Exceptional secular variation recorded in Neolithic hearths, Orkney and Swedish lake sediments

Abstract

Geomagnetic field models constrained by direct observations extend back some 400 years and the magnitude of decadal to centennial variations that they display is often assumed to be typical of the secular variation over longer timescales. Here we present archaeomagnetic directions from layered Neolithic hearths in Orkney, dating to around 3000 BCE, that challenge this assumption. Combining stratigraphic controls with radiocarbon dates allows for a precise chronology, which, in turn, implies directional change of more than 12°/century lasting for 200 years, far in excess of anything seen at this latitude over the era of historical observations. These archaeomagnetic data are complemented by similarly rapid changes in inclination recorded in two high temporal resolution sediment cores from Kälksjön, Sweden. As well as raising the possibility of using archaeomagnetic dating within this important archaeological period, the new data pose questions regarding our understanding of the secular variation, the limitations of the historic field as a proxy for the past, and the underlying dynamical processes within the core. This research highlights the importance of archaeology to the wider scientific community and the potential of archaeological material to further our understanding of Earth processes.