June 13, 2018

Largest ice sheet on Earth was stable throughout last warm period

Antartica drill A drilling rig was used to collect sediment core in Antarctica. (Gavin Dunbar/ANDRILL) Download image

WEST LAFAYETTE, Ind. — The largest ice sheet on Earth was stable throughout the last warm period in geologic time, indicating it should hold up as temperatures continue to rise.

The East Antarctic Ice Sheet is the world’s largest potential contributor to sea level rise (175 feet, if the whole thing melted). Unlike the Greenland and West Antarctic ice sheets, though, it’s been resistant to melt as conditions warm.

New research published in Nature shows that land-based sectors of the East Antarctic Ice Sheet were mostly stable throughout the Pliocene (5.3 to 2.6 million years ago), when carbon dioxide concentrations in the atmosphere were close to what they are today – around 400 parts per million.

“Based on this evidence from the Pliocene, today’s current carbon dioxide levels are not enough to destabilize the land-based ice on the Antarctic continent,” said Jeremy Shakun, lead author of the paper and assistant professor of earth and environmental science at Boston College.  “This does not mean that at current atmospheric carbon dioxide levels, Antarctica won’t contribute to sea level rise. Marine-based ice very well could and in fact is already starting to contribute, and that alone holds an estimated 20 meters of sea level rise. We’re saying that the terrestrial segment is more resilient at current carbon dioxide levels.”

Much of the East Antarctic Ice Sheet is land-based. Coastal regions and floating sheets of ice permanently attached to a landmass, known as ice shelves, are marine-based. This kind of ice is more sensitive to warming temperatures; past studies indicate that these portions of the East and West Antarctic Ice Sheets retreated at times throughout the Pliocene.

The research team measured isotopes produced by the interaction between cosmic rays and the nucleus of an atom, called cosmogenic nuclides, in glacial sediment from Antarctic’s largest ice shelf. If the sediment contained significant concentrations of these nuclides, beryylium-10 and aluminum-26, researchers would know the region wasn’t covered in ice because it had been in contact with cosmic rays.

This was not the case for the East Antarctic Ice Sheet during the Pliocene.

“The concentration of beryylium-10 and aluminum-26 in these sediments is profoundly low. They show no indication of being exposed to cosmic rays,” said Marc Caffee, a co-author of the paper and professor of physics and atronomy at Purdue University.

These extremely rare isotopes of beryllium and aluminum were measured by accelerator mass spectrometry, an extremely sensitive analytical technique for measuring long-lived radionuclides, in Purdue’s PRIME Lab.  

The fingings rule out substantial melting of the East Antarctic Ice sheet during the past 8 million years, putting an upper limit on Pliocene sea level estimates, since melting of all marine-based ice in Antarctica and the Greenland Ice Sheet should contribute 100 feet of sea level rise at most. This agrees with the latest climate models, which predict that variations in Antarctic ice volumes are driven mostly by the melting of sea ice.  

Researchers from Boston College, Lawrence Livermore National Laboratory, Purdue University, the University of Vermont and Victoria University of Wellington collaborated on this work. The study was funded by the National Science Foundation, Boston College, Vermont EPSCoR and the New Zealand Ministry of Business and Innovation Employment.

Writer: Kayla Zacharias, 765-494-9318, kzachar@purdue.edu 

Sources: Marc Caffee, 765-494-2586, mcaffee@purdue.edu

Jeremy Shakun, 617-552-1625, jeremy.shakun@bc.edu


Minimal East Antarctic Ice Sheet retreat onto land during the past 8 million years

Jeremy D. Shakun, Lee B. Corbett, Paul R. Bierman, Kristen Underwood, Donna M. Rizzo, Susan R. Zimmerman, Marc W. Caffee, Tim Naish, Nicholas R. Golledge, Carling C. Hay

The East Antarctic Ice Sheet (EAIS) is the largest potential contributor to future sea level rise, but projections are hindered by uncertainty in how the EAIS responded to past warm periods, for example during the Pliocene (5.3-2.6 Myr ago) when atmospheric CO2 concentrations were last ≥ 400 ppm. Geological evidence indicates that some marine-based portions of the East and West Antarctic Ice Sheets retreated during parts of the Pliocene, but it remains uncertain whether ice grounded above sea level also experienced retreat. This uncertainty persists because global sea level estimates for the Pliocene have large uncertainties and cannot be used to rule out substantial terrestrial ice loss, and also because direct geological evidence bearing on past ice retreat on land is lacking. Here, we show that land-based sectors of the EAIS draining into the Ross Sea were ice covered throughout the past 8 Myr based on extremely low concentrations of cosmogenic 10Be and 26Al in quartz sand extracted from a land-proximal marine sediment core. The sediment we analyzed was eroded from the continent where it experienced only minimal exposure to cosmic radiation, indicating that atmospheric warming during the past 8 Myr was insufficient to cause widespread and/or long-lasting meltback of the EAIS margin onto land. We suggest that Antarctic ice volume variations in response to the range of global temperature experienced over this period – up to 2-3 ̊C above preindustrial, which correspond to future scenarios with CO2 concentrations between 400 and 500 ppm – are instead driven mostly by retreat of marine ice margins, in agreement with the latest models.

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