Dateringen av Västerhus kyrkogård på Frösön i Jämtland. En djupanalys av isotopvärden och dateringar av fisk, däggdjur och människor

The medieval church and cemetery of Westerhus (Västerhus) is situated on the island of Frösön in the province of Jämtland, Mid-Sweden. The site was completely excavated and underwent anthropological analysis, published by Nils-Gustaf Gejvall in 1960. A number of early radiocarbon datings were later obtained (Gejvall 1968). In 2009 a new study of the church, site, burials and humans was presented by Elisabeth Iregren and collaborators, and new attempts were made to date the burial ground. Two series of radiocarbon dating have been performed. One aimed at finding the foundation of the site and one sought to identify the last burials. The datings have, however, been questioned in general and supposed to be severely influenced by a reservoir effect. This problem has now been faced through a detailed analysis of available stable carbon and nitrogen isotope measurements – performed by Iregren and collaborators – from the site, the region and elsewhere. Data from humans, domesticates and wild mammals have been used. In addition, the reservoir effect on fish from the surrounding Lake Storsjön has been measured. Combining all this information and applying the diet analysis program FRUITS we found out that the influence of a reservoir effect on the radiocarbon dates is limited to about 100 years. Based on this, a recalibration of the available radiocarbon dates determines that the beginning of the use of the churchyard with 95% probability falls in the interval 1140–1300 AD and the ending within the interval 1330–1470 AD. We thus conclude that the cemetery was in use from 1140 AD until 1470 AD.