This is one of the female skulls from the Repton charnel. Photo: Cat Jarman

The bones of at least 264 people were uncovered in the 1970s, from within a low mound of pebbles in a church vicarage garden in Derbyshire, England. Further investigation by archaeologists determined that most of the artifacts and other evidence indicated the discovery was a mass grave from the fabled Viking Great Army that invaded England in 865 A.D. – but some of the bones were instead radiocarbon dated to a century or two before that, complicating the interpretations.

But it was the Vikings’ seafood diet, which brings older carbon into the body, have been throwing off the age estimates, according to a new study in the journal Antiquity by a joint team from the University of Bristol and Oxford.

“We take into account marine reservoir effects (MREs) on human bone, and use contextual information with Bayesian modeling to constrain the dates further,” they write. “The results show that all dated remains from the charnel deposit are consistent with a single late ninth-century event.”

The mound was in the vicarage garden of St. Wystan’s church in Repton, Derbyshire. It was first excavated between 1974 and 1980, and showed the disarticulated bones of almost all males, almost all between the ages of 18 and 45. The significance of the various artifacts showed Viking and Scandinavian influence, including a silver Thor’s hammer pendant with one set of remains. Isotope analysis of oxygen and strontium in the tooth enamel from two of the skulls showed a linkage to southern Scandinavia, as well.

Most of the remains and the evidence indicated the bodies were from a Viking Great Army invading force that was wintering between 873 and 874 A.D. But not all the radiocarbon dates matched. Theories indicated that some of the bones may have become dug up and reburied within the mass grave.

The latest study re-tested the bones which showed outlier dates. They found that a particular isotope driven by the consumption of large amounts of seafood in life could throw off the estimates of time of death.

“It is clear from the Carbon-13 values that some individuals has considerable marine dietary input, up to a maximum of 32.9 years plus or minus 10 percent for samples (from the charnel mound),” they write. “The charnel is consistent with a single deposit in the late ninth century… MRE calibrations are vital for accurately dating the Repton material – the early dates from the charnel were caused by a lack of such corrections.”

The additional factor of seafood diet could factor into other dating methods, in other places, they added.

“Beyond the context of Viking Age England, the material presented here demonstrates the need to account fully for MREs, in particular when working with secondary burials or material where Carbon-14 dates seem inconsistent with other forms of evidence,” they conclude.