Asymmetry in the reversal rate before and after the Cretaceous Normal Polarity Superchron

The geomagnetic reversal chronology shows a decreasing reversal rate, λ, from about 160 Ma to about 118 Ma, a cessation in the reversal process (the Cretaceous Normal Polarity Superchron) until about 83 Ma, and then an increasing λ until recent times. Visually, the rate of decrease in λ leading into the superchron appears larger than the rate of increase in λ following the superchron. Statistical tests developed here confirm the visual perception of an asymmetry in the rate of change in λ. This asymmetry could be caused by a relatively rapid breakdown of D″, with thermal energy being advected away followed by a slower thickening of the layer through thermal conduction. However, in the absence of good thermal models as to how different core-mantle boundary conditions affect the geodynamo there are probably many apparently viable speculations as to the origin of the observed rate asymmetry.