Words of warning about radiocarbon dating medieval human bones

Radiocarbon dating medieval human bones can produce misleadingly old ages when people ate substantial amounts of aquatic food. Fish absorb carbon from rivers and lakes that is depleted in carbon-14 compared with the atmosphere; this depletion passes into human bone collagen and makes the remains appear older than they are. Freshwater reservoir effects are especially difficult to correct: unlike the comparatively predictable marine offset of about 400 years, freshwater offsets can range from negligible to roughly 1,000 years, depending on local conditions.

The warning is based on 27 burials from prominent locations in Danish monasteries. Their collagen had elevated nitrogen-15 values, indicating significant freshwater-fish consumption. When the radiocarbon results were calibrated without reservoir corrections, several burials appeared to predate the monasteries in which they were found - by at least a century in one case and more than two centuries in another - an implausible result given the burials’ relationships to the buildings. Carbon-13 measurements suggested only a small marine contribution, while nitrogen-15 pointed to freshwater protein. The authors explain that stable-isotope measurements can help identify dietary sources and estimate offsets, but mixed diets make corrections complex and require specialized models. They urge archaeologists to assess diet and local reservoir effects before interpreting dates from medieval human remains, in Denmark and elsewhere in Europe.