History Hidden Under the Glaciers

Greenland’s ice sheet shed more than 264 gigatons of ice—a single gigaton is 1 billion metric tons—per year from 2002 to 2024, adding to sea level rise around the world. What lies ahead for it—and us—in the coming decades? It’s hard to say, but looking at the past might give clues.

That’s where work being done by Allie Balter-Kennedy, an assistant professor in the Department of Earth and Climate Sciences, comes in. “I’m really interested in understanding how glaciers and ice sheets, like the Greenland ice sheet and the Antarctic ice sheet, have responded to climate change in the Earth’s past, so we can use that as an analog for what might happen in the future,” she says.

To do that, she and her colleagues have been doing fieldwork in Greenland, Antarctica, and other locations, looking for clues to glacial movements in the last two and a half million years. That’s “when the Earth was experiencing big swings in temperatures and ice volume that we know as ice ages,” says Balter- Kennedy, who is one of two Tufts faculty recently named to the inaugural class of 28 Young American Scientists by Scientific American magazine.

Over the past 700,000 years the Earth experienced a warmer interglacial period, when glaciers typically receded, about every 100,000 years. Before that, the cycle occurred about every 40,000 years. The period of warming and melting is relatively short, followed by a gradual buildup into full glacial conditions. 

A Map of the Past 

We are in one of those interglacial periods now. The last ice age ended about 20,000 years ago, when the Earth started to warm again, “which obviously we are now impacting further with our greenhouse gas emissions,” she notes. 

Allie Balter-Kennedy atop a boulder at Nathan Tufts Park in Powerhouse Square. The rock has grooves left from a retreating glacier as the last ice age ended. Photo: Alonso Nichols

To reconstruct past warming and cooling events, Balter-Kennedy examines sediment left behind when glaciers melted in the past. As glacier ice expands, it creates moraines, which are ridges of rock and sediment that build up at glacier edges. As glaciers melt and recede, that rocky sediment is exposed and gets bombarded by high-energy cosmic ray particles that originate in outer space.  The sediment and rock that’s trapped beneath the ice today provides a timeline of when ice was smaller in the past.

“The cosmic rays come into the Earth’s atmosphere and affect us all the time,” she says. When they hit rock surfaces, they create rare isotopes, a type of chemical element, that’s not otherwise found on Earth. 

Balter-Kennedy and her colleagues focus on the rare isotope Beryllium 10, which builds up in the rock over time. “We can use the isotope as a clock to measure how long rock has been exposed on the surface—how long they’ve been ice-free,” she says.

First the researchers have to get to the rock and sediment, which in this case means camping out in the wilds of Greenland and drilling through the ice sheet. Using machines in the lab, they crush the rock into sand-sized particles and separate out the minerals that they are interested in—looking for Beryllium 10 and other cosmogenic nuclides, as the isotopes are called.

“The concentrations of these nuclides are really, really low—we’re talking tens to hundreds of thousands of atoms that we’re measuring,” she says. The concentration of the isotopes in the rock indicates the total duration that it’s been exposed.

Parsing the Cosmic Rays

Then it gets complicated, as they have to figure out how long the rock and sediment was exposed or buried by ice. “If we are measuring two isotopes with different half-lives, then once the ice sheet’s covering the site, the isotopes that have built up continue to decay, but they decay at different rates,” she says.

The ratio between the isotopes can show how long the site has been covered by ice, while the concentration and amount of the isotopes themselves indicate exposure duration. 

Inside the drill tent during a recent Greenland ice sheet drilling season. Photo: Courtesy of Allie Balter-Kennedy

She and colleagues, led by Caleb Walcott-George at the University of Kentucky, have published research about the top part of the Greenland ice sheet core, which shows that the last time that the site was ice-free was 7,000 years ago. 

“This was a time in the Arctic when temperatures were 2 to 3 degrees Celsius warmer than they are now,” she says. “It was a window into what the ice sheet does when climate is warmer—it gets smaller. This is the first direct evidence that the ice sheet was in fact smaller and the exact timing of when it was smaller.”

Balter-Kennedy is continuing her work on the deep past. She notes the rock and sediment found beneath the ice sheet indicate that several of the interglacial periods were ice free in the past. Now she is trying “to tease apart that signal and see if we can learn something about the frequency of ice-free conditions during those interglacial periods.”

Balter-Kennedy, who teaches the classes Earth Surface Processes and Geochemical Cycles, hasn’t been out in the field for more than a year, but has a backlog of samples from her field research projects. Those are not only from Greenland and the Antarctic, but also from Alaska and Colorado. Using those different locations helps scientists understand how glaciers change on different timescales. 

It’s all in the service of understanding the past to help think about the future. What she’s working on “integrates the history of these ice sheet fluctuations over the landscape on millions of years timescale, to get a sense of how often, during these warm interglacial periods, the Greenland ice sheet actually got smaller than today,” she says. The question is, “are we in relatively unprecedented territory now as the Greenland ice sheet is melting away again?”