SAN FRANCISCO, CALIFORNIA—As the Arctic warms, Greenland’s fringe of glaciers is thinning and melting—but the future of the Greenland ice sheet remains a giant question mark. Until recently, that was also true of the ice sheet’s past: Scientists have long debated whether it might have shrunk away to nothing during Earth’s warmest periods. Now, a new study suggests that Greenland was entirely ice free at some point in the last 1.25 million years.
“We should be worried about the Greenland Ice Sheet,” says Joerg Schaefer, a geochemist from Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, and lead author of the findings, presented yesterday at the American Geophysical Union’s annual meeting here.
Scientists have been keeping a wary eye on Greenland’s ice sheet, which holds in its frozen waters the equivalent of 7.4 meters of sea level rise. Many of the glaciers that jut out into the ocean are thinning, but whether the ice sheet itself has remained stable and intact, even during warm interglacial periods, is a matter of considerable debate. So scientists are keen to learn more about the icy island’s past. One period of particular interest is a warm, wet interglacial stage known as the Eemian that occurred from 124,000 to 119,000 years ago, featuring average global temperatures about 2°C warmer than today.
Using data from a 3053-meter-long core of ice and bedrock collected from the center of the island in 1993, Schaefer’s team has found valuable clues to what the period held. In particular, the 1.55 meters of bedrock at the core’s base revealed much about the island’s history of glaciation, Schaefer says, in atoms that chronicle exposure to the elements. Earth’s surface is constantly bombarded by cosmic rays, high energy particles streaming into Earth from space. They collide with atoms in Earth’s atmosphere as well as in the uppermost centimeters of its rocks, producing new particles. Some of those particles have a particularly useful set of properties: They don’t naturally occur in the rocks, and they are radioactive. Thus, they can act as a sort of clock, marking time since the rocks were last ice free and exposed to the atmosphere.
Schaefer and his colleagues measured the abundance of two cosmogenic isotopes, aluminum-26 and beryllium-10, in grains of the mineral quartz that they found within the bedrock. Each isotope is produced at a different rate by cosmic rays and has a different half-life. Once the rocks are no longer exposed to the atmosphere—for example, buried by ice—the ratio of 26Al to 10Be in the rocks changes because of their differing half-lives. Schaefer and his team found that the ratio in the bedrock ?was simply too low for the site to have remained buried continuously over the last 1.25 million years—suggesting that it had been exposed and ice free at least once during that time.
Schaefer says he is certain the findings show that Greenland was ice free at one point—but, he says, “we need to understand what happened and how it happened.” More cosmogenic studies of the bedrock, for example, could further narrow the dates of exposure. He finished his presentation with an appeal to other researchers: If you’re going to retrieve an ice core, consider getting some bedrock while you’re at it.