Le champ géomagnétique est-il influencé par les mouvements orbitaux? – MAG-ORB

Since 1910 and the first evocation of the effect of precession on the Earth's core, it has been suggested that the geomagnetic field could be modulated by the precession of the earth's rotation axis. This hypothesis was supported by theoretical but controversial considerations. It is now supported by revised theoretical considerations by observations of an the presence of orbital periods in the paleomagnetic spectrum and by a preferred occurrence of magnetic events, such as dipole lows linked with excursions and reversals, in the context of interglacial paleoclimates. However such relationship remains questionable since magnetic mineral concentration in sediments is influence by environmental conditions, so that climatic modulation of magnetic records could also result from even slight contaminations. It is thus necessary to use different and complementary signals with good control of their amplitudes and periodicities but also to acquire magnetic and climatic data from the same sediments. The first orginality of this project is to associate paleointensity data derived from the magnetic remanence of sediments with changes in production of Cosmogenic 10Be. Production of cosmonucleide in the atmosphere is modulated by the screening of the magnetosphere to the cosmic rays and thus depends on the strength and orientation of the dipole field. It is now well established that the dipole field intensity is very weak during excursions and reversals and thus these periods are associated with very large 10Be production. The geomagnetic events did not seem to occur randomly over the past 800 ka, but most of them are found during or at the end of interglacial stages. We propose to test further this correlation for the past 780 ka that have been punctuated by a large number of excursions and then over the Matuyama epoch which has been characterized by several reversals. Very different climatic regimes have dominated during the past 2.6 Ma from warm and stable climate to cold and unstable situation. We have selected cores within the equatorial band ' thus with maximum modulation of screening to cosmic rays responsible for 10Be production ' and at different longitudes ' to eliminate redistribution of 10Be by oceanic circulation. Most sites are located on continental margins in order to deal with large deposition rates (at least 5 to 10 cm/ka). The second characteristic of the present project is to acquire magnetic and paleoclimatic data from the same cores in order to compare the occurrence and phase relationships of the various parameters over a single stratigraphic basis. Three major types of analyses will be conducted. 1) Variations of relative paleointensity will be derived by normalizing the remanent magnetization intensity by distinct parameters but also by articial redeposition of the same sediment in laboratory controled field. 2) Cosmogenic 10Be concentrations adsorbed onto the mineral fraction (called authigenic 10Be) will be extracted by specific leaching procedures and measured by accelerator mass spectrometry. The 10Be concentration will be normalized by concentration of lithogenic 9Be as well as by 230Th, to correct from environmental biases (terrigeneous input, ocean transport, scavenging'). The two approaches will provide errors and testing of the suitability of the results 3) Measurements of oxygen isotopes of benthonic and planktonic foramnifera by mass spectrometry will provide 180' records of ice volume changes during the Plio-Pleistocene glacial/interglacial successions. The suitability of the results will be constrained by studies of multiple sequences from various sedimentary environments, different sedimentation rates and different carbonate clay ratio. This will allow checking the responses of each separate signal and establishing errors about their amplitudes and phase relationships. Different techniques of analyses (specifically wavelets) will be used to extract the spectral content and to evaluate the coherence of the signals. These results should provide aditionnal constraints about the influence of precession on core fluid motions.