Minimal East Antarctic Ice Sheet retreat onto land during the past eight million years
Jeremy D. Shakun, Lee B. Corbett, Paul R. Bierman, Kristen Underwood, Donna M. Rizzo, Susan R. Zimmerman, Mark W. Caffee, Tim Naish, Nicholas R. Golledge, & Carling C. Hay
The problem: There has been debate among scientists if the East Antarctic Ice Sheet melted substantially during the Pliocene (~5.3-2.6 million years ago) and Miocene (23-5.3 million years ago) when the amount of carbon dioxide in the atmosphere was higher (and thus the global average temperatures were much warmer). Some scientists think that as the Earth was warmer during this time, the ice melted back substantially, thus exposing some land surface on East Antarctica. Other scientists think this is not possible based on other lines of evidence. This study set out to investigate whether or not the ice sheet melted back and exposed land by measuring the amount of cosmogenic nuclides, Beryllium 10 and Aluminum 26 (written as 10Be and 26Al). Both 10Be and 26Al occur in rocks that have been exposed to the sun (to read more about cosmogenic nuclides, click here).
Methods: First, the researchers of the study needed to obtain rocks and sediment that was underneath East Antarctica. Lucky for them, there was already drilled cores from this area! In 2006-2007, a team of scientists went to Antarctica for the purpose of recovering sediment cores from beneath the East Antarctic Ice Sheet. The team ended up with two cores that were more than 1,200 meters (0.75 miles) in length. The project was called ANDRILL, and you can read more about it here. The cores are stored in a special facility, and any scientist that wants material (rocks and sediment) from the cores can request it.
Once the scientists in this study had the sediment and rocks, they cleaned the rocks of the very fine sediment until they had a good amount of rocks, which were mostly quartz. They then used a certain method to extract and measure the amounts of 10Be and 26Al in the rocks. The idea is that with long-term exposure to sunlight, the rocks would contain high amounts of 10Be and 26Al. This would indicate that at the time the rocks were deposited millions of years ago, the ice on East Antarctica would have to be melted away, and the land surface exposed.
Results: The scientists found little, if any, of 10Be and 26Al in their samples. This indicates that the rocks were not exposed to sunlight, and thus the glacier that covers East Antarctica did not melt back and expose the land surface millions of years ago.
Why is this study important? This study used a novel approach and really cool method to investigate a problem that scientists didn’t agree upon. It also indicates, to some degree, how much the glacier on East Antarctica melted during interglacial (warm periods within an ice age) times over the last millions of years.
Citation: Shakun, J. D., Corbett, L. B., Bierman, P. R., Underwood, K., Rizzo, D. M., Zimmerman, S. R., Caffee, M. W., Naish, T., Golledge, N. R., Hay, C. C. 2018. Minimal East Antarctic Ice Sheet retreat onto land during the past eight million years. Nature. doi:10.1038/s41586-018-0155-6