Exceptional secular variation recorded in Neolithic hearths, Orkney.

Geomagnetic field models constructed from direct observations extend back some 400 years and the magnitude of decadal to centennial variations seen is often taken to be typical of the secular variation over longer timescales. Here we present archaeomagnetic directions from layered Neolithic hearths in Orkney 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 globally at this latitude over the era of historical observations. These archaeomagnetic data are complemented by extreme inclinations recorded in two 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.