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Saturday, May 18, 2024

A Cosmic Ray Event Pinpoints the Viking Landing in Canada

To the untrained eye, the L’Anse aux Meadows archeological site on the island of Newfoundland—since 1978, a UNESCO World Heritage Site—doesn’t look like much. The reconstructed Viking huts and workshops are quaintly photogenic, but they are, after all, reconstructions. All that remains of the original buildings are low protuberances in the surrounding fields: the remains of timber-and-turf structures, covered by lush grass. But the site, modest as it is, represents a key moment in history: It is proof that Europeans crossed the Atlantic some five centuries before Columbus. And while scholars continue to debate how far the Vikings voyaged or why they came to these shores in the first place, they now have a much better idea of when they were here, thanks to a new study that places the Vikings at this spot in AD 1021—exactly 1,000 years ago.

Though the L’Anse aux Meadows site had been studied since the 1960s, only rough estimates of its age had been possible until now. Radiocarbon dating, which was in its infancy when the site was first studied, yielded results with wide margins of error. But a new technique that leverages astrophysics in the aid of archeology has lent the process a far greater degree of precision.

The key to this technique is a “cosmic ray event,” a burst of energetic particles from space—likely from the sun—that struck Earth’s atmosphere in the late 10th century AD. “We think in 992, the sun sent out a big burst—either a solar flare or a coronal mass ejection—of highly energetic particles,” says Michael Dee, a geophysicist and archeologist at the University of Groningen in the Netherlands. When they hit our atmosphere, the barrage of particles triggered the production of carbon-14, a radioactive isotope of carbon, which was absorbed by plants around the world in the following year, AD 993.

Carbon-12, with six protons and six neutrons, accounts for about 99 percent of all the carbon on Earth; the slightly heavier carbon-13, with an extra neutron, accounts for about 1 percent. Carbon-14, which has two extra neutrons and is radioactive, occurs only in trace amounts, accounting for about one out of every trillion carbon atoms in the atmosphere. But the solar outburst caused carbon-14 levels to jump by about 12 percent, the authors say. Trees all over the world, if they were alive at that time, contain a ring documenting this carbon-14 spike. So if you’re lucky enough to find wood from a tree that was alive when one of these extreme solar storms happened, you just need to count outwards from the ring in which the spike was measured to the edge of the tree, to determine the date on which it was felled.

The objects Dee and his colleagues studied, recovered from L’Anse aux Meadows decades ago and carefully preserved in a freezer in a Parks Canada storage facility in Dartmouth, Nova Scotia, fit the bill perfectly. They include a tree stump that may have been pulled from the ground as the land around the Viking site was being cleared—and which, critically, still had its “bark edge” intact. Since there were 28 rings from the carbon-spike ring to the edge, the cutting of the tree can be pegged to AD 1021. (The fact that this is exactly 1,000 years ago is just a coincidence, though a welcome one, Dee says.)

The team of Dutch, German, and Canadian scientists, led by Dee and his Groningen colleague Margot Kuitems, published their study in Nature on October 20. One of their coauthors is Birgitta Wallace, a Canadian archeologist who has worked at the site since the 1960s. Dee credits Wallace, who is now in her late seventies, with having the presence of mind to preserve the bits of wood used in the current study. “A lot of people would have just chucked it away. But she figured science might one day have a use for them, and put them in the freezer to keep them well-preserved for 40 years,” he says.

“It’s a really nice paper—it dates this wood very precisely,” says Timothy Jull, an expert in radiocarbon dating at the University of Arizona, who was not involved with the current study. Previously, studies using dendrochronology—the science of determining a tree’s age from the relative growth rates recorded in its rings—required cross-comparisons involving large numbers of trees, in order to calibrate a new sample and come up with an (often quite rough) estimate of its age. “But in this case, they didn’t need to do that, because they have this spike that tells them precisely where they are [in the timeline]. That’s what makes it such a nice study,” says Jull.

Scientists had long believed that the highly energetic particles produced by solar activity and other astrophysical sources like supernovas arrive on Earth in a more or less steady stream. That would mean that the ratio of carbon-14 to its stable cousins would be fairly constant over time. But in 2012, a Japanese physicist, Fusa Miyake, found trees containing a carbon-14 spike dating from AD 774 to 775. Scientists now believe there have been a handful of these bursts of high-energy particles over the last 10,000 years.

Because these events are so rare, researchers like Dee and his colleagues can be confident they’re not just looking at some random carbon-14 spike, but a specific one—which means they can be confident of the date they attach to it. Other spikes, meanwhile, can be used to pinpoint other historical events. (The same technique was used recently to pin down the date when a medieval church in Switzerland was built, from a study of its roof beams.)

Aside from the archeological evidence, there are also written accounts of the Viking seafaring expeditions preserved in the famous Norse sagas. (Historians often use the term “Norse,” though in popular usage “Viking” has become the more common label.) The Norse homeland was in Scandinavia; they eventually settled in Iceland and, by the late 10th century, in Greenland. They also ventured across Europe and into the Middle East. The sagas describe how the Norse, sailing west and south from Greenland, reached a place they called Vinland, which they described as much greener and more hospitable than the land they had left behind. The sagas also detail encounters—sometimes peaceful, sometimes not—with the indigenous peoples of the lands they were exploring.

While the sagas were written centuries after the events they purportedly describe, the new paper seems to give credence at least to the broad brushstrokes of the tales they recorded. The new paper “effectively demonstrates that the textual tradition is pretty accurate,” says Sturt Manning, an archeologist at Cornell University. “We’re finding that the thousand-year-old literary tradition is basically correct.” Valerie Hansen, a historian at Yale University, is more circumspect: “We have to treat the sagas in the same way we think about Hollywood films based on a true story: The writers start with a germ of truth, which they embellish considerably.”

While the Atlantic crossings undertaken by the Vikings have long intrigued historians, the voyages can also be seen in the broader context of human migration—a story that began when Homo sapiens left Africa, sometime between 50,000 and 75,000 years ago. Some groups ventured east into Asia, while others moved north into Europe. Some of those Asian populations then migrated further eastward, crossing a land bridge into the Americas more than 20,000 years ago. For millennia afterward, Europeans and Americans were relatively isolated, with the Atlantic forming a barrier between their worlds. When Vikings sailed westward across that ocean a thousand years ago, humans had finally circled the globe.

The first encounter between the Vikings and indigenous locals may not have happened at L’Anse aux Meadows, but it surely happened somewhere in the area, says Wallace. Moreover, the archeological evidence, including fire pits and arrowheads, points to the presence of indigenous peoples at the L’Anse aux Meadows site, even if the two groups were not there at exactly the same time.

“The Norse would have known there were other people in L’Anse aux Meadows before them,” says Wallace. “There was definitely contact. But most of the contact was probably much further south—and we know they went at least as far south as New Brunswick.” (That claim rests on fragments of butternuts and butternut wood found at the site; butternut trees have never grown on the island of Newfoundland but were once plentiful in New Brunswick.)

For whatever reason, the Viking experiment in North America, which left its only indisputable evidence on the remote, northernmost tip of the island of Newfoundland, appears to have been short-lived. They explored; they gathered resources—especially timber, which was essential for building and repairing their longships—but they didn’t stay. “We think their main objective was to find a place where they could harvest timber and take it back to Greenland,” says Dee. “But that’s speculation. It could be that they had originally wanted to colonize the place, to stay and live there—but something went wrong. The sagas mention conflict with local indigenous people, but we can’t know that for sure.”

While the grass-covered ruins at L’Anse aux Meadows seem frozen in time, the stories we tell about the Vikings have evolved. A bronze sculpture on a hilltop near the entrance to the site depicts six Vikings (five men and one woman) armed with swords, spears, and shields, boldly seeking new lands. But inside the visitor center, the newer exhibits give a more nuanced view, with more balance between the European and indigenous perspectives. The text in one of the displays reads: “The landscape of Newfoundland and Labrador and the Maritimes of 1,000 years ago was populated by several different cultural groups, possibly with overlapping homelands and trade routes. We do not know for sure whom the Norse encountered, although it must have included the ancestors of the Aboriginal people who still live here today.”

Still, the story is often framed from a European viewpoint. “Our entire history of North America is basically written in terms of the European perspective,” says Manning. “Up until very recently, we’ve dated Indigenous sites on the basis of what European goods they had or did not have.” Today’s archeologists and historians are gradually embracing the need for a broader perspective, he says. “The Norse people arrive—no question about that. But whether you describe it as a heroic crossing of the sea, or as the first signs of an impending invasion, there are different views on that. indigenous people might wonder why we’re celebrating the beginning of a thousand-year disaster.”

Still, he sees the Vikings only as a precursor of what was to come: “It’s really only when you get to people like John Cabot and [English merchant] William Weston at the end of the 15th century that you get that key moment when ‘worlds collide’ in a fateful way.”

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