Bones excavated from medieval Danish monasteries have been radiocarbon dated to decades or even centuries before their final resting places were constructed, according to a study published in npj Heritage Science.
Scientists from the University of Southern Denmark and the Center for Isotope Research investigated why carbon-14 dating placed certain human remains long before the foundations of the convents holding them. The discrepancy stems from the freshwater reservoir effect, which alters carbon isotope ratios in individuals with fish-heavy diets.
“In extreme cases, a fish from the Rhine [River], for example, can appear up to 1,300 years older than a land-based organism that lived at exactly the same time,” Kaare Lund Rasmussen, an archaeometrist at the University of Southern Denmark, told ScienceAlert.
“But that does not mean you can simply subtract 1,300 years from the date of a person who ate freshwater fish,” Rasmussen told ScienceAlert. “Quite the opposite. The particular problem with freshwater is that the effect varies enormously. It can range from zero to 1,000 years.”
How Radiocarbon Dating Measures Bone Age
Radiocarbon dating calculates the age of bones by measuring carbon-14 levels and comparing them to known historical atmospheric levels. Cosmic rays produce carbon-14 in the atmosphere, which enters plants, animals, and humans through the food chain.
However, rivers, lakes, and oceans possess different carbon isotope ratios due to dissolved carbonate runoff or upwelling of older carbon dioxide. This phenomenon causes freshwater to contain roughly 15 percent less carbon-14 than the atmosphere.
Consequently, the bones of humans consuming large amounts of freshwater fish can appear hundreds of years older than their actual age. Regular carbon dating functions reliably for populations eating land-based plants and animals, but aquatic diets skew the results significantly.
Anomalous Finds in Danish Monasteries
The research team examined bones from 27 individuals excavated across medieval Danish monasteries and identified six samples with anomalous ages. Each set of bones appeared older than the buildings where the individuals were buried.
A skeleton recovered in Svendborg yielded a radiocarbon date between 1100 and 1155 CE, despite the local monastery remaining unbuilt until 1236. Another individual in Viborg dated back to 976 CE at the latest, predating the regional monastery by over a century.
“In Christian medieval Europe, meat was restricted during the lent period,” the researchers wrote in the study. “However, the meat of freshwater fish was tolerated as a food source because freshwater fish were considered ‘cold’ or ‘neutral’ in comparison with meat from warm-blooded animals and marine fish. Freshwater fish was even in some Christian circles and cultures considered a prestigious food item.”
Nitrogen-15 Testing Confirms Aquatic Diets
Further analysis confirmed that the reservoir effect drove these dating anomalies through nitrogen-15 testing. Aquatic ecosystems maintain high levels of this isotope, and elevated nitrogen-15 appeared across all six anomalous samples.
Researchers noted similar discrepancies beyond Denmark, pointing to a Stone Age site in the Balkans where human remains appeared up to 500 years older than animal-bone arrows embedded in them. Previous studies indicate that differences in dietary intake between humans and the animals hunted for tools create these offset timelines.
“The word of warning is that it is necessary to assess whether excavated bones are affected by a freshwater reservoir effect, and if so, how large the reservoir effect could be,” the researchers concluded.
Questions About Medieval Skeleton Dating
Why do fish diets change radiocarbon dating results?
Freshwater sources contain lower concentrations of carbon-14 than the atmosphere because of dissolved carbonate runoff and older carbon dioxide upwelling. When humans consume fish from these environments, their bones absorb those altered isotope ratios, making the skeletal remains appear centuries older than they actually are.
How do scientists correct for the freshwater reservoir effect?
Researchers measure nitrogen-15 isotope levels within excavated bone samples to detect heavy aquatic consumption. When elevated nitrogen-15 levels appear, scientists apply specific calibration models to adjust the radiocarbon dating and determine a more accurate age for the remains.
Is this dating distortion unique to medieval Denmark?
The distortion occurs globally wherever populations consume large quantities of freshwater fish. Researchers identified similar dating gaps at archaeological sites across Europe, including Stone Age locations in the Balkans where human remains predated associated animal-bone artifacts by centuries.
Assessing Freshwater Reservoir Impacts
The research team recommends testing nitrogen-15 levels in all excavated medieval bones to identify potential freshwater dietary signatures. Calibrating dates using established models prevents archaeologists from misinterpreting the chronology of historical sites.
The researchers suggest that this phenomenon is probably occurring elsewhere in Europe and the rest of the world.