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

A complexity is emphasized in the distribution of French archaeomagnetic directions during the thirteenth and the fourteenth 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 emphasize this indeterminacy, a reference secular variation curve was constructed for dates between AD 1000 and 1500, 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 the 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.