Why Did These Vikings' Bones Appear Older Than They Are?

Published: May 28, 2019

ONSCREEN: When this mass grave was discovered in England, archaeologists thought the bones belonged to a 9th-century Viking army.

CAT JARMAN: Everything, the bones, artifacts, the coins, everything is really screaming Viking Great Army. But the science, the radio carbon dates said that’s not possible.

NARRATOR:Scientists date bones by measuring the amount of carbon-14 they contain. This radioactive isotope remains in the skeleton after death. It decays over time at a steady rate, so, by measuring what’s left in the bones, scientists can figure out roughly when the person died.

JARMAN: What we didn’t realize 20 years ago, we actually have to take into account how the carbon that we’re dating gets into our bodies - and it actually gets into our bodies through the food that we eat.

NARRATOR: It turns out that people with a diet high in fish absorb older carbon than meat-eaters. That’s because the oceans contain carbon that is hundreds of years old. When fish ingest this, and people in turn eat the fish, the ancient carbon enters their bones. Scientists now know that the bones of people who eat fish appear older than they really are, skewing carbon dating results. Cat has also been able to calculate just how much fish each person has eaten, by using the distinctive chemical markers seafood leaves in human bones.

JARMAN: I looked at all these different bones and it turned out, everybody with a sort of “wrong date”, as it were, had been eating a lot of fish! And that was a really brilliant moment actually to be able to see those dates fitting perfectly and that meant the entire mass grave could now be dated to the late 9thcentury, meaning it’s completely consistent with the Viking Great Army.