There is something strange about some skeletons buried in monasteries in Denmark.
Some appear to date back decades – even centuries – before their final resting place was even built.
So what’s going on?
Scientists from the University of Southern Denmark and the Isotope Research Center believe they have found the answer.
In a commentary article published in npj Heritage Sciencesthey have issued “words of warning” to archaeologists when dating medieval human remains.
Radiocarbon dating is a fairly reliable method for calculating the age of bones and artifacts, up to about 50,000 years old.
The process begins in space: cosmic rays interact with nitrogen in the atmosphere, producing small amounts of a specific isotope called carbon-14, which then binds with atmospheric oxygen to form carbon dioxide.
Trees and other photosynthetic plants then absorb this carbon from the air into their own cells, and in turn, the animals that eat those plants (as well as the animals that feed on those plants) do so as well. other animals that eat plants).
When these animals eventually die, they obviously stop eating and therefore stop adding new carbon-14 to their cells. From there, carbon-14 levels can only decrease over time through radioactive decay.
By analyzing the levels of carbon-14 inside the bones and comparing them to known historical levels of this isotope in the atmosphere, scientists can calculate the age of these bones quite accurately.
But there is a problem.
Regular radiocarbon dating works well for humans and animals that eat largely land animals and plants, but it’s a different story for those who eat a lot of fish, thanks to a phenomenon known as the “reservoir effect.”
Rivers, lakes and oceans have different ratios of carbon isotopes, due to the runoff of dissolved carbonates or the upwelling of much older carbon dioxide. This means that rivers, for example, can contain around 15% less carbon-14 than the atmosphere.
As a result, the bones of a person eating a diet high in fish may appear to be hundreds of years older than they really are.
“In extreme cases, a Rhine fish, for example, can appear up to 1,300 years older than a terrestrial organism that lived at exactly the same time,” says Kaare Lund Rasmussen, an archaeometrist at the University of Southern Denmark.
“But that doesn’t mean you can simply subtract 1,300 years from the date a person ate freshwater fish. Quite the opposite. The particular problem with freshwater is that the effect varies enormously. It can range from zero to 1,000 years.”
The team analyzed the bones of 27 individuals discovered in medieval Danish monasteries and, sure enough, six samples had particularly abnormal ages. In each case, the bones appeared to be decades or centuries older than the buildings they were buried in or near.
In one case, for example, a skeleton buried in the Danish town of Svendborg had been radiocarbon dated to between 1100 and 1155 CE – but the monastery was not erected until 1236.
Another, in the town of Viborg, was dated to no later than 976 CE, well over a century before the monastery was built on this land.
“In medieval Christian Europe, meat was restricted during the Lent period,” the researchers write.
“However, freshwater fish meat was tolerated as a food source because freshwater fish were considered ‘cold’ or ‘neutral’ in comparison to the meat of warm-blooded animals and marine fish. Freshwater fish was even in some Christian circles and cultures considered a prestigious food.”
There was more direct evidence that the reservoir effect also played a role.
The team analyzed the samples for an isotope called nitrogen 15, found in high quantities in aquatic ecosystems – and sure enough, high levels were detected in all samples.
Researchers suggest that this phenomenon is not unique to Denmark and is likely occurring elsewhere in Europe and the rest of the world. They use an example from a Stone Age site in the Balkans, where human remains were found pierced by arrows made from animal bones.
One might expect the two sets of bones to be about the same age, but previous studies have shown that human bones appear to be up to 500 years older than animal bone arrows.
Either a Stone Age prankster went and stuck then-modern arrows into bones that had been lying around for half a millennium, or humans ate fish and animals didn’t.
“The word of caution is that it is necessary to assess whether the excavated bones are affected by a freshwater reservoir effect and, if so, how large this reservoir effect might be,” the researchers conclude.
The team recommends measuring nitrogen-15 levels in bones, particularly those from the medieval period. If these levels are high, there are models that can help calibrate a more accurate age for the bones.
The commentary article was published in npj Heritage Sciences.
This article was fact-checked by Rachel Garner and edited by Peter Dockrill. Although we are proud of our process, we are only human. If you spot an error, please let us know.
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