Tracking Ice Sheets When They Were Smaller than Today
Tracking Ice Sheets When They Were Smaller than Today
Glaciers leave telltale clues about their past on the landscape. As the ice retreats, it leaves behind boulders and rocky debris called moraines that mark the edges of the ice sheet through time. What’s harder to figure out is what ice sheets looked like during ancient warm periods when they were smaller than today.
It’s valuable information. As global temperatures rise, knowing just how far Greenland’s ice sheet shrank in the past could help scientists predict sea level rise in the future.
Nicolás Young is on the trail of an answer. Young, a 31-year-old geochemist at Columbia University’s Lamont-Doherty Earth Observatory, was just named a winner of a 2015 Blavatnik Award for post-doctoral scientists for his work measuring ice sheets in changing climates and their contributions to sea level rise. His current projects are taking glacier tracking to the next level.
Young is putting two innovative methods to work mapping the outlines of Greenland’s glaciers during a warm period 6,000 years ago called the Holocene Thermal Maximum, when temperatures were as warm or slightly warmer than today. One method uses lakes at the edges of the Greenland ice sheet. The other method, Young dubs “measuring the suntan on a rock.”
“That period may be a good analog for future warming that we’re likely to see by 2080-2100 and beyond,” Young said. “We know the Greenland ice sheet was smaller than today during that window, but we don’t know how much smaller. This new project is going to try to figure out how small the ice sheet was and how much its melting contributed to sea level rise.”
Blue lakes and black lakes
When you look at aerial photos of Greenland, you’ll notice lakes near the edge of the ice sheet that are either grey/blue or black. Young saw those lakes and recognized a new way to measure the history of the expanding and retreating Greenland ice sheet.
He is currently co-leading a four-year project, financed by the National Science Foundation, that will use sediment cores from those lakes to determine when their catchment basins were covered by ice.
When an ice sheet overlaps a lake’s catchment basin, the water flowing into the lake leaves a layer of minerals on the lake floor. The grey lakes are those still receiving mineral-laden water from the ice sheet. When the catchment basin is ice-free, organic material dominates the lake and the lake appears black.
Scientists can take a sediment core from the bottom of a lake, date the sediment layers and determine when the lake was receiving meltwater from the ice sheet based on the mineral content of each layer. Pairing that information with mapping of each catchment basin under the current ice sheet, they can reconstruct the changing size of the ice sheet over time. Ice sheet modelers will then use that data to gauge sea-level rise tied to the melting Greenland ice sheet through history. Because the Greenland ice sheet is entirely on land rather than floating, any melting adds to sea level rise.
Measuring the suntan on a rock
The “suntan on a rock” method provides a different way of tracking the advance and retreat of glaciers. Of course, this isn’t a suntan in the traditional sense.
When rocks are exposed to the atmosphere, they are bombarded with cosmic ray neutrons – the same neutrons raining down on all of us right now, day and night. In quartz, a mineral commonly found in Greenland’s rocks, those neutrons create isotopes including beryllium-10 and carbon-14.
Scientists know how quickly the isotopes form in rock and how quickly they decay, so they can tell how long the rocks have been exposed since they were last covered by an ice sheet. By comparing the fast-decaying carbon-14, which has a half-life of about 5,730 years, and much slower-decaying beryllium-10, with a half-life closer to 1.4 million years, they can also tell if the rocks were exposed in the past, covered by an ice sheet, and re-exposed. The process is called cosmogenic dating.
Joerg Schaefer, who heads the Cosmogenic Nuclide Group at Lamont, puts Young’s skills and accomplishments into perspective: Young was hired at Lamont to bring in Arctic glacier and geology expertise that is normally found well above his career level, and he has lived up to the challenge, Schaefer said. Young currently runs the highly complicated carbon-14 lab, one of only a few such labs in the world.
“Nicolás is outstanding in breadth and depth of scientific excellence for his career level,” Schaefer said. “He is very savvy in climate and climate geology questions, he has quantitative skills, and he has hardware skills on top of that. He’s in a leader in all of these different fields.”
How sensitive are ice sheets?
Another area of Young’s work that drew the award committee’s attention came from his dissertation. He was looking at short periods of cooling, including one about 8,200 years ago. His work found that at least parts of the Greenland ice sheet responded to abrupt cooling events by pausing their retreat and briefly advancing.
“It tells a lot about how sensitive the Greenland ice sheet is,” Young said. “Where some people think you would need a cooling event of several hundred years to millennia, this shows that a very brief, at least geologically speaking, cooling event can trigger a response from the ice sheet.”
“I thought of that as being somewhat hopeful for the future,” Young said. “We know the ice sheet is retreating now, but if we somehow got our act together and temperatures leveled off or went down, we might be able to elicit a response from the ice sheet.”
Young has worked on glaciers in the Western United States, Alaska, Bhutan, and Baffin Island, as well as Greenland. He was inspired to focus on ice while working on a study near Alaska’s Columbia glacier as a student at the College of Wooster in Ohio, and he credits his advisors there, at the University of New York, Buffalo, and now at Lamont for helping him build his success.
Young will receive the Blavatnik Regional Award in the physical sciences category, which carries a $30,000 prize, on November 9 at the New York Academy of Sciences Annual Gala. The award was created in 2007 by the Blavatnik Family Foundation to honor young postdoctoral and faculty scientists in the New York, New Jersey and Connecticut area.