Résumé
This study addresses several issues of using calcite speleothems as a high temporal resolution archive of the directional fluctuations of the Earth's magnetic field. This was achieved by collecting 3 stalagmites in the Theoreticians' cave (St Benoit, French Alps). This cave, although hardly accessible, presents two advantages. First, there are recent speleothems as evidenced by their still active growth. Thus the remanent magnetization can be directly compared with a secular variation model such as ARCH.3k. Second, the cave is carved in lutetian limestones that are directly overlain by iron-sulfide-rich priabonian marlstones. So potentially, drip water can contain iron that is a major asset for a magnetic study. We estimated by means of various rock magnetic experiments that the remanent magnetization is carried by single-domain magnetite. The recent age of the stalagmites is confirmed with U/Th dating suggesting that our sampling covers the last 3 millennia. These ages yield a mean growing rate of about 0.1 mm/yr. Remanent magnetization measurements are carried out with a 2G cryogenic magnetometer. The challenging issue is to measure samples of small size (cylinder of 1 inch in diameter and about 2 mm in height, i.e. a sample of about 1 gram). We worked on improving measurement protocols as well as on the development of a new type of sample holder. Before each series of measurements, the magnetometer must be perfectly tuned in order to have the lowest possible baseline and a minimum drift. In the best conditions, encountered during the nights or during the weekends, the background noise is around 1 X 10-12 Am2. Such conditions are necessary to measure the NRM that is characterized in most cases by intensity less than 1X10-8 Am2/kg (i.e one order of magnitude above the sensitivity limit of our magnetometer). With such NRM intensities, the major challenge is to determine the direction of the ChRM by stepwise AF treatment. The results obtained are very encouraging in particular with respect to the evolution of the inclination of the field. It fits perfectly on the ARCH.3k model. Even if the measurement and sampling protocols still require developments, our results open new perspectives on the study of high-resolution record of the Earth's magnetic field fluctuations.