Complex Interpretation of the Earth Despinning History

The length of day (LOD) values deduced from fossils and tidal deposits strongly suggest that the Earth’s despinning rate was on average about five times smaller in the Proterozoic than in the Phanerozoic. Moreover, these data indicate that between 250 and 100 million years ago, i.e. during Mesozoic there was a non-linear variation superimposed on the overall linear trend of the Earth’s rotation rate as a function of time. In order to understand these observations within a geodynamical framework, we investigated the variations throughout geological time of the oceanic tides, of tectonic plate speeds, and of geomagnetic paleointensities. In agreement with other authors, we found that in the Mesozoic, the geomagnetic moment underwent a minimum, but no statistically significant change could be inferred for the Proterozoic and the Archean. On the other hand, during the Mesozoic, concomitantly with the geomagnetic paleointensity minimum, the average oceanic tidal torque and the average lithospheric plate speeds, were significantly smaller than before and after this epoch.We partly ascribe the difference of the despinning rate in the Proterozoic and Phanerozoic to a more intensive mass redistribution within the Earth remote geological past. Less deep oceans and/or the exsistence of a supercontinent during most of the Proterozoic could also account for the observed LOD data. As far as the Mesozoic data are concerned, we show that they can almost certainly be explained by the tectonic regime of the epoch and the exsistence of the supercontinent Pangea. In any case, our work suggests that there exsists a complex interplay between geomagnetic, tectonic and rotational processes.