stones 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, researchers could figure out how long it has been exposed. This technique set a date for the ice sheet to retreat. It turns out that the Laurentide had retreated to the current ice limit 3,600 years ago and probably continued to shrink for about 3,000 years.
The second dating method involved examining the dead plants. “There were all these plants growing, just like they do today. And that included mosses, these little mosses that are rooted in the ground,” Young says. “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 it.”
By radiocarbon dating the moss’ remains, the team was able to determine the time when it died, establishing what glaciologists call the kill date – the time at which the ice arrived at that location. By stringing together multiple kill dates from moss samples taken from different locations, scientists reconstructed a map of an advancing ice edge.
About 70 percent of the team’s radiocarbon dates date from around 1800 AD, the height of the Little Ice Age. “It’s all one and the same,” he says. The Little Ice Age marks the point at which the Northern Hemisphere’s glaciers reached their greatest extent over the past millennia. In Barnes, Young argues, it was just a late impulse in a push that had been underway for two thousand years.
However, the most important plants for Young’s team are still hidden under ice. The edge of Barnes has retreated 200 to 800 meters since 1960, exposing mosses that were killed off from about 1000 AD. Everything older remains buried. To determine the exact moment the ice began to advance, the team had to take a look at Barnes’ neighbors.
Nail the curve
Scattered across the same plateau are other small ice caps that are close enough to share the same climate. Previous studies collected nearly 200 killing dates among them – none of which are older than 2,000 years. “That means all those ice caps up there didn’t advance or expand again until about 2,000 years ago,” Young says. “Since the Barnes Ice Cap is right next door, it’s probably a good bet that it also began expanding 2,000 years ago.”
