Strong secular variation in Central Europe during the Early Medieval Ages

The study of geomagnetic field variations provides information on the Earth's inner dynamics, helps understanding the role of the Earth's magnetic field as the primary shield against cosmic radiations and is also used as a geochronological tool for dating archaeological artefacts. Geomagnetic field variations during the Early Medieval Age (EMA) in Central Europe are generally poorly constrained due to the scarcity of archaeological sites. While a rapid intensity increase in the 6th century, along with high intensity values for the 7th to 9th centuries, have been reported for Western Europe, new archaeointensity data from other regions is thus needed in order to reconstruct more closely the spatio-temporal geomagnetic field evolution.

This work focuses on the study of the secular variations during the EMA period for selected regions in Central Europe : Germany, Austria and Poland. We analyzed potsherds from Ternitz and Unterrohrbach, and baked clay from Frauenkogel in Austria. In Poland, we examined potsherds from Klenica and Chobienia from two different locations; for the latter also daub and baked clay of a drying pan have been investigated. Finally, we studied kiln rocks from Schnapsweg and baked clay of a rampart from Fergitz in Germany.

For setting up the archaeointensity measurements, we used thermal demagnetization of the NRM and thermal κ(T) cycling to determine the unblocking temperature spectra and alteration behavior. The MT4 protocol – a Coe variant of the Thellier method - was used, including pTRM, tail checks and additivity checks, as well as corrections for anisotropy and cooling rate effects. Modified selection criteria sets TTA and TTB were applied. For Ternitz and Fergitz sites, we also used the multi-specimen domain-state corrected (MSP-DSC) protocol. Rock magnetic experiments comprised hysteresis and backfield curve measurements.

Between 500 and 700 AD, results of Unterrohrbach and Ternitz yield palaeointensities around 50 µT. While the MT4 site mean for Ternitz is characterized by high scatter, MSP-DSC experiments revealed a reliable archaeointensity. For Unterrohrbach site, a similar value with a lower scatter is determined. Finally, results from Frauenkogel site, suggest a rapid and strong increase of the archaeointensity within 100 to 150 years to high values around 85 µT. Similar high values were obtained in France. After this maximum, a strong intensity decline is indicated by the results from the remaining sites.