Beneath the surface of LaSalle Lake, the deepest natural lake fully in Minnesota, is a hidden archive of climate data going back thousands of years. It’s stored in varves, or annual layers of sediment deposits on the lake’s bottom. Just like tree rings, varves indicate the age of the sediment and can be studied to reconstruct past environmental conditions.

"The idea is to figure out how climate change is impacting extreme precipitation and flooding here in Minnesota, and using analysis of the sediment in lakes like LaSalle to do that," said Byron Steinman, PhD, a professor of Earth and Environmental Sciences at the University of Minnesota Duluth (UMD).

But there’s one problem … These delicate sediment layers are challenging to pull to the surface without compromising their structural integrity. As a result, Steinman and a team of researchers from UMD are using a technique called “freeze coring.” The process involves filling a large steel wedge with ethanol and dry ice, cooling the metal to about minus 100 degrees Fahrenheit. The wedge is then slowly lowered into the lake, where it forms dramatic plumes of carbon dioxide bubbles on the surface. Once it hits the bottom, the wedge sinks into the mud where sediment freezes to its sides, keeping its layers intact.

“It’s a lot like sticking your tongue on a pole in the winter. That same principle applies here, where the sediment sticks to the side of the coring device,” Steinman said.

Steinman and his team are partnering with the United States Geological Survey (USGS) to retrieve freeze cores from six lakes across Minnesota and Wisconsin over the next five years.  "We are constantly aware of ongoing droughts and floods. The sediment varve records will be particularly useful for improving flood frequency predictions for the region," said William Daniels, PhD, an assistant professor of Earth and Environmental Sciences at UMD.

The goal is to extend the record of flood and drought data in the region beyond modern instrumental or observational limits, “and establish a robust year-by-year hydrologic reconstruction for the region over the past 1,000 years," Daniels said. The findings could improve flood modeling and guide infrastructure decisions.

“A lot of times decisions are made based on what's referred to as the 100-year flood, which isn't based on 100 years’ worth of data,” said Jessica Rodysill, PhD, a research geologist with the USGS. “What we want to get out of this is a thousand years worth of flood data from these lakes that we can use to better calculate the 100-year flood, the 500-year flood, and so on.”

On a warm July evening, the researchers pulled the first ever freeze core from LaSalle Lake. They excitedly noted its layering as it emerged from the bubbling water. The core was about a foot and a half long, equivalent to about 150 years of climate history. “It was like going after walleyes and catching a handful of nice 25-inchers,” Steinman said. “It was an excellent day of coring.”

Back in the lab at UMD’s Large Lakes Observatory, the core is carefully smoothed by hand with a wood-planer before it’s run through an X-ray fluorescence scanner that measures the chemical composition of the sediment. From there, the team hopes to determine relative rainfall amounts from each individual year and reconstruct past climate history. The core from LaSalle Lake represents the upper-most part of the sediment record, so the team will visit the lake again to dig even deeper into that historical archive.

For Steinman, it’s rewarding work that reminds him of his childhood spent playing in the mud and connecting with nature.

“I feel really fortunate that I get to go to beautiful settings like this and literally play with mud and … produce information that's actually helpful for people,” he said. “UMD provides the support and the foundation for research that directly impacts the lives of Minnesotans.”

Header image caption: UMD students Abe Underhill and Cole Barre work together to deploy a freeze coring device in Lake LaSalle. “One of my favorite things about my graduate studies is that I get to do my own field work,” Underhill said.