Ancient Ice Melt Unearthed in Antarctic Mud

July 22, 2013
 
 



Ancient sediments recovered off the coast of East Antarctica suggest that the ice sheet repeatedly melted back 3 to 5 million years ago, contributing to sea level rise. (Cathy Stickley)
 
The East Antarctic Ice Sheet repeatedly melted back several hundred miles inland during several warming periods 3 million to 5 million years ago in the Pliocene Epoch, according to a new study in the journal Nature Geoscience.
 
While most of the West Antarctica and Greenland ice sheets are thought to have melted during this time, the East Antarctic Ice Sheet was considered more stable due to its larger size and higher elevation. The study shows that the East Antarctic ice sheet is also vulnerable to substantial melting under warmer climates than today.
 
During the Pliocene, seas are thought to have risen up to 70 feet higher than today due to the melting ice sheets, about half of which may have come from East Antarctica. It is not well know how fast sea levels rose, but it is currently understood to take millennia.
 
The research team say understanding this glacial melting during the Pliocene Epoch may give us insights into how sea levels could rise as a consequence of current global warming. This is because the Pliocene Epoch had carbon dioxide concentrations similar to now and global temperatures comparable to those predicted for the end of this century.

“We suspected from the sea level estimates that East Antarctic ice was at risk from warming, but this is the first good evidence from East Antarctica itself that the ice on low-lying areas melted back repeatedly during the Pliocene,” said study coauthor Trevor Williams, a marine geologist at Columbia University’s Lamont-Doherty Earth Observatory. 

Roughly the size of Australia, the East Antarctic Ice Sheet is the largest ice mass on Earth. Its size has ebbed and flowed from its formation 34 million years ago to about 14 million years ago, when scientists believe the ice sheet stabilized. But the study results suggest that the East Antarctic Ice Sheet was more sensitive to Pliocene warming than previously thought. Carbon dioxide levels at the time were roughly the same as they are today, at 400 parts per million, while temperatures were 3 to 5 degrees F higher—within the range projected for 2100.
“Our study underlines that these conditions have led to a large loss of ice and significant rises in global sea level in the past,” said study coauthor Tina Van De Flierdt, a scientist at Imperial College London. “Scientists predict that global temperatures of a similar level may be reached by the end of this century, so it is very important for us to understand what the possible consequences might be.”
 
The evidence comes from columns of mud retrieved from the sea bed off the coast of Wilkes Land, East Antarctica, on a 2010 research expedition organized by the Integrated Ocean Drilling Program. In the mud, researchers identified a chemical fingerprint that allowed them to identify where on the continent the mud originally came from. In the sedimentary layers deposited under warm climates, they tracked the mud back to rocks currently hidden beneath the ice sheet; the only way so much mud could have been carried off to sea is if the ice sheet had retreated inland, eroding those rocks, they concluded.
 
The researchers suggest that the region’s low-lying topography, including basins over a mile deep, made the overlying ice sheet susceptible to melting. The ice came into direct contact with seawater and as the ocean warmed it ate away at the ice.
 
In the next phase of the project, researchers will further analyze sediments recovered off the coast of East Antarctica to estimate how rapid and extensive the ice sheet’s advances and retreats were during the Pliocene. “We were surprised by the varying composition of the fine sediments, and want to know what the different grain sizes and types of minerals can tell us about the ice sheet’s movements in the past,” said study coauthor Sidney Hemming, a geochemist at Lamont-Doherty. 
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