G. Hulot and Y. Gallet
D\'epartement de G\'eomagn\'etisme et Pal\'eomagn\'etisme, CNRS UMR 7577, Institut de Physique du Globe de Paris, France.
The mainly dipolar geomagnetic field has reversed its polarity many times in the past. This succession of intervals of alternate polarity defines the Geomagnetic Polarity Time Scale (GPTS), usually interpreted as resulting from a time-varying Gamma renewal process . The GPTS shows that while the reversal rate associated to this process must have been of 2 to 3 rev/Myr at the Upper Jurassic (~160 Myr ago), it must have fallen to a near zero value at the onset (~120 Myr ago) of the so-called Cretaceous superchron, a ~35 Myr long normal polarity interval. It is often thought that this decrease of the reversal rate must have been progressive over the corresponding 40 Myr, testifying for changes in the boundary conditions imposed by the mantle. Here we reconsider this conclusion. We use the most recent GPTS and show that the reversal rate before the superchron has more likely dropped within less than 10 Myrs, a very short time scale with respect to mantle processes, and maybe even instantaneously. This superchron could thus have occurred with little, if any, paleomagnetic warning. This suggests that the onset of superchrons could be a direct manifestation of the non-linear nature of the geodynamo rather than the consequence of fundamental changes in boundary conditions.