Fish-Heavy Diets Made Medieval Danish Monks Look Centuries Too Old

Some skeletons found at medieval monasteries in Denmark appear to be older than the monasteries themselves, and researchers now say a diet heavy in freshwater fish is the likely reason. According to ScienceAlert, scientists from the University of Southern Denmark and the Center for Isotope Research have warned archaeologists that radiocarbon dates for medieval human remains can run far too old.

Their warning appears in a comment article in the journal npj Heritage Science. It is a commentary, not a full new study, and it draws on a set of bones the team analyzed.

How radiocarbon dating works, and where it slips

Radiocarbon dating is considered a fairly reliable way to date bones and artifacts up to about 50,000 years old. Cosmic rays interact with nitrogen in the atmosphere to produce small amounts of carbon-14, which combines with oxygen to form carbon dioxide. Plants absorb that carbon, and animals pick it up by eating plants or other animals.

When an animal dies, it stops taking in carbon-14, and the amount already in its tissues slowly falls through radioactive decay. By comparing the level left in a bone with known historical levels in the atmosphere, scientists can estimate its age.

The method works well for people and animals that eat mostly land plants and land animals. It runs into trouble with those who eat a lot of fish because of what is called the reservoir effect. Rivers, lakes and oceans carry different ratios of carbon isotopes than the air, for instance because of dissolved carbonate runoff or upwelling of much older carbon dioxide. Rivers can hold around 15 percent less carbon-14 than the atmosphere, ScienceAlert reported, so a fish-heavy eater can look hundreds of years older than they really were.

What the Danish bones showed

The team analyzed bones from 27 individuals excavated from medieval Danish monasteries. Six samples had especially odd ages, appearing decades or centuries older than the buildings where they were buried or nearby.

A skeleton from Svendborg was radiocarbon dated to between 1100 and 1155 CE, yet the monastery there was not built until 1236.
A skeleton from Viborg was dated to 976 CE at the latest, well over a century before the monastery was built on the site.

The researchers found more direct evidence of the reservoir effect. They measured nitrogen-15, an isotope that is high in aquatic ecosystems, and detected elevated levels in all the samples.

Why monks ate so much freshwater fish

The authors point to religious practice. In Christian medieval Europe, meat was restricted during Lent, they write, but freshwater fish was tolerated because it was considered cold or neutral compared with the meat of warm-blooded animals and marine fish. In some Christian circles it was even seen as a prestigious food.

Kaare Lund Rasmussen, an archeometrist at the University of Southern Denmark, said 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. He cautioned against a simple fix: researchers cannot just subtract 1,300 years from the date of someone who ate freshwater fish. Freshwater is a particular problem, he said, because the effect varies enormously, from zero to 1,000 years.

Why it matters

For anyone who reads about archaeological finds, the lesson is that a radiocarbon date is not always the final word on when someone lived. A date for a person's bones can come out too old if their diet leaned on fish, which could lead to wrong conclusions about when a grave, a burial ground or a community existed.

The researchers suggest the problem is probably not unique to Denmark and likely occurs elsewhere in Europe and the rest of the world. As an example, they cite a Stone Age site in the Balkans, where human remains were found pierced by arrows made of animal bones. Earlier studies found that the human bones looked up to 500 years older than the animal-bone arrows, even though both would be expected to be roughly the same age. One explanation, according to ScienceAlert, is that the humans ate fish and the animals did not.

What researchers recommend

The team says archaeologists should assess whether excavated bones are affected by a freshwater reservoir effect, and estimate how large it could be. They recommend measuring nitrogen-15 levels in bones, especially medieval ones. If the levels are elevated, there are models that can help calibrate a more accurate age.

Around Prague, based on reporting by ScienceAlert

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