FUNDAMENTAL TRANSITIONS IN THE GEODYNAMO AS SUGGESTED BY PALEOMAGNETIC DATA

Analysis of the reversal chronology shows that the reversal process effectively ceases during the Cretaceous Superchron. The geodynamo appears to have two basic states: a reversing state and a non-reversing state. This must reflect a fundamental transition in the geodynamo process at the onset and termination of the Superchron. We suggest that this transition, and indeed the long-term variations in reversal rate, may reflect a change from a predominantly oscillatory dynamo to a steady one (which might, for example, be caused by a change in the relative roles of the omega- and alpha-effects in alpha(2) omega-dynamos). If so, this should probably have been reflected in reduced secular variation during the Superchron. Unfortunately, data are not available to test this prediction directly, but we use the surface integral of the angular standard deviation of paleomagnetic field directions as a proxy. This shows a significantly reduced secular variation during the Superchron as predicted.