Fossil plants deep in Greenland ice sheet warn of future melt

Fossil plants deep in Greenland ice sheet warn of future melt

Vegetation covered the area in the recent geological past

In 1966, US Army scientists drilled nearly a mile of ice in northwest Greenland and extracted a 15-foot-long sediment core 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 discover that it not only contained the usual sand and rock found under glacial ice, but well-preserved remains of twigs and leaves – the first discovery of ancient plant life beneath this long-seemingly 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 has melted once or more in the past million years or so, allowing vegetation or even forests to develop oneself. The discovery strongly reinforces a 2016 study of bedrock extracted from the bottom of an even deeper core that indicated the ice had largely disappeared in the same time frame. Taken together, the studies suggest that Greenland is much more unstable than long thought and could undergo drastic melting in the coming decades due to human-influenced climate change.

The discovery of intact plant fossils at the bottom of the Greenland Ice Sheet suggests that it completely melted in the recent geological past. Here, a relatively new expanse of tundra adjoins the rapidly losing edge of the sheet near the southwest coast. The ice is about half a mile away in this photo. (Kevin Krajick/Earth Institute)

“What we found were delicate, perfectly preserved plant structures,” said study lead author Andrew Christ of the University of Vermont. “They are fossils, but they look like they died yesterday. It’s a time capsule of what once lived in Greenland. The researchers estimate that the remains are at most 1.1 million years old, but that the vegetation may have developed only a few hundred thousand years ago.

“We now have two studies with surprisingly similar results, although in both cases we drilled blind,” said Joerg Schaefer, geochemist at Columbia University’s Lamont-Doherty Earth Observatory. , who led the 2016 study and is a lead author of the new paper. . “The consistency between the two signals is stunning.”

The material for the new study comes from Camp Century, a massive Cold War military base dug into the northwestern 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 near the Soviet Union. As a cover, the army presented the camp as a polar science station.

twig on ice

A twig, possibly of the evergreen shrub genus Empetrum, recovered under nearly a mile of ice. (Dorothy Peteet/Lamont-Doherty Earth Observatory)

The military mission failed, but the science team did important research, including drilling an ice core 4,560 feet deep. Scientists focused on the ice itself, part of the nascent effort at the time to understand the deep history of Earth’s ice ages. They were apparently less interested in the dirt on the bottom. In the 1970s, these lowest samples were moved from an army facility at the University at Buffalo, then in the 1990s to a freezer at the University of Copenhagen, Denmark. A few decades later, scientists examining the contents of the freezer realized the material could be significant.

The new study clearly indicates that the deep ice at Camp Century, about 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 maybe spruce and spruce. firs. The findings line up perfectly with data from a single bedrock sample drilled in 1993 below the deep interior top of Greenland Ice, below 10,000 feet of ice and more than 800 km from Camp Century.

Using techniques not available at the time of drilling, Schaefer’s group showed in their 2016 study that the bedrock of the summit site contained isotopes of aluminum and beryllium that only form when the Earth’s surface is exposed to cosmic rays which constantly bombard the planet. They estimated that the rock had been exposed in the open for several tens of thousands of years, over the past 1.1 million years, the same period estimated by the new study. There may have been only one episode without ice cream, or repeated episodes, they said. It was the first direct evidence that the ice had disappeared in the recent geological past.

black and white photo of engineers with an ice core

Engineers from the US Army Cold Regions Engineering and Research Laboratory capture an ice core at Camp Century, Greenland, circa 1940. 1966. (US Army Corps of Engineers)

In the new study, the team used a series of advanced analytical techniques to probe the sediments, fossils and waxy leaf coatings found at the bottom of the Camp Century ice core. To help establish the date of the deposits, they measured the same isotopes in the rock debris used in the previous study. Another test used rare forms of oxygen, found in sediments, which revealed that precipitation must have fallen at altitudes much lower 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 shows that the overlying ice is relatively young, in geological terms, the researchers say; otherwise, the remains would have been erased into oblivion by the ice, which is constantly moving in slow motion towards the coast. Instead, pieces of the landscape were captured and simply frozen in place. A team is currently analyzing DNA from the samples to see if they can learn more about the identity and age of the plants.

Scientists have long thought that for much of the Pleistocene – the ice age spanning the past 2.6 million years – parts of Greenland’s ice likely melted during warmer periods called interglacials. But so far, most of this general story has been pieced together from circumstantial evidence in mud and rock washed off the island and collected by offshore ocean drilling. The extent of the melt and the types of ecosystems that existed there before the last interglacial, which ended around 120,000 years ago, have been hotly debated and poorly understood.

searchers cross rocky terrain near the edge of the ice sheet

A search party traverses bare ground newly exposed by retreating ice sheet. Eventually, the flora will recolonize this site, unless the ice returns. (Kevin Krajick/Earth Institute)

Last year, a separate team announced the discovery, the first of its kind, of an intact frozen lake bed under the Greenland ice, which may contain many layers of fossils similar to those at 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 US National Science Foundation, to drill several other sites that may provide additional evidence for recent ice sheet history.

“Our study shows that Greenland is much more susceptible to natural global warming than we thought – and we already know that humanity’s uncontrollable warming of the planet far exceeds the natural rate,” Christ said.

Greenland’s ice has been melting at an accelerating rate in recent years and is responsible for an increasing share of continued sea level rise. If the entire sheet were to melt, it would add about 24 feet to global sea level “Greenland may seem distant, but it can melt quickly, spilling into the oceans enough for New York, Miami, Dhaka – pick your city – to go underwater,” said Paul Bierman, co-head of the study at the University of Vermont. “It’s not a problem of 20 generations. This is an urgent problem for the next 50 years.

Dorothy Peteet of Lamont-Doherty contributed to analyzes of 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 US National Science Foundation.

Adapted in part from a University of Vermont press release.

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