Paleointensity of the geomagnetic field recovered on archaeomagnetic sites from France

Abstract

Twenty new paleointensity determinations have been obtained from archaeomagnetic sites from France. Time intervals investigated range from 65 to 380 AD and from 1370 to 1700 AD. Two hundred forty-two samples of tiles and bricks were treated with the classical Thellier method for paleointensity determination. Rock magnetic studies identify magnetite, substituted maghemite and hematite as the main magnetic minerals. As is usual with archaeomagnetic samples, especially tiles, magnetic anisotropy was observed in all samples. The effect of thermoremanent magnetization (TRM) anisotropy upon the paleointensity values was corrected after the determination of the tensor of TRM anisotropy in each sample. These determinations allow corrections of the field strength obtained, which are usually lower than 20. The effect of the cooling rate on the TRM intensity acquired was also determined and seems relatively low on our samples. Most frequently eight to 10 samples per site were used to calculate the mean intensity. Standard deviations are typically lower than 7% of the average value. During the Roman period, small variations of the field intensity in Paris are observed between 62 and 70 μT, while during the more recent period, the intensity of the field decreases from 62 to 52 μT. An analysis of the paleointensity data available for the last 2000 years, obtained from sites in Western Europe, was performed using a weighing factor, which takes into account the number and type of the samples studied as well as the technique used for the paleointensity determination. This weighing factor clearly shows that most of the existing data might not be reliable. The analysis performed indicates that more data is needed, especially from the time interval 500 to 1400 AD, before it would be possible to determine a reference curve of the time variations of the archaeomagnetic intensity of the field in Western Europe.