New archeomagnetic secular variation data from Central Europe for the Early Medieval Ages

Summary

During the Early Medieval Ages, unusually strong and rapid geomagnetic field variations have been reported in several European regions; however, archeomagnetic data from Central Europe remain scarce. To help filling this gap, we present new archeointensity results from eight archeological sites in Germany, Austria, and Poland, dated between 500 and 1200 AD. The investigated materials mainly consist of potsherds, together with two in situ baked clay structures that also provided archeodirectional information. Archeointensities were determined using the MT4 protocol, a Thellier-type technique including cooling-rate and anisotropy corrections. For two sites, the multi-specimen domain-state-corrected paleointensity protocol was additionally applied. Rock magnetic experiments indicate that the main remanence carriers are low-coercivity magnetite and high-coercivity ε-maghemite and hematite. The presence of these phases suggests incomplete transformation to hematite during firing. To further assess the archeointensity determinations, the Bias Corrected Estimation of Paleointensity (BiCEP) method was applied, particularly for specimens showing curved Arai plots. This analysis confirmed the reliability of the Thellier results for most investigated structures, whereas no reliable BiCEP outcome could be obtained for one structure from Chobienia (Poland). For four structures, the classical Arai plot evaluation agrees with the BiCEP results and, for one site, also with the independently obtained multi-specimen results. In another case, the comparison between Thellier and BiCEP estimates allowed a more realistic assessment of intensity uncertainty. One site mean value (~ 50 µT) around 600 AD yields a lower geomagnetic field intensity than other contemporaneous European records. Overall, the data suggest an increase in field intensity between 600 and 800 AD, with values becoming more consistent with previously published regional results after this period. However, given the relatively large uncertainties and the still limited number of available studies, additional archeointensity data from 500-800 AD are needed to determine whether the observed regional differences reflect genuine geomagnetic field heterogeneity during this period. Furthermore, new chronological constraints were obtained through archeomagnetic dating approaches applied to the two available full-vector records.