On the resolution of regional archaeomagnetism: Untangling directional geomagnetic oscillations and data uncertainties using the French archaeomagnetic database for between 1000 and 1500 AD as a guide

A complexity is emphasised in the distribution of the French archaeomagnetic directions during the 13th and the 14th centuries AD. Data uncertainties, and the smoothing introduced when estimating an average secular variation curve prevent scrutiny of the very nature of this complexity. It might correspond to a directional yaw, the nature of which would be compatible with the recent geomagnetic field evolution as traced by the gufm1 model. In order to emphasise this indeterminacy, a reference secular variation curve was constructed for between 1000 and 1500 AD, including the yaw in question, and synthetic databases that mimic the accuracy and density characteristics of the true French archaeomagnetic database were considered for some of these. The synthetic curves hence obtained show that the dating accuracy of archaeomagnetic data is the crucial parameter for constructing a detailed secular variation path. The significant impact of the experimental data accuracy is also illustrated. Even more crucial is the fact that precision of the data dating required to describe the directional variability over the century timescale largely exceeds the precision of the archaeological dates available for the structures generally studied. This highlights the intrinsic limitation of archaeomagnetism for regional reconstruction of century-scale geomagnetic field variations.