Now, two first-of-their-kind studies provide new insight into the deep history of the Greenland Ice Sheet, looking back millions of years farther than previous techniques allowed. However, the two studies present some strongly contrasting evidence about how Greenland‘s ice sheet may have responded to past climate change – bringing new urgency to the need to understand if and how the giant ice sheet might dramatically accelerate its melt-off in the near future.
The two new studies were published in the journal Nature on December 8, including one led by University of Vermont geologist Paul Bierman. The other led by Joerg Schaefer of Lamont-Doherty Earth Observatory and Columbia University.
Bierman and four colleagues – from UVM, Boston College, Lawrence Livermore Laboratory, and Imperial College London – studied deep cores of ocean-bottom mud containing bits of bedrock that eroded off of the east side of Greenland. Their results show that East Greenland has been actively scoured by glacial ice for much of the last 7.5 million years – and indicate that the ice sheet on this eastern flank of the island has not completely melted for long, if at all, in the past several million years. This result is consistent with existing computer models. Since the data the team collected only came from samples off the east side of Greenland, their results do not provide a definitive picture of the Greenland ice sheet.
The other study in Nature – led by Joerg Schaefer of Lamont-Doherty Earth Observatory and Columbia University, and colleagues – looked at a small sample of bedrock from one location beneath the middle of the existing ice sheet and came to what appears to be a different conclusion: Greenland was nearly ice-free for at least 280,000 years during the middle Pleistocene – about 1.1 million years ago. This possibility is in contrast to existing computer models.
“These results appear to be contradictory” UVM’s Bierman says. He notes that both studies have “some blurriness,” he says, in what they are able to resolve about short-term changes and the size of the ancient ice sheet. “Their study is a bit like one needle in a haystack,” he says, “and ours is like having the whole haystack, but not being sure how big it is.”
Both Studies Analyze Cosmic Ray Bombardment in Bedrock
Both teams of scientists used, “a powerful new tool for Earth scientists,” says Dylan Rood, a scientist at Imperial College London and a co-author on the Bierman-led study: isotopes within grains of quartz, produced when bedrock is bombarded by cosmic rays from space. The isotopes come into being when rock is at or near Earth’s surface – but not when it’s buried under an overlying ice sheet. By looking at the ratio of two of these cosmic-ray-made elements – aluminum-26 and beryllium-10 caught in crystals of quartz, and measured in an accelerator mass spectrometer – the scientists were able to calculate how long the rocks in their samples had been exposed to the sky versus covered by ice.
