Geomagnetic field variations and low success rate of archaeointensity determination experiments for Iron Age sites in Bulgaria

Bulgarian archaeomagnetic database is the longest local geomagnetic field record covering almost completely the last 8000 years. However, very poorly constrained periods in need of elucidation still exist. Among the most problematic are the last 1200 years BC corresponding to the Iron Age in Bulgarian lands. In contrast to the relatively well studied Neolithic, Eneolithic and Bronze Age settlements, the Iron Age sites (especially in the Early Iron Age phase) are not sufficiently investigated from the archaeological point of view. The lack of precise stratigraphic frames significantly hampers the specification of sites chronology, and as a result, baked clay features that could serve as reliable reference points are rather scarce.

The most recent compilation of the Bulgarian archaeomagnetic database contains 18 intensity and 16 directional reference points belonging to the Iron Age, which have very uneven temporal distribution. To extend the coverage, 26 baked clay structures from nine different archaeological sites were sampled and archaeomagnetically studied producing ten new directional, but only five intensity data. It seems quite often materials from Iron Age combustion structures to possess magnetic properties unfavorable for archaeomagnetism, generally reflected in non-linear and concave Arai plots. Hereby, the lowest success rate of archaeointensity determination experiments is registered for this period within the whole database. Usually it is much easier to determine the past geomagnetic field direction compared to the intensity and the failure of the Thellier experiment is not surprising. Nevertheless, more than 90% of the investigated features belonging to all the other epochs except Iron Age normally display success rates over 50%, as only for 1-2% the archaeointensity determination experiments fail completely. In contrast, in the Iron Age the successful features comprise only 56% of the total number studied with 28% failure. A possible explanation for this observation was sought in some specific inappropriate conditions during firing/burial time that affect magnetic properties of baked clay materials during and/or after their thermoremanence acquisition.

This study is supported by the grant KP-06-Russia-10 from the Bulgarian National Science Fund and Russian Foundation of the Basic Research grant 19-55-18006.