HR: 0800h
AN: GP11B-0833    [Abstracts]
TI: Geomagnetic Dipole Strength and Reversal Rate
AU: * Valet, J
EM: valet@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, Place Jussieu 75252, Paris, 75252 France
AU: Meynadier, L
EM: meynad@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, Place Jussieu 75252, Paris, 75252 France
AU: Guyodo, Y
EM: Yohan.Guyodo@lsce.cnrs-gif.fr
AF: LSCE, Avenue de la Terrasse, Gif/Yvette, 91190 France
AB: We present a first 2 million years long composite curve obtained after stacking records of relative paleointensity from a selection of sedimentary sequences. The composite Sint-2000 record was calibrated using the virtual dipole moments (VDMs) of the 2004 updated volcanic database of absolute paleointensity over 0.1 Myr long intervals. The value of the time-averaged VDM (7.46+/-1.16 x 1022 Am2) for the past 0.8Myr was used for calibration and the mean values of the successive 0.1 Myr intervals were found in very good agreement with the relative paleointensity for the same periods. A striking characteristic of this Sint-2000 curve is the succession of periods with different mean values of paleointensity. During the Brunhes chron the dipole oscillated around a value of 7.51+/-1.66 x 1022 Am2, which was significantly larger than during the previous 400 kyrs (5.3 +/- 1.5 x 1022 Am2). To provide a more quantitative picture of field strength as a function of reversal frequency, we calculated successive running averages of the field intensity over 100 kyrs long intervals and found that the time-averaged field was higher during periods without reversals. We also observe that the amplitude of the short-term oscillations remained the same. As a consequence, less intervals of very low intensity are expected during periods associated with a strong average dipole moment, whereas more excursions or other instabilities are produced during periods of weak field intensity. The relation between the mean dipole strength and the frequency of reversals suggests also the existence of a large field during long periods without reversals, under the assumption that they would be governed by the same processes. Prior to reversals, the axial dipole decays during 60 to 80 kyrs, but rebuilds itself in the opposite direction much more rapidly, in a few thousand years at most. These time constants suggest that the decay phase is caused by diffusion while advection would dominate the dipole recovery.
DE: 1521 Paleointensity
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting