Rocks and plants
The first method measures beryllium-10, an isotope that accumulates in the top few centimeters of rock that is exposed to the sky. By sampling a boulder’s upper surface and counting the beryllium, researchers could learn how long it has been uncovered. That technique put a date on the ice sheet’s retreat. It turned out the Laurentide had withdrawn to within today’s ice margin by 3,600 years ago, probably shrinking further by around 3,000.
The second dating method looked at the dead plants. “There were all these plants growing, just like today. And included in that are mosses, these little mosses that are rooted in the ground,” Young says. “So they’re there, they’re living their life, and eventually the Laurentide Ice Sheet starts to re-expand. And when that ice margin covers the moss, it literally kills it.”
By radiocarbon dating the remnants of the moss, the team could pinpoint the moment it died, yielding what glaciologists call a kill date—the moment the ice arrived at that spot. By stringing multiple kill dates of moss samples taken from different spots, scientists reconstructed a map of an advancing ice margin.
About 70 percent of the team’s radiocarbon dates cluster around 1800 CE, at the culmination of the Little Ice Age. “It’s all sort of the same thing,” he says. The Little Ice Age marks the point where Northern Hemisphere glaciers were at their most extensive in recent millennia. At Barnes, Young argues, it was just one late pulse within an advance that had been running for two thousand years.
The most important plants for Young’s team, however, are still hidden under ice. Barnes’ margin has retreated 200 to 800 meters since 1960, exposing mosses killed from about 1000 CE onward. Anything older remains buried. To pinpoint the exact moment the ice started advancing, the team had to look at Barnes’ neighbors.
Pinning down the turn
Scattered across the same plateau are other small ice caps, close enough to share the same climate. Earlier studies collected nearly 200 kill dates from beneath them—not one is older than 2,000 years. “What that means is that all of those ice caps up there didn’t start advancing or re-expanding 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 started expanding 2,000 years ago.”