U.S. geologists have discovered what they say is a Pleistocene landscape preserved about 3 km beneath the Greenland Ice Sheet.
“We found organic soil that has been frozen to the bottom of the ice sheet for 2.7 million years,” said Dr Paul Bierman, a geologist with the University of Vermont and the lead author of the paper appearing online in the journal Science.
“The ancient soil under the Greenland Ice Sheet helps to unravel an important mystery surrounding climate change – how did big ice sheets melt and grow in response to changes in temperature?” said co-author Dr Dylan Rood from the Scottish Universities Environmental Research Center and the University of California, Santa Barbara.
The discovery indicates that even during the warmest periods since the ice sheet formed, the center of Greenland remained stable. This allowed a tundra landscape to be locked away, unmodified, under ice through millions of years of global warming and cooling.
“The traditional knowledge about glaciers is that they are very powerful agents of erosion and can effectively strip a landscape clean. Instead, we demonstrate that the Greenland Ice Sheet is not acting as an agent of erosion; in fact, at it’s center, it has performed incredibly little erosion since its inception almost three million years ago,” said co-author Lee Corbett, a graduate student at the University of Vermont.
Dr Bierman added: “rather than scraping and sculpting the landscape, the ice sheet has been frozen to the ground, a refrigerator that’s preserved this antique landscape.”
The scientists tested 17 dirty ice samples from the bottommost forty feet of the 3,053-meter GISP2 ice core extracted from Summit, Greenland, in 1993.
From this sediment, they 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.
The researchers expected to only find soil eroded from glacier-scoured bedrock in the sediment at the bottom of the ice core.
They planned to work diligently to find vanishingly small amounts of the beryllium – since the landscape under the ice sheet would have not been exposed to the sky.
“On a global basis, we only find these sorts of beryllium concentrations in soils that have developed over hundreds of thousands to millions of years,” said co-author Joseph Graly from the University of Wyoming.
The findings show that the soil had been stable and exposed at the surface for somewhere between 200,000 and 1 million years before being covered by ice. To help interpret them, the scientists also measured nitrogen and carbon that could have been left by plant material in the core sample.
“The fact that measurable amounts of organic material were found in the silty ice indicates that soil must have been present under the ice,” said co-author Dr Andrea Lini from the University of Vermont.
“Greenland really was green! However, it was millions of years ago. Greenland looked like the green Alaskan tundra, before it was covered by the second largest body of ice on Earth,” Dr Rood said.
To confirm their findings about this ancient landscape, the researchers also measured beryllium levels in a modern permafrost tundra soil on the North Slope of Alaska.
“The values were very similar, which made us more confident that what we found under Greenland was tundra soil,” Dr Bierman concluded.
Paul R. Bierman et al. Preservation of a Preglacial Landscape Under the Center of the Greenland Ice Sheet. Science, published online April 17, 2014; doi: 10.1126/science.1249047