Glaciers and ice sheets are commonly thought to work like a belt sander. As they move over the land they scrape off everything — vegetation, soil and even the top layer of bedrock. So a team of university scientists and a NASA colleague were greatly surprised to discover an ancient tundra landscape preserved under the Greenland Ice Sheet, below two miles of ice.
“We found organic soil that has been frozen to the bottom of the ice sheet for 2.7 million years,” said University of Vermont geologist and lead author Paul Bierman. The finding provides strong evidence that the Greenland Ice Sheet has persisted much longer than previously known, enduring through many past periods of global warming.
Greenland is a place of great interest to scientists and policymakers because the future stability of its huge ice sheet — the size of Alaska — will have a fundamental influence on how fast and high global sea levels rise from human-caused climate change.
“The ancient soil under the Greenland ice sheet helps to unravel an important mystery surrounding climate change,” said Dylan Rood, a co-author on the new study, from the Scottish Universities Environmental Research Centre and the University of California, Santa Barbara. “How did big ice sheets melt and grow in response to changes in temperature?”
The new discovery indicates that even during the warmest periods since the ice sheet formed, the center of Greenland remained stable. “It’s likely that it did not fully melt at any time,” Bierman said. This allowed a tundra landscape to be locked away, unmodified, under ice through millions of years of global warming and cooling.
“Some ice sheet models project that the Greenland Ice Sheet completely melted during previous interglacial periods. These data suggest that did not happen,” said co-author Tom Neumann, a cryospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “We don’t know how much of the ice sheet remained — to estimate it, we’d have to study other ice cores in Greenland that have sediment in the bottom to see if ancient soil is preserved under those sites as well.”
The scientists tested seventeen samples of “dirty ice” — ice with sediment mixed in — from the bottommost 40 feet of the 10,019-foot GISP2 ice core extracted from Summit, Greenland, in 1993. From this sediment, Bierman and a team at the University of Vermont’s Cosmogenic Nuclide Laboratory extracted a rare form of the element beryllium, an isotope called beryllium-10. Formed by cosmic rays, it falls from the sky and sticks to rock and soil. The longer soil is exposed at Earth’s surface, the more beryllium-10 it accumulates. Measuring how much is in soil or a rock gives geologists a kind of exposure clock.
Photo shows a piece of the GISP2 ice core that the researchers analyzed for the isotope beryllium-10, showing silt and sand embedded in ice. Soon after this picture was taken, the ice was crushed in the University of Vermont clean lab and the sediment was isolated for analysis.
The researchers expected to only find soil eroded from glacier-scoured bedrock in the sediment at the bottom of the ice core. But the silt they did find had very high concentrations of beryllium-10 when the team measured it on a particle accelerator at Lawrence Livermore National Laboratory, in Livermore, Calif.
Article by Roger Greenway
