Archaeomagnetic dating of vitrified Broborg hillfort in southeast Uppsala, Sweden

Abstract Hundreds of European hillforts from the Bronze and Iron Ages show evidence of extreme heating of the stone ramparts, which has caused melting of the rocks which partially turned to glass on cooling. The cause and age of wall vitrification at these hillforts has long been debated in the archaeological community. This work aims to determine the age of vitrification at the Broborg hillfort (Uppsala, Sweden) using archaeomagnetic dating. Rock magnetic investigations verified the suitability of the vitrified materials for such studies, with titanomagnetites being the major magnetic signal carriers. Despite the challenges involved in sample collection at the Broborg site, a total of eight oriented samples (five vitrified, three non-vitrified) were collected and used in this study. The mean paleodirection and paleointensity results were combined to compare with a geomagnetic field model for Europe. Three possible archaeomagnetic age intervals for Broborg vitrification were found: 389–579 CE, 602–752 CE, and 965–1300 CE, among which the 389–579 CE date range is the most consistent with the published 14C ages and known historical context of the site. This study shows that paleointensity data alone could be used to date such sites, requiring smaller unoriented samples, potentially removing some of the barriers to sample acquisition at some archaeological sites.