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# Freshwater fish in the diet makes radiocarbon dates of bones from Danish monasteries look older
- URL: https://theparnassian.com/en/rybnaia-dieta-mozhet-udrevniat-radiouglerodnuiu-datirovku-kostei/
- Published: 2026-11-12T12:21:15.000Z
- Updated: 2026-11-12T14:11:00.000Z
- Author: Alexandr Radayev
- Tags: Archaeology, Faith, Science, Nature

**A fish diet can make radiocarbon dates of bones look older**

A Danish-Dutch team of scientists has checked the radiocarbon dating of 27 burials in medieval Danish monasteries. One would naturally suppose that monks and noble laymen could not have been buried at a monastery before the brethren had built it. Yet radiocarbon analysis of the bones placed the death of a man from the Franciscan friary in Svendborg between 1100 and 1155, although the friary itself came into being only in 1236\. The researchers identified five more such cases, including the monasteries of Viborg and Ebeltoft.

A similar error arose in dating the bones of the English king Richard III, which the analysis assigned to 1412–1460, although the king perished in 1485.

The authors name freshwater fish as the cause of this discrepancy.

Let us see what is going on here. Fish from rivers and lakes contain less carbon-14, because ancient carbon from bottom sediments and banks enters the water; scientists call this the reservoir effect. A person who ate a great deal of such fish accumulated old carbon in his bones, and his remains may appear older than their true age.

Accordingly, in such cases a certain calibration is required. For fresh water the shift ranges from zero to a thousand years, so researchers cannot introduce a single correction. For sea fish the effect is predictable: in the North Sea it amounts to about 400 years.

In Catholic Europe there were about 180 fast days a year on which meat was forbidden but freshwater fish was permitted. Catholic tradition allowed fish on all days of fasting, Lent included. If one were to try applying this pattern to Russian material, admittedly with an allowance for considerably smaller consumption of fish, much of interest might come to light.