Scientists drilled through 500 metres of Greenland's ice — here's what they found at the bottom - Nature.com

Scientists drilled through 500 metres of Greenland’s ice - here’s what they found at the bottom
Melt water has pooled to form a lake on the surface of Greenland’s ice sheet.Credit: Martin Zwick/Reda&Co/Universal Images Group via Getty

San Francisco, California

The longest core of rock ever extracted from beneath Greenland’s thick ice could hold clues about how quickly the island’s frozen covering will melt as the planet warms.

Preliminary analysis of the rock and associated sediment suggests that, at some time in the past three million years or so, the material in this core was exposed to air. That means that the ice atop it had melted away, at least temporarily.

The work adds to a small but growing cadre of studies that use Greenland’s bedrock to illuminate how unstable the overlying ice has been in the past. This core is particularly important because it is the first such material to be collected in decades, and because it contains much more bedrock material than has ever been gathered from beneath Greenland’s ice.

“This core holds a lot of information about past exposure,” says Allie Balter-Kennedy, a glacial geologist at the Lamont-Doherty Earth Observatory in Palisades, New York. She presented initial findings from the drilling project, called GreenDrill, on 11 December at a meeting of the American Geophysical Union in San Francisco, California.

What lies beneath

The melting of Greenland’s ice sheet is a significant contributor to current sea-level rise. Many researchers have tried to explore the past and future of the ice sheet by drilling into it, to extract the environmental history preserved there. But very few drilling campaigns have penetrated all the way through the ice and into the bedrock underneath, because of the technical challenges involved.

Using the latest equipment, the US$7-million GreenDrill project succeeded this year. It drilled through 509 metres of ice at a site called Prudhoe Dome, and pulled up 7.4 metres of frozen sediment and rock.
Glacial geologist Allie Balter-Kennedy holds an ice core extracted from Prudhoe Dome, Greenland.Credit: Ryan Vachon

To understand whether ice covered a particular location in the past, researchers look for radioactive isotopes that are produced in rock when it is exposed to air and cosmic rays, the high-energy particles from space that constantly bombard Earth. The GreenDrill team’s preliminary analysis suggests that the core, especially the portion that is sediment, contains high levels of beryllium-10, which is one of the key isotopes used to study bedrock exposure, Balter-Kennedy reported at the meeting. “That is way more than we’ve seen anywhere else,” says Paul Bierman, a geoscientist at the University of Vermont in Burlington who was not involved in the project.

The amount of beryllium-10 corresponds to around 40,000 years of exposure to air, Balter-Kennedy says. That exposure could have been a single, continuous event, or, more likely, multiple episodes spread over the past few million years. The researchers’ calculations suggest that, if the Prudhoe Dome site was indeed ice-free, whether for short periods of time or a long one, then Greenland must have melted enough to contribute between 19 and 73 centimetres of global sea-level rise.

Ice on the rocks

The findings are preliminary because the sediments could have been disturbed or moved around, and the measurements still need to be confirmed, Balter-Kennedy cautioned. But there are smaller amounts of beryllium-10 in the rock beneath the sediment, which would support the idea that it was all exposed to air.

The few rock and sediment cores that have been retrieved from beneath Greenland’s ice include the bottom of the GISP2 core, which was extracted from central Greenland in 1993. That core shows signs that the site was ice-free several times in the past 2.6 million years 1— roughly the same time frame suggested by the Prudhoe Dome core. And a core drilled in 1966 in northwest Greenland suggests that that site was ice-free 2 for an unknown period of time around 400,000 years ago.