Eating a lot of freshwater fish could make a medieval skeleton appear hundreds of years older than it really was, a Danish-Dutch research team has warned after re-examining the radiocarbon dates of 27 people buried in Danish monasteries. The team found six of the 27 skeletons had been dated incorrectly, including one from Svendborg whose bones were dated to before the monastery where it was buried had even been built.

The skeleton, known as III9B, was given a prestigious burial at the Franciscan monastery in Svendborg, on the island of Funen. Previous radiocarbon dating placed the person's death sometime between AD 1100 and 1155. But Svendborg's Franciscan monastery was not founded until 1236, almost a century later. The two dates clearly cannot both be right.

A date that could not be right

Professor Emeritus Kaare Lund Rasmussen, from the Cultural Heritage and Archaeometric Research Team at the University of Southern Denmark, led the work with Johannes van der Plicht, from the Center for Isotope Research in Groningen in the Netherlands, and Jesper Lie Boldsen, from the Department of Forensic Medicine at the University of Southern Denmark.

Together they re-examined the dating of 27 people buried at some of Denmark's approximately 140 medieval monasteries, either in monastic cemeteries or inside the churches themselves. Rasmussen said they could say with certainty that another five of the 27 skeletons had also been incorrectly dated. The six incorrectly dated skeletons include one from the Monastery of Our Lady in Viborg, where previous dating put even the latest end of its estimated range at AD 976, more than 200 years before the monastery was built. At the Cistercian monastery at Om, also in Denmark, the same problem occurs in three burials.

Why fish makes bones look older

The explanation lies in something these people ate while they were alive: freshwater fish. Fish from lakes and rivers can contain less carbon-14 than food from terrestrial sources, because old carbon can be released from sediments along the shores of lakes and the banks of rivers. When people consume this carbon in their food, some of it becomes incorporated into their bones. Centuries later, radiocarbon dating can therefore make those bones appear older than they really are.

The size of the effect varies enormously, which is what makes it so difficult to correct. Rasmussen explained that the effect can range from zero to 1,000 years, and that in extreme cases a fish from the Rhine can appear up to 1,300 years older than a land-based organism that lived at exactly the same time. That does not mean researchers can simply subtract 1,300 years from the date of a person who ate freshwater fish.

Saltwater is easier to correct

Saltwater fish can also make human bones appear older than they really are. Here too the effect varies geographically between different parts of the world's oceans, but it is considerably more predictable than the freshwater effect. In the North Sea, for example, the effect is well established at around 400 years, according to the researchers, so for an organism that obtained all its carbon from the marine food chain the date can be corrected using a known reservoir effect.

To draw attention to the problem, Rasmussen and his colleagues have published a warning in the journal npj Heritage Science, titled Words of warning about radiocarbon dating medieval human bones. They hope archaeologists and others working with radiocarbon dating of fish-eating humans and animals will take heed, and make the necessary corrections. Rasmussen said the results made it abundantly clear that this was a major problem, and not just in Denmark, but probably across the rest of Europe too.

Frequently asked questions

What is radiocarbon dating? A method that estimates the age of materials containing carbon by measuring the radioactive isotope carbon-14, which decays at a known rate after an organism dies.

Why does eating fish affect the result? Freshwater fish can contain less carbon-14 than land-based food, because old carbon is released from lake and river sediments, so the bones of heavy fish eaters can look older than they are.

Does this mean all old radiocarbon dates are wrong? No. The problem affects bones where the diet had a large freshwater fish component; the researchers urge archaeologists to check for it and apply corrections.