Fossil Plants at Bottom of the Greenland Ice Sheet Warn of Future Melting
In 1966, U.S. Army scientists drilled through nearly a mile of ice in northwestern Greenland and pulled up a 15-foot-long core of sediment from the bottom. The sample, abandoned and largely forgotten in a series of freezers for decades, was accidentally rediscovered in 2017. Researchers who later examined it were stunned to find it contained not just the usual sand and rock found under glacial ice, but well-preserved remains of twigs and leaves—the first discovery of onetime plant life under this apparently long-frozen part of the world.
In a new study published this week in the Proceedings of the National Academy of Sciences, scientists from a dozen institutions say the discovery indicates that most or all of Greenland’s ice melted one or more times within about the last million years, allowing vegetation or even forests to grow. The finding strongly bolsters a 2016 study of bedrock retrieved from the bottom of an even deeper core that indicated the ice had largely disappeared within the same time frame. Taken together, the studies suggest that Greenland is far more unstable than long thought, and could undergo radical melting in coming decades due to human-influenced climate change.
“What we discovered was delicate plant structures, perfectly preserved,” said the study’s lead author, Andrew Christ of the University of Vermont. “They’re fossils, but they look like they died yesterday. It’s a time capsule of what used to live on Greenland.” The researchers estimate that the remains are at most 1.1 million years old, but that the vegetation could have developed as recently as a few hundred thousand years ago.
“Now we have two studies with strikingly similar results, even though in both cases we drilled blindly,” said Joerg Schaefer, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory who led the 2016 study, and is a leading author of the new paper. “The consistency between the two signals is astounding.”
The material for the new study came from Camp Century, a huge Cold War military base dug into the northwest part of the ice sheet in the 1960s. The real 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 4,560-foot-deep ice core. The scientists were focused on the ice itself—part of the burgeoning effort at the time to understand the deep history of the earth’s ice ages. They apparently took less interest in the dirt from the bottom. In the 1970s, these bottom-most samples were moved from an Army facility to the University of Buffalo, then in the 1990s to a freezer at the University of Copenhagen, Denmark. A couple of decades later, scientists examining the freezer’s contents realized the material might be important.
The new study makes clear that the deep ice at Camp Century—some 75 miles from the coast and only 800 miles from the North Pole—had melted long enough for the ground to become covered with vegetation, including mosses and lichens, and perhaps spruce and fir trees. The findings line up neatly with data from a unique sample of bedrock drilled in 1993 from beneath the deep-interior summit of the Greenland ice, under 10,000 feet of ice, and more than 500 miles from Camp Century.
Using techniques not available at the time of the drilling, Schaefer’s group showed in their 2016 study that the bedrock from the summit site contained isotopes of aluminum and beryllium that form only when the earth’s surface is exposed to cosmic rays that constantly bombard the planet. They estimated that the rock had been exposed to open sky for many tens of thousands of years, some time within the last 1.1 million years—the same time frame estimated by the new study. There might have been only one ice-free episode, or repeated ones, they said. It was the first direct evidence that the ice had disappeared in the recent geologic past.
In the new study, the team used a series of advanced analytical techniques to probe the sediments, fossils and waxy coatings of leaves found at the bottom of the Camp Century ice core. To help establish the date of the deposits, they measured the same isotopes in rocky debris used in the earlier study. Another test used rare forms of oxygen, found within the sediment, which revealed that precipitation must have fallen at much lower elevations than the height of the current ice sheet. This implied that the ice sheet was absent at the time, the researchers say.
The fact that there are intact plant remains at all shows that the overlying ice is relatively young, in geologic terms, say the researchers; otherwise, the remains would have been scoured into oblivion by the ice, which is in constant slow motion toward the coast. Instead, pieces of the landscape were captured and simply frozen in place. A team is now analyzing DNA from the samples to see if they can learn more about the identities and ages of the plants.
Scientist have long thought that for much of the Pleistocene—the icy period covering the last 2.6 million years—portions of the ice on Greenland probably melted during warmer spells called interglacials. But up to now, most of this general story was pieced together from indirect evidence in mud and rock washed off the island and gathered by offshore ocean drilling. The extent of the melting and what kinds of ecosystems existed there before the last interglacial, which ended about 120,000 years ago, have been hotly debated and poorly understood.
Last year, a separate team announced the first-of-its kind discovery of an intact frozen lake bed under the Greenland ice, which could hold many layers of fossils similar to those from Camp Century. The lake-bed site is considered a prime candidate for drilling. Schaefer is also co-leading an upcoming project called GreenDrill, funded by the U.S. National Science Foundation, to drill several other sites that could yield further evidence of the ice sheet’s recent history.
“Our study shows that Greenland is much more sensitive to natural climate warming than we used to think—and we already know that humanity’s out-of-control warming of the planet hugely exceeds the natural rate,” said Christ.
Greenland’s ice has been melting at an accelerating rate in recent years, and is responsible for an increasing portion of ongoing sea-level rise. If the entire sheet were to melt, it would add about 24 feet to global sea levels. “Greenland may seem far away, but it can quickly melt, pouring enough into the oceans that New York, Miami, Dhaka—pick your city—will go underwater,” said study co-leader Paul Bierman of the University of Vermont. “This is not a 20-generation problem. This is an urgent problem for the next 50 years.”
Dorothy Peteet of Lamont-Doherty contributed to analyses of the plant remains. The study also included authors from other universities and scientific institutions in the United States, Canada, Belgium and France. It was supported by the U.S. National Science Foundation.
Adapted in part from a press release by the University of Vermont.
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