Canada Excellence Research Chair, Dr. Dorthe Dahl-Jensen (right), led the team that discovered the Cold War-era ice core.
Scientists stunned to discover fossil plants beneath mile-deep Greenland ice, indicating an ice-free landscape in a warmer climate
Long-lost ice core provides direct evidence that a giant ice sheet covered Greenland probably within the last million years, but was gone in the past in a warmer climate, study shows
A new study led by the University of Manitoba has found that most or all of Greenland was ice-free for a period of time earlier than believed, indicating that it is more sensitive to climate change than previously understood.
In 1966, US Army scientists drilled down through nearly 1,390 meters of ice in northwest Greenland, and pulled up a more than three meter tube of dirt from the bottom. The frozen sediment was moved to a freezer in Copenhagen in 1994 and forgotten. In 2017, the sample was again moved to a new freezer and the frozen sediments were accidentally rediscovered.
In 2019, two samples of sediments were studied by a team of scientists from Denmark and the US, and the team couldn’t believe what they saw: twigs and leaves instead of just sand and rock. That suggested that the ice was gone in the recent geologic past—and that a vegetated landscape, perhaps a boreal forest, stood where a mile-deep ice sheet stands today.
Over the last year, an international team of scientists – led by Dorthe Dahl-Jensen at the University of Manitoba and University of Copenhagen, Andrew Christ and Paul Bierman at University of Vermont, and Jean-Louis Tison at Université Libre de Bruxelles – studied the one-of-a-kind fossil plants and sediment from the bottom of Greenland. Their results show that most, or all, of Greenland has been ice-covered the last million years and ice-free for a period before this time.
“Ice sheets freeze and preserve material in a very pristine way,” says Dorthe Dahl-Jensen, Canada Excellent Research Chair at University of Manitoba.
“But it is a miracle to directly discover delicate plant structures perfectly preserved. They’re fossils, but they look like they died yesterday. It’s a time capsule of what used to live on Greenland that we wouldn’t be able to find anywhere else.”
The discovery helps confirm a new and troubling understanding that the Greenland ice has melted off entirely during recent warm periods in Earth’s history—periods like the one we are now contributing to with human-caused climate change.
Understanding the Greenland Ice Sheet in the past is critical for predicting how it will respond to climate warming in the future and how quickly it will melt. Since some seven meters of sea-level rise is tied up in Greenland’s ice, every coastal city in the world is at risk. The new study provides the strongest evidence yet that Greenland is more fragile to climate change than previously understood—and at grave risk of irreversibly melting off.
“This is a very urgent problem,” says Dahl-Jensen. “Sea level change will impact a significant part of the global population within the next 50 years.”
Beneath the Ice
The material for the new PNAS study came from Camp Century, a Cold War military base dug inside the ice sheet far above the Arctic Circle in the 1960s. The hidden purpose of the camp was a super-secret effort, called Project Iceworm, to hide 600 nuclear missiles under the ice close to the Soviet Union. As cover, the Army presented the camp as a polar science station.
The military mission failed, but the science team did complete important research, including drilling a 1,390 meter-deep ice core. The Camp Century scientists were focused on the ice itself, being the first deep ice core ever drilled. The stable water isotopes measured by professor Willi Dansgaard was the first climate record from ice cores and became an important part of the burgeoning effort at the time to understand the deep history of Earth’s ice ages. They apparently took less interest in a bit of dirt gathered from beneath the ice core. Then, in a truly cinematic set of strange plot twists, the ice core was moved from an Army freezer to the University at Buffalo in the 1970s, to another freezer in Copenhagen, Denmark, in the 1990s, where it languished for decades—until it surfaced when the cores were being moved to a new freezer.
For much of the Pleistocene—the icy period covering the last 2.6 million years—portions of the ice on Greenland persisted even during warmer spells called “interglacials.” But most of this general story has been pieced together from indirect evidence in mud and rock that washed off the island and was gathered by offshore ocean drilling. The extent of Greenland’s ice sheet and what kinds of ecosystems existed there before the last interglacial warm period—that ended about 120,000 years ago—have been hotly debated and poorly understood.
The new study makes clear that the deep ice at Camp Century—some 120 km inland from the coast and only 1300 km from the North Pole—entirely melted at some time within the last million years and was covered with vegetation, including moss and perhaps trees. The new research lines up with data from two other ice cores from the center of Greenland, collected in 1990s. Sediment from the bottom of these cores also indicate that the ice sheet was gone for some time in the recent geologic past. The combination of these cores from the center of Greenland with the new insight from Camp Century in the far northwest give researchers an unprecedented view of the shifting fate of the entire Greenland ice sheet
The team of scientists used a series of advanced analytical techniques—none of which were available to researchers fifty years ago—to probe the sediment, fossils, and the waxy coating of leaves found at the bottom of the Camp Century ice core. For example, they measured ratios of rare forms—isotopes—of both aluminum and the element beryllium that form in quartz only when the ground is exposed to the sky and can be hit by cosmic rays. These ratios gave the scientists a window onto how long rocks at the surface were exposed vs. buried under layers of ice. This analysis gives the scientists a kind of clock for measuring what was happening on Greenland in the past. Another test used rare forms of oxygen, found in the ice within the sediment, to reveal that precipitation must have fallen at much lower elevations than the height of the current ice sheet, “demonstrating ice sheet absence,” the team writes. Combining these techniques with studies of luminescence that estimate the amount of time since sediment was exposed to light, radiocarbon-dating of bits of wood in the ice, and analysis of how layers of ice and debris were arranged—allowed the team to be clear that most, if not all, of Greenland melted at least once during the past million years—making Greenland green with moss and lichen, and perhaps with spruce and fir trees.
In a 1960s movie about Camp Century created by the Army, the narrator notes that “more than ninety percent of Greenland is permanently frozen under a polar ice cap.” This new study makes clear that it’s not as permanent as we once thought. “Our study shows that Greenland is sensitive to natural climate warming, but significant warming is needed to melt the full Greenland ice sheet, a state we will reach in the future warming climate,” says Dahl-Jensen.
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