New Insights on Long Term Geomagnetic Moment Variation from Cosmogenic Nuclide and Paleointensity Signatures along Ocean Sediment Cores.

Abstract:

Some numerical and experimental simulations suggest that precession might supply enough power to influence planetary dynamos. The demonstration of a causal relationship between the Earth’s orbital motion and variations of the geomagnetic field intensity, would open interesting perspective for modelling the past and future geomagnetic field behaviour and its eventual relationships to past and future orbitally constrained, climatic changes. Although pristine geomagnetic signals can be extracted by filtering and stacking multiple normalized intensity records, the reconstruction of high resolution geomagnetic field variations still raises questions. Namely, significant variance at orbital frequencies in relative paleointensity (RPI) records are generally considered as clues of residual contamination by paleoclimatically induced variations of magnetic carriers size ranges or mineralogy. Such questions can be adressed using other indicators of the geomagnetic dipole moment variation, such as the cosmogenic production modulated by the magnetospheric shielding. During the MAGORB project (ANR-09-BLAN-053-001) cosmogenic nuclide geochemistry, d¹⁸O, and paleomagnetic records were constructed along thick clayey-carbonate sequences deposited in the equatorial pacific and indian oceans over the last million of years. Authigenic ¹⁰Be/⁹Be ratio and RPI variations generally exhibit similar ranges of oscillations. However significant offsets appear between some RPI lows and their corresponding ¹⁰Be/⁹Be peaks, suggesting delayed lock-in of the remanent magnetization. After transfer on time scales the new geomagnetic moment series can be compared with the PISO-1500 and SINT-2000 stacks, and with the ¹⁰Be ice core record of EPICA Dome C. These new authigenic ¹⁰Be/⁹Be ratio records provide new opportunities to: 1) assess the validity of high resolution RPI records, 2) evaluate address the question of the presence of orbital periods in the paleo-field geomagnetic spectrum, and 3) to precise dipole moment rates of changes before and after the dipole moment reductions linked with excursion and polarity reversals.