernmost continent and in the Southern Ocean.


The new work has major implications for the study of ice-sheet dynamics, which, in turn, are important in understanding how the vast Antarctic ice sheets will behave, as global temperature increase.


Ice shelves play a vital role in the behavior of the enormous mass of freshwater ice that covers the Antarctic content, acting as dams that regulate the flow of ice into the sea.


One known and relatively well-understood means by which rising temperatures can cause the erosion of ice shelves is that relatively warm ocean water cuts away at the underside of the shelf itself, at the grounding line, where the ice shelf and sea floor meet. By cutting away at the grounding line, and warming the underside of the ice, the shelf is weakened from below and may eventually break way.


But, according to the newly published research, an entirely different mechanism caused the Larsen B to collapse.


“Satellite observations that were made [of the Larsen B] suggested that maybe there was another mechanism at work here, based on melt water ponding, crevasse propagation, and cracking of the ice. The sudden disintegration removes the buttress that held back the land-based ice, and allowed it to surge into the ocean,” Domack said.


He added “What we found was that evidence from the sea floor showed that very little has changed with respect to melting by the ocean in this area since the end of the last ice age. That tells us that the 2002 event was almost entirely driven by surface melting and a warmer climate.”


The new research, Domack noted, does not negate the model in which ice shelves are undercut at the grounding line, but simply includes a new possibility for scientists to consider when studying ice-shelf dynamics.


“Grounding line systems need to be studied in more detail and need to be sampled directly in order to understand how a large ice mass behaves under changes in ocean temperature and sea level rise,” he noted.


“International efforts are being made to sample, also by drilling, such grounding line systems, in both hemispheres” explains Michele Rebesco, an OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale) researcher involved in the project. “It’s important to understand the mechanism, the development and the time frame invoved in this process. This is related to a recent cruise we made in 2013 in North Western Barents Sea”.


Domack also noted that the collapse of the Larsen B shelf has, itself, contributed greatly to scientific knowledge in a range of fields.


Previously, researchers including Domack have looked at the biological implications of the Larsen B collapse, including the surprising find of a seafloor community, surviving in the sunless depths of the ocean without any apparent source of nutrients and energy.


But, he added, “it’s the unusual experience of collapse of the Larsen B that allowed us to get in there and sample the grounding line, which led to this new finding.”