By Annika Schmidt
Medill Reports, Dec. 22. 2023
Dating how glaciers retreated over the Cordillera Darwin mountain range at the end of the last Ice Age is taking a team of researchers and postdoctoral students to the southern tip of South America on a detective hunt investigating climate change. Their findings are crucial to understanding accelerating climate change today.
The collaborative effort between top paleoclimatologists reveals how quickly — and the question is why — glaciers melted in the past, even without the human forcing associated with present-day climate change. Brenda Hall, a glacial geologist and geochronologist with the University of Maine, presented preliminary findings at the Comer Climate Conference in southern Wisconsin in fall 2023 documenting the rapid change from the data collected in Chile.
Her research is mapping how quickly glaciers in the Cordillera Darwin melted using exposure dating, a method that examines how long ago the glaciers lumber off the landscape and left it free of ice in the retreat. Researchers use rocks and sediment samples to date the melt.
“Our initial conclusions are that the ice retreat was very early, at the start of the termination” Hall said, referring to the end of the last Ice Age some 20,000 years ago.
“It was very fast and it appears to me, at 20,000 — there may be disagreement in this audience in places — but it appears to me at least that it is a global event,” Hall said. She explained that this would require a climate change mechanism that affects the entire globe quickly. With accelerating ice melt today pushed by fossil fuel emissions, understanding how that could amplify past conditions of rapid retreat becomes critical.
Hall and Tom Lowell, a geosciences professor at the University of Cincinnati, are working under a grant from the National Science Foundation to pursue this research that will continue in coming years. Last March, Hall, Lowell, a colleague from Chile, a team of postdoctoral students and a student journalist from Medill spent one month on a sailboat navigating icy waterways and hiking into the glacier fields. They collected clues that, back in the lab, were analyzed in an ongoing process to reveal how the ice behaved at the end of the last Ice Age.
The last glacial period is a time when global ice sheets advanced to their maximum before terminating in a retreat that lasted several thousand years.
“All aspects of our research really have some implications for the current day or future behavior of climate,” Hall said. “A lot of our research is looking at the timing of the last termination really gives us an idea of what the mechanisms behind rapid global warming are.”
Early results show that glaciers in the Cordillera Darwin mountain range retreated 15,000-17,000 years ago. But moraines — ridges at the mountainous foothills — still had some ice up against them within the last few hundred years, and as recently as the 1980s, Hall said. Quartz in the boulders along the moraine generate isotopes of beryllium, caused by collisions with cosmic rays and the concentration collected in the samples dates when the boulder was exposed to air – exposed as the glaciers threw it aside in their retreat.
In the five times she’s visited the landscape for fieldwork since 2006, Hall said she can see differences in the glaciers within nearly two decades. “The glaciers have retreated,” she said. The retreat in that time span is faster than what is considered rapid retreat in geological time perspectives.
In March, Hall and Lowell brought rock samples back to labs in the U.S. to analyze their contents. The rocks — collected by removing parts of large boulders deposited by melting glaciers — are ground up and chemically analyzed.
When cosmic rays from space strike quartz in the moraine rocks, the collision creates the isotope beryllium-10, but the reaction can only take place when the rock is exposed to the atmosphere and not when it is inside a glacier. By measuring the amount of beryllium-10 in a rick, scientists can determine how long ago that rock was uncovered as a glacier melted.
Hall and Lowell’s team also collected sediment cores, which contain deep layers of the earth that correlate to parts of natural history. These cores contribute to exposure testing by showing how much organic matter like leaves or wood are present in each layer.
The rock and sediment core data support each other because, as Hall explains, plants may not have grown in a certain spot immediately after the ground was exposed. But dating both rocks and plants generate a more complete picture of historic glacier movements over the landscape, as they should be relatively similar.
Meredith Kelly, a researcher and earth sciences professor at Dartmouth College, studies glaciers and also presented research at October’s climate conference. She said by sharing work at the conference on ice sheets, scientists are piecing together a more composite view of past climate change. “It’s the accumulation or conglomeration of all these things that provides this global picture that’s important,” she said.
“I think it’s such an important site,” Kelly said of the Cordillera Darwin mountain range. “Being the southernmost landmass in [South America] really tells us about things like what’s going on in the Southern Ocean and what’s happening with the frontal boundaries around Antarctica and the westerly winds. And it’s such a critical state. It’s just amazing to be able to get data from there.”
The grant from the National Science Foundation is funding Hall and Lowell’s research through 2025. Hall and Lowell will return to the Cordillera Darwin mountain range this march to collect more samples, filling in geographic gaps that remain from their 2023 fieldwork.
Photo at top: The Cordillera Darwin mountain range of southern Chile during fieldwork in March 2023 as ice melt accelerates. Photo by Kala Hunter/Medill School of Journalism.