Geomagnetic field variations in Western Europe from 1500 BC to 200 AD. Part II: New intensity secular variation curve

Abstract

In order to extend the secular variation curve (SVC) of archaeointensity in Western Europe to the first millennium BC, we studied 24 kilns and hearths in place, two displaced hearths and six sets of pottery sherds from French archaeological sites. Archaeological artefacts, radiocarbon and dendrochronology dated the acquisition of the thermoremanent magnetization (TRM) carried by the studied objects. Rock magnetism experiments suggest that the main carrier of the magnetization is a Ti-poor titanomagnetite. Archaeointensity was determined by the Thellier–Thellier classical protocol with pTRM-checks. A strict criteria set was applied to select only the most reliable results with linear NRM–TRM diagrams (55% of total specimens). This study demonstrates that pottery sherds with two TRMs give reliable archaeointensities in the low-temperature interval, if the NRM–TRM diagram is adequately adjusted. Eighteen new mean archaeointensities (14 corrected from the anisotropy of TRM and 16 from cooling rate) were computed. The comparison with previously published Western Europe paleointensities show a strong dispersion between data primarily due to their variable quality. Western Europe data were weighted following the archaeointensity protocol, the number of specimens per site and the type of studied materials, in order to better highlight the secular variation of archaeointensity during the first millennium BC. The SVC, built with sliding windows of 160years shifted every 50years, presents (at Paris) a maximum of 90μT around 800BC and a minimum of 60μT around 250BC. These archaeointensity maximum and minimum correspond to cusps of the geomagnetic field direction in Western Europe. This new curve is consistent with Mesopotamian and Eastern Europe data. The archaeointensity secular variation in Western Europe predicted by global geomagnetic models CALS3k.4, ARCH3k.1 and ARCH3k_cst.1 is smoother than our SVC. We used our directional dataset (Hervé et al., 2013) to build a new Western Europe VGPs and VDMs mean curves. Comparison with the predictions given by the global models points out a possible persistent non-dipole fields effect over Europe between 1000BC and 600–500BC. Finally, we note that the strong variations of intensity of the geomagnetic field (with a mean decrease rate per century close to 6μT) will be useful for archaeomagnetic dating purposes.