By Grace Rodgers, Dec. 18, 2020 –
Researchers have long tracked the timing and retreat patterns of the North American Laurentide ice sheet, the greatest ice sheet to exist in the Ice Age. During that time, nearly two-thirds of the rise in the global sea level was caused by the melting of the Laurentide — the majority of which occurred over 10,000 years.
“That’s an interesting, dynamic problem. In many senses, they’re very few other elements that size on the planet that change that rapidly,” said Thomas Lowell, a geology professor at the University of Cincinnati.
For over 40 years, Lowell has studied the Laurentide and tracked two key behaviors the ice sheet exhibited during climate changes: the amount of meltwater and ice margin retreat. Lowell presented his most recent research on these two behaviors at the annual Comer Climate Conference, an annual summit where climate scientists from around the world gather to present emerging research.
Over a five-year research process, Lowell and a team of scientists assembled the first-ever annual chronology tracking the amount of meltwater released and the rate at which the Laurentide retreated. The chronology begins nearly 12,700 years ago and spans a 1,500-year transition between the late Younger Dryas, a geologic interval of colder temperatures, and the Holocene boundary, a geologic interval of warmer temperatures.
Lowell predicted the measurements would show a slower retreat rate and less meltwater during the cold interval, and a faster retreat rate and more meltwater during the warm interval. However, the chronology revealed the meltwater doubled at the transition between the cold and warm interval, while the ice sheet retreated at a constant rate.
“If you melt a glacier more, you expect it to shrink faster. But the interval that we recorded, [the retreat rate] didn’t seem to be paying attention [to the temperature],” said Lowell. “Even though it was melting faster, it was still backing up and retreating at the same rate.”
While melting in warmer temperatures was expected, Lowell was surprised the glacier retreated at the same rate despite the higher meltwater.
“There’s nothing super exciting about the [meltwater] findings, but coupled with the [retreat rate] findings, they seemingly conflict with each other,” said Lowell.
To quantify the amount of glacial meltwater, Lowell and the team of scientists recovered roughly 1,000 meters total of sediment cores, one meter at a time across lakes in Central North America. Back in the lab, scientists measure the core’s “varves”, the thickness of sediment layers deposited in one year. Each varve indicates the amount of melting that occurred in one year: the thicker the layer, the more meltwater.
Altogether, the sediment cores serve as a chronology of meltwater. Measuring varve units can be critical for assessing the response of both past and present ice sheets to climate change.
“[Varves] were an archive that we really needed to understand the ice,” said Dr. Andy Breckenridge, lead author and a geology professor at the University of Wisconsin. “We knew the record was there and we designed a strategy where we were going after modern lakes to reconstruct it.”
Breckenridge measured each sediment core, beginning by taking high-resolution scans to survey the varve thickness and color. Lighter layers indicate summer seasons, meaning the sediment is coarser due to warmer temperatures. Darker layers indicate winter seasons, meaning the sediment finer grain due to colder temperatures. Once tallied on a spreadsheet, Breckenridge identifies thickness patterns to organize the varve chronology.
While one half of the cores are used for data collection, the other half is stored in a refrigerated warehouse allowing researchers from around the world to come and study the chronological cores.
“[Other researchers] are going to be able to request samples from our cores,” Breckenridge said. “And that’s already happened at some of our sites. I know some of our cores have been used for a carbon storage question”
Now that these findings are published, Lowell’s current research question asks: are these melting patterns a behavior of the whole ice sheet or just a small part of it, and if so, is that small part important. Further research into past and present glacial melting is critical to help us understand sea level change that threatens the livelihood of millions around the world.
“The ice sheets have the potential to change sea level, enough to really matter to society,” said Lowell.
Grace Rodgers is health, environment and science reporters at Medill. You can follow her on Twitter at @gracelizrodgers.