When and how the first humans reached North America is a complicated puzzle. Scientists believe these people arrived in Alaska after crossing the Bering Strait, only to be stopped by the enormous Laurentide and Cordilleran ice sheets. The migrants would eventually make their way down to the Americas via one of two paths: the western coast of Canada or an opening between the two ice sheets.
The question, then, is which path opened first? According to a new study in Geophysical Research Letters, ice on the western coast of Canada was retreating by 18,100 years ago, potentially thousands of years before the inland route.
“This means that while there was probably still a lot of ice around, there was dry ground in some places, so humans could have found refuge during their journey,” said Marc Caffee, a professor of physics and astronomy at Purdue University and co-author of the paper.
Scientists don’t know exactly when the corridor between the ice sheets opened up, but they think it was between 13,000 and 15,000 years ago. Remains of a human settlement in Chile suggest people were there 14,500 years ago, which means if they had taken this route, they would’ve had to hurry. Evidence for migration along the coast has started to accumulate over the last few years, with these findings being the most recent addition.
The research team, led by Christopher Darvill, a researcher at the University of Manchester, collected rock samples from remote islands along Canada’s western coast before bringing them back to the lab. Chipped from the tops of boulders and bedrock, these samples were examined for concentrations of beryllium-10, a rare isotope of beryllium often used as a proxy for solar intensity. Measurements were made in Purdue’s PRIME Lab, a research facility for accelerator mass spectrometry.
“The isotope accumulates in certain rocks over time as they are bombarded by high-energy particles – cosmic rays – from outer space,” Darvill said. “Measuring the isotope tells you how much time has passed since the rock was exposed by retreating ice.”
This beryllium-10 chronology revealed that if the western margin of the Cordilleran Ice Sheet reached the continental shelf during the last glacial period, it did so before 18,100 years ago. The following retreat would have exposed several islands that could have been used by early people migrating south.
The Cordilleran Ice Sheet was about the same size as the Greenland Ice Sheet, and they’re both marine-based. For these reasons, the Cordilleran is often used as a proxy for what might happen to the Greenland Ice Sheet as climate warms.
The research was supported by funding from the Tula Foundation, Natural Sciences and Engineering Research Council of Canada Discovery grants, Canada Research Chairs Program and the Canada Foundation for Innovation.
Source : Purdue University