03/19/2021

Under A Mile Of Ice, A Climate Clue

12:12 minutes

5 images of various twigs photoshopped together. the twigs all look slightly different, one is triangle-shaped and several are brown and stubby
Twig samples. Credit: Andrew Christ and Dorothy Peteet

Scientists studying sediment taken from a core sample of the Greenland ice sheet just 800 miles from the North Pole have found remnants of ancient plants, freeze-dried under more than a mile of ice. Using several different dating techniques, they say the soil, twigs, and leaves date to sometime within the last million years—probably on the order of several hundred thousand years ago—a time when Greenland’s massive ice cap did not exist.

The finding that the ice sheet may have been missing so recently in geologic time provides clues to the stability of the ice, and just how sensitive it might be to modern global warming.

The samples themselves have an unusual history. In the 1960s, the US Army set out to build a base under the surface of the ice in Greenland. Ostensibly, the outpost, named Camp Century, was to be used for research into polar conditions, and how best to work in them. In reality, the US also hoped to secretly bury nuclear missiles under the ice cap within close reach of the Soviet Union. As part of that effort, codenamed Project Iceworm, core samples were taken of the ice and sediment. Year later, those samples would become the basis for this climate study, reported in the journal Proceedings of the National Academy of Sciences.

Drew Christ, one of the authors of that report and a geologist at the University of Vermont, joins Ira to talk about the study, and explain what ancient dirt can teach us about the future climate.

two white men wearing blue gloves grin into the camera while holding bits of twigs
Researcher Drew Christ (right) and Landon Williamson pull bits of fossil plants out of their sample. Credit: Andrew Christ and Dorothy Peteet

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Segment Guests

Drew Christ

Drew Christ is a Gund Postdoctoral Fellow & Lecturer in the Department of Geology at the Gund Institute for Environment at the University of Vermont in Burlington, Vermont.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, a look at privacy in the digital age and the science of rainbows. But first, a trip to the frozen north.

SPEAKER: On the top of the world, below the surface of a giant ice cap, a city is buried.

IRA FLATOW: It was called Camp Century, built in the 1960s, when this army propaganda film was made.

SPEAKER: Today, on the island of Greenland, the United States Army has established an unprecedented nuclear-powered Arctic Research Center.

IRA FLATOW: The Army also hoped to secretly bury nuclear missiles there.

SPEAKER: This is an ideal Arctic laboratory, for more than 90% of Greenland is permanently frozen under a polar ice cap which covers all but a few coastal areas of the island.

IRA FLATOW: As part of Project Iceworm, they drilled core samples deep into that ice cap.

SPEAKER: In this remote setting, less than 800 miles from the North Pole, Camp Century is a symbol of man’s unceasing struggle to conquer his environment, to increase his ability to live and fight, if necessary, under polar conditions.

IRA FLATOW: Today, of course, those polar conditions are changing. The ice is melting. The camp itself is no more. And those core samples are tools in an effort to better understand climate change.

Writing this week in the Proceedings of the National Academy of Sciences, researchers describe what they saw when they looked closely at the core samples taken from deep under the ice in the 1960s. They saw not barren rock, but bits of freeze-dried twigs and plants. It meant that sometime within the last million years, the ice was gone, and the climate quite different than today.

Joining me to talk about this is one of the researchers on that project. Drew Christ is a Gund postdoctoral fellow and lecturer in the Department of Geology at the Gund Institute for Environment of the University of Vermont in Burlington. Welcome to Science Friday.

DREW CHRIST: Thanks so much for having me, Ira.

IRA FLATOW: You’re welcome. When you looked at those samples under the microscope, give us an idea of what you saw.

DREW CHRIST: Oh, gosh. Well, it was just one of the most exciting days of science that I’ve ever gotten to be a part of. We were preparing these samples of frozen soil. And we weren’t expecting to find any biologic remains.

And so Paul Bierman, my coauthor, and I were in the lab rinsing away these soil samples. And we saw these little floating black specks in the water. And we’re like, what is that, and took a little pipette, sucked a little bit up, and looked at it under a microscope.

And we just were totally shocked because we saw these twigs and delicate little leaves that were just perfectly preserved. And they had been under almost a mile of ice. And it looked like they died yesterday. It was amazing.

IRA FLATOW: Really?

DREW CHRIST: Really, really.

IRA FLATOW: But in fact, how many years ago had they died?

DREW CHRIST: So our estimates right now place it to less than the last million years, but likely hundreds of thousands of years.

IRA FLATOW: Could you actually make out these cells inside the plants?

DREW CHRIST: Yeah. So with a more high-powered microscope, we were able to see the actual cellular structure in the leaves of these ancient tundra plants. And we actually went to use a scanning electron microscope, and we were able to see just incredible texture of the woody tissue and leaves that were on these freeze-dried fossils.

IRA FLATOW: Could you tell anything about what kind of plants they were?

DREW CHRIST: With the help of some of the authors. On our study who are experts in Arctic plants, they determined that these are Arctic tundra. So they are small Arctic willows and mosses that grow in the High Arctic today.

IRA FLATOW: Just to get back to your statement about freeze-dried fossils, these are not the kinds of fossils we normally think of, dinosaurs, pieces of rock. These are actual just– the plants that were freeze-dried?

DREW CHRIST: It is not like going to the Museum of Natural History and seeing a giant dinosaur bone in rock. These are more like twigs that you might see in your hiking boot if you went for a hike in the mountains or something and got stuff from the forest in your boot. It really looked like it was from yesterday. But we know it was so much older.

IRA FLATOW: That’s just incredible. I know you didn’t go into this looking for plants. What were you after?

DREW CHRIST: That’s right. That was the surprise. What we were really after is we were trying to figure out how old the sediment was. And we did a variety of different tests to do that.

And one of those ways was to figure out how long the soil has been buried. And the way that we do that is by counting atoms that accumulate or form in rocks and sediment when they’re exposed to radiation hitting the Earth from outer space. And over time, those little atoms accumulate to a point that we can measure it. And if you bury it under an ice sheet for hundreds of thousands of years, they decay away. And we can use that radioactive decay to figure out how long ago they formed.

IRA FLATOW: And why couldn’t you use carbon dating for this? We always hear about carbon dating.

DREW CHRIST: We tried carbon dating on our little fossil twigs. But carbon dating relies on the amount of radioactive carbon that is in dead organic material. And all of that radioactive carbon will decay away after 50,000 years. So when we tried that analysis, we got answers saying, there’s no more radioactive carbon left. These are older than 50,000 years.

IRA FLATOW: There’s a technique you can use to tell how long it’s been since some soil has been exposed to light, I understand. How does that work?

DREW CHRIST: Yeah. This is a really cool technique. And the way that this works is that there is background radiation in all of the materials in Earth’s surface.

And when a crystal is buried below the surface, there are these little holes inside of that crystalline structure that will become occupied by the radiation from the background. We can go into a lab in the dark and then re-illuminate that sediment with light and measure how much radiation comes out of that sediment and figure out how long ago it has been exposed to sunlight. It’s a pretty involved and technical method, but it’s another way that we have determined how old these soils are.

IRA FLATOW: I understand there’s a kind of analysis that you can do on the water that tells you about the elevation. Can you describe what you found there?

DREW CHRIST: Yeah. So what we did there is that this soil, when it came out at the bottom of this ice core, it’s like sands that are held together by frozen ice. And we thawed that ice and then took the meltwater from it. And we can analyze different isotopes of oxygen in that water. And those isotopes of oxygen will tell us things about how warm or cold precipitation was in the past as well as the elevation that it fell at.

And so the results we got from that analysis tell us that the precipitation that fell at this part of Greenland was much lower in the past than it is today. Right now, the elevation of the ice sheet is about 1,900 meters. And our analyses tell us that elevation was about 900 meters lower, based on these analyses of the water.

IRA FLATOW: So that ice sheet was really much thinner back then?

DREW CHRIST: It was gone. So all of that ice must have disappeared in the past to allow precipitation to fall at that elevation. And it was slightly warmer, too. So that’s one line of evidence that this part of Greenland was ice-free in the geologic past.

IRA FLATOW: And how long would that geologic past be?

DREW CHRIST: Within the last million years from the other ways that we determine the age of this soil. And we are still working to figure out exactly how long it was gone. And that’ll be really important information for understanding how sensitive the Greenland ice sheet is to climate warming.

IRA FLATOW: Does that mean that the ice sheet melts and refreezes many, many times since the last– well, a million years ago?

DREW CHRIST: Potentially. That is an important thing to consider. Because if the Greenland ice sheet has been able to survive through fluctuations in Earth’s climate over the last million years, then that means it is more stable. But if it disappears, and we know it’s disappeared at least once, and perhaps multiple times, that means it’s quite sensitive to minor changes in climate.

Over the last million years, carbon dioxide in the atmosphere hasn’t gotten much higher than about 285 parts per million. And right now, because of human burning of fossil fuels, we are in excess of 410 parts per million. So we have completely changed the climate system. And let’s just say we are going into uncharted territory.

IRA FLATOW: Well, we know that Antarctica is melting now, and that Greenland is melting quite rapidly. Why is it so important to know that this happened before?

DREW CHRIST: It’s really important to know that this happened before because it allows scientists to understand what climate conditions caused the ice sheets to melt in the past. And if we know what conditions allowed ice to melt in the past, it helps us understand in the future what the limit is to climate warming. So that’s why it’s so important to understand how these ice sheets have been sensitive to climate change in the past.

IRA FLATOW: Do we know if that past melting and freezing took place at the same rate that it’s happening today, as quickly as it’s happening today, or was it much slower?

DREW CHRIST: Well, that’s also a really important field of research, as well, is trying to figure out in the geologic past, how quickly ice sheets have melted. In the past, these melting events would take thousands of years to occur. But as we move into the future and we increase the temperatures on this planet much more rapidly than would naturally occur in the past, those thinning and retreat rates of the ice sheets could be faster. And that’s something we want to avoid because it would result in more sea level rise over a shorter amount of time.

IRA FLATOW: The absence of a layer of ice in Greenland that you talked about before, do we know if that was connected to what might have been global warming in those days?

DREW CHRIST: So the absence of the ice in Greenland within the last million years may be due to periods of time where Earth’s climate was slightly warmer than it is today, but it was warm for really long periods of time. And so what we are worried about in the future is that if we warm the planet so much for such a long period of time, that we could cause additional melting of these ice sheets. An analogy that I think is helpful for people to think about is if you go to a barbecue in the summer and there’s a cooler of drinks with ice in it and you leave that cooler open for maybe half an hour, you’ll probably still have ice in the cooler. But if you leave that cooler open for eight hours, and then it gets up to 95 degrees, you’re probably going to melt all of the ice, and you’ll have warm drinks. So that’s what we are worried about, is keeping the Earth too warm for too long and melting the ice.

IRA FLATOW: We’ve run out of time, Drew.

DREW CHRIST: Thank you for your great questions.

IRA FLATOW: Well, Thank you for your great answers. Drew Christ is a Gund postdoctoral fellow and lecturer in the Department of Geology at the Gund Institute for the Environment at the University of Vermont in Burlington.

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