Radiocarbon dating error: freshwater fish made monks look older

How does the freshwater fish problem in radiocarbon dating work?

Living things take in radioactive carbon-14 and start losing it when they die. Count what is left and you can estimate how long ago that happened. Rivers and lakes complicate this. Older sediments dissolve carbonates into the water, leaving it with roughly 15% less carbon-14. This is called the reservoir effect, and it passes into the fish and then into whoever eats them.

A fish from the Rhine can look up to 1,300 years older than a land animal that lived at the same time. But first author Kaare Lund Rasmussen warns you cannot just subtract 1,300 years from a fish eater's date. The effect "can range from zero to 1,000 years," he said, depending on the water.

How did the team confirm it?

Historical and archaeological records suggested these people ate mostly fish. Measurements of nitrogen-15, which gives clues about diet, showed that the six misdated individuals very likely ate a lot of freshwater fish.

Rasmussen calls it "a major problem," and not only in Denmark but probably across Europe. The team calls it the "freshwater trap" and says it should be considered whenever medieval remains are dated. Because it varies by region, checking indicators like nitrogen-15 alongside the carbon dates is a sensible start.