Cosmic Rays and Perturbed Boulders Suggest First People to Reach Americas Came by Coast

Cosmic Rays and Perturbed Boulders Suggest First People to Reach Americas Came by Coast
Boulders transported by glaciers. (Photo: Ian Watkinson/EGU)

The opening of the ice-free corridor that linked Beringia to the North American interior happened potentially thousands of years after the first human migrations to the continent, according to new evidence. Scientists say this finding should bolster the idea that ancient humans travelled to the Americas along a coastal route, but other researchers remain sceptical.

New research in Proceedings of the National Academy of Sciences pinpoints the appearance of an ice-free corridor linking Beringia to the Great Plains to around 13,800 years ago. Prior estimates suggested the corridor appeared around one thousand years earlier, as the last ice age was coming to an end. According to the new estimate, the first human migrations into the North American continent happened around 15,000 to 16,000 years ago, and possibly 20,000 years ago. The authors of the new paper say their findings strengthen the coastal migration hypothesis, in which the first people to reach the Americas did so by travelling along the Pacific coast.

“The ice-free corridor has long played a key role in hypotheses about the peopling of the Americas, but our results provide robust evidence that the ice-free corridor was not open and available for this,” Jorie Clark, the first author of the new paper and a researcher from the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University, explained in an email. “This has been inferred before, but the evidence for the age of the ice-free corridor opening was very uncertain and could not be used conclusively to address this question one way or the other.”

Clark and her colleagues used a dating method known as “cosmogenic nuclide surface exposure dating,” which works by “dating a boulder that was deposited by the ice sheet when it first retreated from the site, with the date telling us how long ago that boulder was first deposited by the ice sheet and exposed to the atmosphere,” she said. In simpler terms, they counted hits by cosmic rays to determine how long a boulder’s been sitting on Earth’s surface.

In an email, Ben Potter, an archaeologist from the Arctic Studies Centre at Liaocheng University in China who wasn’t involved with the new research, said he was “unconvinced” by the paper. Cosmogenic exposure dating provides minimum ages, not maximum ages, he said, adding that the researchers failed to provide reasons for rejecting other efforts to date the opening of the ice sheets, including research showing the emergence of a deglaciated and lake-free corridor by at least 15,000 years ago.

Establishing the timing of an overland route linking Eurasia to North America is important, as it carries implications for the Clovis-first hypothesis. That theory holds that people living in Alaska and Yukon travelled southward along the interior to the Great Plains, where they established the Clovis culture, named for their distinctive stone tools. Recent archaeological and genetic evidence has challenged this theory, pointing instead to a pre-Clovis migration to the Americas prior to the receding of the massive Cordilleran and Laurentide ice sheets. “Resolving this debate” over migration routes is “important for addressing the questions of when and how the first Americans arrived,” the scientists write in the new study.

Previous studies using other dating techniques are limited, said Clark, in that they only show that the ice-free corridor emerged some time before the acquired date. For example, “a radiocarbon date on a piece of organic fossil material only dates the time of when that fossil material lived, which could be any time after the ice-free corridor opened — we simply don’t know how long before the date that the IFC opened.” As for prior research that used cosmogenic exposure dating to date the ice-free corridor, they’re limited in terms of geographic scope and the amount of samples analysed, she added.

For the new analysis, Clark and her team studied glacially displaced boulders along 1,200 kilometres of the Cordilleran-Laurentide ice sheet suture zone, allowing them to sample 64 cosmogenic exposures. The team was able to “evaluate various potential uncertainties in the dates and derive a robust average date for each site,” she explained. Using cosmic rays to date boulders might sound weird, but Clark likened it to a suntan.

“When the boulder is first deposited by the retreating ice sheet, it is exposed to the atmosphere for the first time, including the cosmic rays which come from space and pass through the atmosphere and hit the surface of the Earth,” Clark explained. “This would be analogous to sitting outside for the first time after being indoors all winter and starting to be exposed to the sun. As soon as the boulder is first exposed, cosmic rays penetrate the boulder and produce new elements — cosmogenic nuclides — in the boulder, so with time, the concentration of these elements increases.”

Scientists can measure the concentration of these elements in the lab, and since they know how many new elements are produced each year, they can “calculate the time since the boulder was first exposed by retreat of the ice sheet,” Clark said. “Some people might question our dating method, but we feel confident that any adjustments to our ages will not change our bottom-line conclusion,” said Clark, to which she added: “We are also very confident in our results.”

Potter doesn’t share this confidence, saying the team used only one standard deviation for their interpretation when two were required. When using the more conservative value, the new evidence would indicate minimum ages for the opening of the ice sheets to some time between 13,000 and 15,600 years ago, he said. This range of uncertainty, Potter said, is consistent with numerous optically stimulated luminescence and infrared stimulated luminescence dating efforts that point to the appearance of an ice-free corridor by at least 15,000 years ago.

A key finding of the new paper is that a viable passageway for the first wave of humans to enter into North America by land did not exist until at least 13,800 years ago, and that the humans who migrated earlier must have done so by travelling along the Pacific coast. That this might be the case is not a huge surprise given other clues, such as 15,000-year-old archaeological evidence at the Cooper’s Ferry site in Idaho.

Potter believes we shouldn’t discount the interior route just yet. He said there’s “no widespread consensus that the oldest ages of scattered charcoal at Cooper’s Ferry relates to the occupations,” which some scientists have dated to 11,500 and 14,000 years ago. Thus, “the ice free corridor cannot be ruled out as a potential route for the earliest unequivocal sites south of the ice sheets” after 15,000 years ago, Potter wrote. And as he also explained, there’s still no unequivocally dated sites along the north Pacific coastal route prior to 12,600 years ago, and none from the Kuril Islands to the Aleutians and south central Alaska that date prior to 9,000 years ago, which is a fair point.

On this last issue, Clark would seem to agree. “While we may have addressed one question about the first peopling of the Americas, there is still a lot to learn about whether they actually did come down the coastal route, and if so, how did they travel — we need to find archaeological sites from this area,” she told me in her email.

The question as to when an interior corridor emerged and how the first humans managed to make their way into the continent remains unresolved. As it typical of archaeology, we simply need more evidence if we’re to truly understand this fascinating period in human history.