rocks and plants
The first method measures beryllium-10, an isotope that accumulates in the top few centimeters of rock exposed to the sky. By sampling the surface of a boulder and counting the beryllium, the researchers were able to learn how long it had been exposed. This technique put a date on the retreat of the ice sheet. It turned out that the Laurentide retreated 3,600 years ago in today’s ice margin, probably shrinking by about 3,000 more.
The second dating method looked at the dead plants. “There were all these plants growing, just like today. And included are mosses, the little mosses that are rooted in the soil,” says Young. “So they’re there, they’re living their lives, and eventually the Laurentide ice sheet starts to expand again. And when that ice edge covers the moss, it literally kills them.”
By radiocarbon dating the remains of the moss, the team was able to determine the moment it died, what glaciologists call a kill date – the moment the ice arrived at that location. By drawing multiple chill dates from moss samples from different spots, the scientists reconstructed a map of an advancing ice margin.
About 70 percent of the team’s radiocarbon dates cluster around 1800 CE, at the peak of the Little Ice Age. “It’s all kind of the same thing,” he says. The Little Ice Age marks the point when the glaciers of the Northern Hemisphere were at their most extensive in the last millennium. At Barnes, Young argues, it was just one late pulse in a progression that had been going on for two thousand years.
However, the most important plants for Young’s team are still hidden under ice. The Barnes Margin has retreated 200 to 800 meters since 1960, exposing mosses that were killed by about 1000 CE. Everything older remains buried. To identify the exact moment when the ice began to advance, the team had to look at Barnes’ neighbors.
Push the dirt
Scattered across the same plateau are other small ice caps, close enough to share the same climate. Early studies collected nearly 200 kill dates from among them—none older than 2,000 years. “What that means is that all those ice caps up there didn’t start advancing or expanding until about 2,000 years ago,” says Young. “Since Barnes Ice Cap is right next to it, it’s probably a good bet that it also started to expand 2,000 years ago.”