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Ice sheet expansion may have started 2,000 years ago, but we canceled that
The tiny remains of the Laurentide Ice Sheet began getting somewhat less tiny.
Barnes Ice Cap sits on the central plateau of Baffin Island, in the Canadian Arctic. It is the last surviving fragment of the Laurentide Ice Sheet, the largest of the Pleistocene ice sheets, which at its peak buried most of Canada under two miles of ice and locked away around 80 meters of sea level. That remnant has now yielded direct physical evidence that ice sheets might have reversed their retreat and started to grow again thousands of years ago.
In a recent Science study, researchers show that the Laurentide reached its smallest size about 3,600 years ago, then started re-expanding roughly 2,000 years ago, which the team interprets as the opening move of the next glaciation. Apparently, this brand-new ice age was on its way until about 1960, when human-caused climate warming reversed it.
“Barnes is the last remaining piece of one of the domes of the Laurentide Ice Sheet,” says Nicolás Young, a geologist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the study. As that enormous dome thinned through the Holocene, its margin withdrew up onto Baffin Island, and what’s left today is the deflated core of something that was once two miles thick. The reason Barnes can testify about its own history, Young explains, comes down to the temperature at its base.
Most glaciers are warm-based, meaning meltwater at the bed lets them slide, and sliding ice grinds up everything beneath it, erasing the record. Barnes is cold-based, and thus frozen directly onto the bedrock beneath it, which prevents it from sliding.
“If there is an ice event where you live and you have to go and scrape off your windshield, that ice that’s plastered to the windshield of your car, that’s a good example of how cold-based ice operates. It’s sort of just stuck to what it’s on,” Young says.
This stuck ice preserves what’s beneath instead of eroding it. As the Laurentide retreated across the plateau, tundra plants colonized the newly bare ground. When the ice came back later, it simply smothered them in place and held them there, intact, for two millennia. To pinpoint the exact moment of this turnaround, Young’s team used two independent dating methods.
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.