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Ancient leaves preserved under a mile of Greenland ice – and lost in a freezer for years – teach lessons about climate change
Remains of the ancient Greenland tundra have been preserved in the ground beneath the ice cap. Andrew Christ and Dorothy Peteet, CC BY-ND In 1963, inside a secret US military base in northern Greenland, a team of scientists began digging the Greenland ice sheet. Piece by piece, they extracted a core of ice 4 inches in diameter and almost a mile long. At the very end, they pulled out something else – 12 feet of frozen ground. Ice told a story of the Earth’s climate. The frozen ground was examined, put aside and then forgotten. Half a century later, scientists rediscovered this soil in a Danish freezer. He is now revealing his secrets. Using laboratory techniques unimaginable in the 1960s, when the core was drilled, we and an international team of fellow scientists were able to show that Greenland’s enormous ice cap had melted there over the past million years. Radiocarbon dating shows that this would have happened over 50,000 years ago. This probably happened at a time when the climate was hot and sea level high, perhaps 400,000 years ago. And there was more. As we explored the soil under a microscope, we were amazed to discover the remains of a tundra ecosystem – twigs, leaves and moss. We were looking at northern Greenland as it was the last time the area was ice free. Our peer-reviewed study was published March 15 in the Proceedings of the National Academy of Sciences. Engineers tear off a 4,560-foot-long section of the ice core at Camp Century in the 1960s. US Army Corps of Engineers Paul Bierman, a geomorphologist and geochemist, describes what he and his colleagues found in the ground. Without the ice cap, sunlight would have warmed the ground enough for tundra vegetation to cover the landscape. Oceans around the world would have been more than 10 feet taller, and possibly even 20 feet taller. The land on which Boston, London and Shanghai now stand would have been under the waves of the ocean. All of this happened before humans started to warm the Earth’s climate. The atmosphere then contained much less carbon dioxide than it does today, and it was not rising as quickly. The ice core and the ground below is sort of a Rosetta Stone for understanding how durable the Greenland ice sheet has been during past warm periods – and how quickly it could melt again when the climate warms. . Danish Secret Military Bases and Freezers The story of the Ice Core begins during the Cold War with a military mission called Project Iceworm. Beginning in 1959, the U.S. military transported hundreds of soldiers, heavy equipment, and even a nuclear reactor through the northwestern Greenland ice sheet and dug a base of tunnels inside the ice cream. They called it Camp Century. It was part of a secret plan to hide nuclear weapons from the Soviets. It was known to the public as an arctic research laboratory. Walter Cronkite even visited and filed a report. Workers built the snow tunnels at Camp Century research base in 1960. The US Army Corps of Engineers’ Camp Century did not last long. Snow and ice slowly began to crush the buildings inside the tunnels below, forcing the military to abandon it in 1966. During its short life, however, scientists were able to extract the core from ice and start analyzing the history of Greenland’s climate. As the ice accumulates from year to year, it picks up layers of volcanic ash and changes in precipitation over time, and it traps air bubbles that reveal the past composition of the ice. atmosphere. One of the original scientists, glaciologist Chester Langway, kept the samples of frozen carrots and soil at the University of Buffalo for years, then shipped them to a Danish archive in the 1990s, where the soil became quickly been forgotten. A few years ago our Danish colleagues found the soil samples in a box of glass cookie jars with faded labels: “Camp Century Sub-Ice”. Geomorphologist Paul Bierman (right) and geochemist Joerg Schaefer of Columbia University for the first time examine the pots containing the sediment from Camp Century. They were in a Danish freezer set at -17 F. Paul Bierman, CC BY-ND A surprise under the microscope On a hot day in July 2019, two soil samples arrived at our laboratory at the University of Vermont, frozen solid. We started the painstaking process of splitting up the precious few ounces of frozen mud and sand for different analyzes. First, we photographed the stratification in the ground before it was lost forever. Then we chiseled out small pieces to examine them under a microscope. We melted the rest and saved the old water. Then came the biggest surprise. While we were mopping the floor, we spotted something floating in the rinse water. Paul grabbed an eyedropper and filter paper, Drew grabbed tweezers and turned on the microscope. We were absolutely amazed looking at the eyepiece. We looked back at leaves, twigs and mosses. It wasn’t just dirt. It was an ancient ecosystem perfectly preserved in the natural frost of Greenland. Glacial geomorphologist Andrew Christ (right), along with geology student Landon Williamson, brandishes the first twig spotted as they were washing a sediment sample from Camp Century. Paul Bierman, CC BY-ND Dating from millennial moss How old were these plants? Over the past million years, the Earth’s climate has been punctuated by relatively short warm periods, usually lasting around 10,000 years, called interglacials, when there was less ice at the poles and the sea level was higher. The Greenland ice sheet has survived all of human history during the Holocene, the current interglacial period of the past 12,000 years, and most of the interglacials in the last million years. But our research shows that at least one of these interglacial periods was warm enough for a period long enough to melt large portions of the Greenland ice sheet, allowing a tundra ecosystem to emerge in northwest Greenland. . We used two techniques to determine the age of the soil and plants. First, we used cleanroom chemistry and a particle accelerator to count the atoms that form in rocks and sediments when exposed to natural radiation that bombards Earth. Next, a colleague used an ultra-sensitive method to measure the light emitted by the grains of sand to determine the last time they were exposed to the sun. Greenland maps show the speed of the ice sheet as it sinks (left) and the hidden landscape below (right). BedMachine v3; Copernicus Climate Change Service (C3S), CC BY-ND The concentration of carbon dioxide in the atmosphere today is well above previous levels determined from ice cores. On March 14, 2021, the CO2 level was around 417 ppm. NASA Jet Propulsion Laboratory, CC BY-ND The million-year delay is significant. Previous work on another ice core, GISP2, mined from central Greenland in the 1990s, showed that ice was also absent there for the last million years, possibly around 400,000 years ago. . Lessons for a world facing rapid climate change The loss of the Greenland ice sheet would be catastrophic for humanity today. The melted ice would raise the sea level by more than 20 feet. It would reshape the coasts of the whole world. About 40% of the world’s population lives within 60 miles of a coast and 600 million people live within 30 feet of sea level. Antarctica will pour more water into the oceans. Communities will be forced to relocate, climate refugees will become more common and expensive infrastructure will be abandoned. Already, rising sea levels have amplified flooding from coastal storms, causing hundreds of billions of dollars in damage each year. Tundra near the Greenland ice cap today. What was Camp Century like before the ice came back in the last million years? Paul Bierman, CC BY-ND The story of Camp Century crosses two critical moments in modern history. An arctic military base built in response to the existential threat of nuclear war has inadvertently led us to discover another threat from ice cores – the threat of sea level rise due to man-made climate change . Today, his legacy helps scientists understand how the Earth responds to a changing climate. This article has been updated to correct the legend for the chart at 417 ppm. [Deep knowledge, daily. Sign up for The Conversation’s newsletter.]This article is republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Andrew Christ, University of Vermont and Paul Bierman, University of Vermont. Read more: Shrinking glaciers have set a new standard for the Greenland ice sheet – constant ice loss for the foreseeable future The Arctic hasn’t been this hot in 3 million years – and that portends big changes for the rest of the planet Andrew Christ receives funding from the Gund Institute for the Environment and the National Science Foundation. Paul Bierman receives funding from the US National Science Foundation and the UVM Gund Institute for Environment.
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