GEOMAGNETIC MOMENT VARIATIONS IN THE LAST 70,000 YEARS, IMPACT ON PRODUCTION OF COSMOGENIC ISOTOPES

On various time scales, the intensity of the Earth's magnetic field presents large amplitude oscillations due to secular variation of the non-dipole field (short term regional influences) or to variations of the moment of the main dipole (long term global influence). Here we introduce variations of the relative intensity of the local geomagnetic field reconstructed from a sedimentary sequence deposited during the Late Pleistocene in a maar crater lake located in Western Europe: Lac du Bouchet, Massif Central, France. The comparison of these results with absolute palaeointensities and virtual dipole moment (VDM) values obtained from volcanic rocks, as well as with relative palaeointensity variations measured on marine sedimentary sequences, lead to conclude that the geomagnetic dipole was drastically reduced between 43 and 30 ka. In the second part of this article, we discuss the relation of this geomagnetic crisis with the increase of atmospheric concentration of cosmogenic isotopes, recently documented for the relevant time interval. The modulation of the magnetospheric shielding by geomagnetic dipole fluctuations can be retained as the probable origin of such global atmospheric changes.