“It’s taken up with photosynthesis,” Dr. Dee said.
The vast majority of the carbon in the atmosphere is carbon 12, a stable atom with six protons and six neutrons. Only a fleeting fraction is radioactive carbon 14, also called radiocarbon. That isotope of carbon is produced when cosmic rays — high-energy particles from the sun or beyond the solar system — interact with atoms in Earth’s atmosphere.
Scientists who study cosmic rays used to think that these particles arrived in a relatively constant barrage, meaning that the ratio of carbon 14 to carbon 12 in the atmosphere has largely remained steady over time. But then in 2012, researchers found two cedar trees in Japan that recorded inexplicably high levels of radiocarbon in their rings dating A.D. 774 to 775. That spike is now known as a Miyake event for its discoverer, Fusa Miyake, a cosmic ray physicist at Nagoya University in Japan. Other Miyake events have since been spotted in tree ring records, but they remain exceedingly rare.
“At the moment we only have three or four in all of the last 10,000 years,” Dr. Dee said.
But it just so happened that another Miyake event occurred during the Viking Age, in A.D. 992 to 993. Trees found worldwide record an uptick in carbon 14 around that time, and wood found at L’Anse aux Meadows should be no exception. In the hopes of pinning down the age of the Americas’ only confirmed Viking settlement, Dr. Dee and his colleagues turned to the unlikely marriage of dendrochronology — the study of tree rings — and astrophysics.
“We realized that this could change the game,” Dr. Dee said.
The researchers found that their three pieces of wood all exhibited a pronounced increase in radiocarbon that began 28 rings before their outer bark. Ring 28 must correspond to the year A.D. 993, the team concluded. They ruled out earlier and later Miyake events based on the carbon 14 to carbon 12 ratios measured in the wood, which vary in known ways over centuries.
With a date now pinned to an inner tree ring, “all you need to do is count to when you get to the cutting edge,” Dr. Dee said. The three pieces of wood the team analyzed were all felled in 1021, the researchers calculated.
Until now, estimates of when L’Anse aux Meadows was occupied have very much been “guesstimates,” said Sturt Manning, an archaeologist at Cornell University and the director of the Cornell Tree Ring Laboratory, who was not involved in the research. “Here’s hard, specific evidence that ties to one year.”