Paleoclimatic context of geomagnetic dipole lows and excursions in the Brunhes, clue for an orbital influence on the geodynamo?
Résumé
The hypothesis of an influence of the astronomical precession on the geodynamo energy budget was recently reappraised by theoreticians. Paleomagnetic indications of such an influence remain controversial because reconstructions of paleointensity variations from sediments are suspected to be contaminated by lithological, paleoclimatically induced influences. Three sets of complementary indicators are however available: 1) records of the direction of magnetization in sediments, 2) records of magnetic anomalies of the deep sea floor basalts and 3) records of production variations of cosmogenic isotopes from sediment and ice cores. These records confirm the genuine geomagnetic origin of paleointensity lows and their narrow link with excursions or short reversals recorded in various materials and often dated by radiometric methods. The analysis of these time series and their comparison with δ 18 O records of the paleoclimate suggest that such globally recorded geomagnetic dipole lows have preferentially occurred in the context of interglacial or transitional paleoclimates at the time of low or decreasing obliquity. The dominant periods, extracted with the complex continuous wavelet transform technique, range from 40 to 125 ka, further suggesting a link with orbital parameters. These results encourage future efforts of research to improve the precision, the resolution and the dating of the time series of geomagnetic dipole low, in order to better decipher orbital signatures and understand their origin. An important implication of this topic is that the next geomagnetic dipole low should be related with the present interglacial.
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