Ancient records of the geomagnetic field intensity provide the unique source of information on the evolution of the geodynamo. The paleomagnetic data contain a broad spectrum of dipole moment fluctuations with polarity reversals and excursions that typically occur during periods of very low field intensity, but the amplitude and the timing of the variations as well as critical features remain debated. The variability of the dipole with rapid fluctuations combined with long-term changes must be clarified to understand what controls the dipole strength, why it fluctuates and what is the cause of polarity reversals. Much has been learned for the past 30 years from records of paleointensity relying on natural remanent magnetization of sediments and lava flows, but large uncertainties persist and major features of the field remain poorly documented, pointing out the limits of the approach. As an alternative, changes in geomagnetic intensity can be reconstructed from the production of cosmogenic 10Be. Test studies have shown that 10Be production can be measured with confidence from sedimentary sequences. Our main objective is to build up a worldwide database of the dipole field changes for the past 5 Ma by acquiring high resolution records of 10Be production from a worldwide set of selected sediment cores.
The Accelerator mass spectrometry national facility « ASTER» at CEREGE dedicated to 10Be measurements offers this unique opportunity. Accurate time control will be obtained by astronomical calibration of paleoenvironmental records. In parallel, we will focus on the short-term field changes during geomagnetic reversals. This will be addressed by combining paleomagnetic records of reversals from volcanic sequences with high resolution 10Be measurements from marine cores that recorded the same reversals. Predictions of numerical geodynamo simulations will be tested against the data.