The first permanent settlement of Vikings in North America—a seaside outpost in Newfoundland known as L’Anse aux Meadows—has tantalized archaeologists for more than 60 years. Now, scientists at last have a precise date for the site: Tree rings show a Viking ax felled trees on the North American continent exactly 1000 years ago, in 1021 C.E. The result is a star example of a relatively new dating method using a spike in solar radiation that left its mark in tree rings around the world.
“The precision is astounding,” says Rachel Wood, a radiocarbon scientist at the Australian National University who wasn’t involved in the new study. “The idea to use these short-term sharp fluctuations in radiocarbon … has been around for a few years, but it is great to see it actually being used to date an important archaeological site.”
The Vinland sagas, a pair of Icelandic texts written in the 13th century, describe the Norse explorer Leif Erikson’s expeditions to a land referred to as Vinland. Although the texts contain their fair share of embellishment, most historians agree the sagas show Vikings sailed southwest from Greenland and reached the North American continent sometime at the turn of the millennium. The discovery of a Viking-era archaeological site in 1960 featuring the remains of distinctive Norse-style buildings, a bronze cloak pin, iron nails, and other Viking artifacts bolstered such evidence.
Scientists have previously dated bits of wood from the site using radiocarbon dating, which measures the decay of the radioactive isotope carbon-14 in organic material and often gives dates within a couple hundred years. Early radiocarbon-dating efforts at L’Anse aux Meadows dated the artifacts to between 793 and 1066 C.E.—not much help to historians looking for an accurate timeline of the Vikings’ arrival in North America.
A breakthrough in 2012 promised to refine those dates with the help of abnormally strong cosmic ray bursts. In the year 993 C.E., a large cosmic burst—probably a solar flare—caused a pulse in the production of carbon-14 in Earth’s atmosphere, which was taken up by plants around the world through photosynthesis. Every tree that was alive in 993 C.E. has a telltale ring with higher than usual carbon-14 content. By counting out from that ring, researchers can arrive at the precise year a tree died. A similar cosmic burst in 775 C.E. has already helped scientists precisely date the construction of a chapel in Switzerland and a volcanic eruption on the Chinese–North Korean border.
In the new study, researchers led by radiocarbon scientist Michael Dee at the University of Groningen applied this technique to a collection of wooden chunks that had been excavated from L’Anse aux Meadows through the 1960s and ’70s. Based on cut marks in the wood, archaeologists know they were chopped by metal axes, suggesting Vikings, rather than Indigenous people of North America, were responsible. For decades, those chunks were kept in a freezer by co-author Birgitta Wallace, an archaeologist with Parks Canada who has spent her career at site.
“[The artifacts] are not some beautiful objects or Viking artwork or anything like that,” Dee says. “They are really just the offcuts or refuse of Viking activity. … It’s a real tribute to [Wallace] that she had the foresight to do such a thing.”
Working with three chunks of wood with recognizable edges, the researchers radiocarbon dated the rings in each piece, looking for a telltale spike in carbon-14. In all three pieces, they found it in the 29th ring from the edge, indicating the trees had stopped growing 28 years after the 993 C.E. solar flare, or the year 1021 C.E., the researchers report today in Nature. Of course, that just confirms the Vikings were present in North America by that year, Dee notes, and it’s possible they arrived even earlier.
Another possibility is that the Vikings simply cut up wood that had been lying on the ground for years. But that’s unlikely, Dee says, because fallen wood quickly loses its strength—and historians think Vikings were seeking timber to bring back to relatively treeless Greenland. “There was no reason for them to pick up something and whack at it, rather than just cut down a brand new, solid tree.”
Lukas Wacker, a physicist who studies radiocarbon dates at ETH Zurich’s Laboratory of Ion Beam Physics, agrees that’s the most likely explanation. “[It’s convincing] that different artifacts—not all from the same tree—gave consistently the same result,” he says. “It is very unlikely that just by chance they have the same age.”
Source: Science Mag