Jacques Thiriot

New archeointensity data from Spain and the geomagnetic dipole moment in western Europe over the past 2000 years

Archeomagnetic studies on 14 kilns, a group of jar fragments, and a collection of baked bricks dated between 1000 and 1959 AD plus one Roman pottery kiln have been conducted in order to obtain high-quality archeointensity data to enhance the western European database. The Thellier method with corrections for anisotropy of thermoremanent magnetization (TRM) and for cooling rate dependence upon TRM acquisition was used. The effect of TRM anisotropy is only important for the bricks where the corrected and uncorrected mean intensities differ by more than 5%. Cooling rate correction factors determined per sample are up to 15% and per site up to 7.7%. Our 17 new data together with 62 previously published results were used to obtain, by Bayesian modeling, the geomagnetic field intensity over the past two millennia for western Europe. Our results indicate that geomagnetic intensity remained more or less constant between the 1st and 4th centuries and between the 14th and 16th centuries (mean values, at Paris, around 65 and 57 μT, respectively), whereas an important decrease occurs between 1600 and 1800 AD. The detailed evolution of geomagnetic field intensity during the High Middle Ages is not yet well established. Despite their differences, geomagnetic global models predict our results reasonably well. This work indicates the need to obtain a number of archeointensity data for each time interval in order to reliably record variations of the geomagnetic field and to test whether any relationship exists between field intensity and climate