Archaeologists are recalibrating their understanding of medieval chronology after discovering that human remains in Danish monasteries appear to predate the buildings themselves.
Researchers from the University of Southern Denmark warn that the “reservoir effect” in freshwater fish consumption is causing inaccurate radiocarbon dating, sometimes by over a millennium.
The Reservoir Effect Distorts Carbon Dating
Radiocarbon dating, the gold standard for chronometric analysis of organic matter, relies on the predictable decay of carbon-14. When organisms die, the intake of atmospheric carbon ceases, allowing scientists to calculate the time elapsed since the organism’s death. However, this model assumes a terrestrial diet. For populations relying on freshwater fish, the internal clock is skewed.
Rivers and lakes often contain carbon that has been sequestered for centuries, often through dissolved carbonate runoff. This results in aquatic ecosystems containing significantly less carbon-14 than the atmosphere.
Consequently, an individual consuming a high-fish diet incorporates “older” carbon into their skeletal structure. In extreme scenarios, such as fish from the Rhine, this discrepancy can make an organism appear up to 1,300 years older than a land-based counterpart living at the same time.
Archaeologists Find Skeletal Remains Predating Medieval Danish Monasteries
The research team, publishing their findings in npj Heritage Science, analyzed 27 individuals excavated from medieval Danish monasteries. The data revealed six samples with anomalous ages, appearing to predate the monastic foundations by decades or even centuries.
In one instance, a skeleton in Svendborg was radiocarbon dated to between 1100 and 1155 CE, despite the monastery not being constructed until 1236. A similar discrepancy occurred in Viborg, where remains dated to 976 CE were found in a site established over a century later. The researchers attribute this to the medieval Christian practice of fasting, where freshwater fish was permitted as a “cold” or “neutral” food source during Lent.
“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 in their article.”
Nitrogen-15 as a Diagnostic Metric
To confirm the reservoir effect, the team analyzed the samples for nitrogen-15, an isotope that accumulates in higher concentrations within aquatic food chains. Elevated levels were detected in all anomalous samples, providing a clear chemical signature of a fish-heavy diet. This methodology allows researchers to identify which remains are likely to suffer from carbon-14 age inflation.
This phenomenon is not isolated to Denmark. Similar discrepancies have been observed in previous studies of Stone Age sites in the Balkans. In those cases, human remains appeared 500 years older than animal-bone arrows found in the same context.
“The particular problem with freshwater is that the effect varies enormously. It can range from zero to 1,000 years,” said Kaare Lund Rasmussen, an archeometrist at the University of Southern Denmark, noting that the effect size is highly variable, ranging from zero to 1,000 years, preventing a simple linear subtraction of time.
Calibrating Future Archaeological Data
| Factor | Impact on Radiocarbon Age | Diagnostic Marker |
|---|---|---|
| Terrestrial Diet | High accuracy (Baseline) | Standard Carbon-14 ratio |
| Freshwater Fish Diet | Appears artificially older | Elevated Nitrogen-15 levels |
| Marine Fish Diet | Variable (Reservoir dependent) | Isotopic carbon-nitrogen shift |
