An updated archeomagnetic directional variation curve for France over the past two millennia, following 25 years of additional data acquisition

Abstract

Nearly 40 years ago, Emile Thellier published an article summarizing the archeomagnetic data he had obtained during his career, which had allowed him to recover the main features of the directional variations of the geomagnetic field in France for the last two millennia. This database went on to be significantly completed 25 years ago by Ileana Bucur, who had taken over Thellier's work on archeomagnetism; this forms the current basis of our knowledge of the directional evolution of the geomagnetic field in France. Since then, archeomagnetic studies have been continued at Thellier's historical laboratory in Saint Maur. This article presents the directional archeomagnetic data obtained in France over the past 25 years in this same laboratory. A total of 528 new data are presented, which, together with the 170 results obtained on the French territory previously listed in Bucur (1994), constitute the French directional database (698 data in all). All but two of these data were obtained using the experimental protocol developed by E. Thellier based on the analysis of large samples and on a magnetic viscosity test that has proved its reliability on numerous occasions. The directions from the entire French database have been precisely defined, with 95% of the α 95 values being less than 1.9°, and ~50% being less than 0.8°. The selection of 286 data with dating

Global archaeomagnetic data: The state of the art and future challenges

“…Several curves have been frequently updated with new data as they become available, e.g., France (e.g., Thellier, 1981;Bucur, 1994;Chauvin et al, 2000;Genevey and Gallet, 2002;Genevey et al, 2009Genevey et al, , 2016Gallet et al, 2002;Hervé et al, 2013a,b;Le Goff et al, 2020). With efforts to improve data coverage for other regions, there are now curves for China (Cai et al, 2017), the Near East (Gallet et al, 2015;Stillinger et al, 2015;Shaar et al, 2020;Livermore et al, 2021), Mexico (Soler Arechalde et al, 2019Mahgoub et al, 2019) and South America (Goguitchaichvili et al, 2019).…”

The dawn of archeomagnetic dating

“…Brunhes's analogy with the train (Figure 6) seems somewhat bold, but it expresses well the recurrence of the geomagnetic directions and intensities known today for a given region at certain periods and indicates that archeomagnetists, when they want to provide chronological constraints, must take into consideration the help of archeologists or other dating tools, such as radiocarbon. In France (and more generally in Western Europe), for instance, recurrences in directions occur between the Roman and present periods and between the 7th-8th centuries and the 17th century [Figure 7a; Le Goff et al, 2020]. Recurrence is more frequent for intensities, such as between the 6th and 9th-10th centuries or between the 14th and 15th centuries [Figure 7b; Genevey et al, 2016Genevey et al, , 2021.…”

“…Even if the approach used by Chevallier was based on an invalidated hypothesis about the periodic behavior of the geomagnetic field (see also in Forel's letter; Text S1), and on the incorrect dating of his dated results [while the magnetic measurements were fairly good; see discussion in Tanguy et al, 1999], these are Goff et al [2020]. The data are exhibited in light blue.…”

“…The mean curve (thick line in red) and its 95%-confidence intervals (gray ovals) are determined from the method developed by Le Goff et al [2002] based on the bivariate extension of Fisher's statistics. See details in Le Goff et al [2020]. The pale green line indicates the directional variations after 1650 AD determined using instrumental measurements [after Le Goff and Gallet, 2017].…”

“…Geomagnetic secular variation in France revealed by archeomagnetic data. (a) Directional variations since the first century BC derived from the database compiled by LeGoff et al [2020]. The data are exhibited in light blue.…”