UF Research Shows Antarctic Ice Sheet Was Unstable In Past Warming Periods

The findings, which will appear in an article in the journal Science on Friday, do not address the current debate over global warming and its impact on the polar ice caps. But the research adds to a growing body of evidence suggesting polar ice can undergo huge and rapid changes that may be tied to climate, said Sharon Kanfoush, a UF graduate student and lead researcher on the project.

"The fact that the ice sheet has behaved very dynamically in the past, or undergone very rapid changes in the past, implies that such changes are possible in the future," Kanfoush said.

Geologists have known for several years that the Northern Hemisphere ice sheet has shed huge chunks of ice in the form of icebergs at intervals of 1,000 years or more during the last period of glacial advancement, which extended from about 60,000 to 20,000 years ago. Researchers believe the ice sheet "calved" virtual armadas of icebergs just before extreme climatic periods that, over Greenland, resulted in as much as a 60-degree atmospheric temperature increases in 50 years. The Northern Hemisphere ice sheet instability and subsequent warming spikes have spurred considerable discussion and debate. Until the UF work, however, no one had sought to broaden the issue by probing what happened to the Antarctic ice sheet in the same glacial period, said David Hodell, professor of geological sciences and the other UF researcher on the team.

The team discovered that parts of the Antarctic ice sheet were unstable during the warming spikes in the Northern Hemisphere.

Two factors may be at work, Hodell said. One, the higher temperatures may have caused the Northern Hemisphere glaciers to melt, raising sea levels and causing grounded ice in the seas near Antarctica to become unstable. Alternatively, the higher temperatures may have altered the circulation of global ocean currents, bringing more warm water to Antarctica and causing its ice shelf to begin to break up.

Four massive icebergs broke off from the Ross Ice Shelf in Antarctica this spring, the latest of several iceberg events that have focussed attention on global warming and its possible impact on Antarctica. Hodell said the UF team's research has no direct bearing on the recent events because the climate during the glacial-advancement period was fundamentally different from the climate in the modern era of glacial retreat.

But Hodell added that if water temperature increases or rising sea levels caused the Antarctic ice instability in the past, similar increases could do the same today.

"You could make the analogy that finding these events during the last glacial period suggests that these grounded ice shelves are inherently unstable and susceptible to either temperature or sea level change," he said.

The team came to its conclusions based on analysis of sediment cores from a series of sites in the southern Atlantic obtained on two separate research cruises, including one cruise on the JOIDES Resolution, a scientific research ship operated by the International Ocean Drilling Program. The cores, samples of material from as far as 4,600 meters beneath the sea surface, contained large grains of quartz derived from Antarctica as well as fragments of volcanic material. The researchers concluded the minerals were deposited by melting icebergs that had split off from Antarctica near the Weddell Sea, then drifted north and east in the south Atlantic before melting. The Ocean Drilling Program is an international research partnership of scientific institutions and governments.

The other members of the research team are Christopher Charles and Graham Mortyn, both with the Scripps Institute of Oceanography at the University of California at San Diego; Thomas Guilderson of the Lawrence Livermore National Laboratory in Livermore, Calif.; and Ulysses Ninnemann of the Lamont-Doherty Earth Observatory of Columbia University.