Intensity of the Earth's magnetic field: Evidence for a Mesozoic dipole low

Abstract

The variation of the strength of the Earth's magnetic field throughout geological time is known only poorly. Large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks. Even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units, fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations. These uncertainties, combined with well-known experimental difficulties, can explain the large scatter in paleointensity data reported in most previous review papers. Because experimental difficulties are more rigourously dealt with using the Thellier method, which also offers some opportunities to minimize some fundamental uncertainties, we carried out a compilation of all Triassic and younger paleointensity determinations made using this method. This selected data set indicates the occurrence of large, long-term changes of the geomagnetic dipole strength with time. The average dipole strength seems to have been approximately constant since Late Cretaceous time, but a dipole strength only one third of the Cenozoic value prevailed during most of Mesozoic time, following a period of larger strength ending possibly during Early Jurassic times. The Mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle. The terrestrial dynamo seems therefore to exhibit different long-term stable states, which are probably controlled by the thermal structure in the lowermost mantle.