In a surprise find, scientists have discovered an ancient tundra landscape preserved under the Greenland Ice Sheet for almost three million years.
"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 Paul Bierman, lead author of the study in the journal Science.
This provides strong evidence that the Greenland Ice Sheet has persisted much longer than previously known, enduring through many past periods of global warming.
The new discovery indicates that even during the warmest periods since the ice sheet formed, the centre 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.
The scientists tested seventeen "dirty ice" samples from the bottommost forty 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.
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.
The team found that the silt had very high concentrations of the isotope when the team measured it on a particle accelerator at Lawrence Livermore National Laboratory.
The new research shows that "the soil had been stable and exposed at the surface for somewhere between 200,000 and one million years before being covered by ice," said Ben Crosby, a member of the research team from Idaho State University.
To help interpret these unexpected findings, the team 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 Andrea Lini at the University of Vermont.
Its composition suggests that the pre-glacial landscape may have been a partially forested tundra, Lini said.
To confirm the 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," said Bierman.
