In 1966, scientists from Camp Century, a now-abandoned U.S. military base in the Arctic, drilled deep into the Greenland Ice Sheet and retrieved a cylinder of ice nearly a mile long along with 12 feet of the frozen sediment trapped beneath.
“That was a pretty amazing engineering feat that was really hard to replicate,” said Andrew Christ, a geoscientist who recently completed a postdoctoral fellowship at the University of Vermont.
The sample was the first deep ice core scientists had ever collected, and in the decades that followed, the ice became the subject of intensive scientific research, yielding critical clues about the planet’s climate history. The same cannot be said of the sediment, which was largely overlooked before disappearing completely.
In 2017, the sediment was rediscovered in a freezer in Denmark. Now a study of the frozen monsters is shedding new light on Greenland’s past and perhaps providing an ominous warning for the future. The findings, published Thursday in Science, suggest that about 400,000 years ago the Camp Century site in northwestern Greenland was temporarily free of ice. They add to the growing evidence that the Greenland ice sheet has not been stable for the past 2.5 million years, as scientists once assumed.
“The main message from this is that Greenland is vulnerable,” said Paul Bierman, a geoscientist at the University of Vermont and an author of the new study. “The ice sheet has melted in the past and may therefore melt again.”
Dr. Bierman and an international team of collaborators first started studying the sediment several years ago and soon made a surprising discovery. The top layer of the monster, where they had expected to find little more than a jumble of compressed rock, was full of plant material: twigs, leaves, small bits of moss. The discovery, which the scientists published in 2021, suggested that the area was not always covered in ice.
“But the question we didn’t answer at the time was how old were these plants and the sediment of this landscape without ice?” said Dr. Christ, who is also an author of the new analysis. “This new study in Science tells us when that happened, that was 400,000 years ago.”
To arrive at that date, the scientists used a technique known as luminescence dating. Since minerals are in the soil, they are exposed to ambient radiation and accumulate free electrons. Those electrons build up over time, but exposure to sunlight essentially wipes out the electrons, as a washing machine can remove the layers of dirt that accumulated on a piece of clothing during a week-long camping trip, said Dr. Christ.
By measuring the signal the accumulated electrons give off, the researchers were able to calculate when the top layer of sediment was last exposed to the sun – and thus how long ago the site was ice-free.
(Tammy Rittenour, a geoscientist at Utah State University who led this part of the study, had to analyze the samples in the dark to avoid resetting the electron clock.)
After the scientists estimated the approximate date of the thaw, they modeled several scenarios that could have resulted in an ice-free sampling site 400,000 years ago, calculating that the ice sheet should have melted enough to raise sea levels by at least four and a half feet.
That “is a lot of sea level rise,” said Dr. Christ. “And that’s something that we should really consider as a worst-case scenario for future climate change.”
The temperature then was not much higher than it is now, he noted, and the carbon dioxide level in the atmosphere was much lower.
Still, many uncertainties remain about how the ice sheet will respond to continued warming, said Elizabeth Thomas, a geologist at the University at Buffalo and an author of the new study. And it’s hard to extrapolate from that one sampling site, which is “close to the edge of the ice sheet and also not in a particularly sensitive part of the ice sheet,” she said.
Samples from parts of the ice sheet known to be less stable could provide more information about what could happen as the planet warms, she said.
“We have these amazing samples collected in the 1960s,” said Dr. thomas. “It’s so cool that we get to work on it.” Still, she added, it would be nice to “go back in time and say, ‘Hey, First Ice Core Drilling Team, could you please choose another location?'”
