Nonrandom geomagnetic reversal times and geodynamo evolution

•We apply Shermanʼs test to the Phanerozoic Geomagnetic Polarity Time Scale.•We find evidence of nonrandom behavior in geomagnetic reversal times.•Nonrandom geomagnetic reversal times indicate irregular evolution of the core.

Shermanʼs ω-test applied to the Geomagnetic Polarity Time Scale (GPTS) reveals that geomagnetic reversals in the Phanerozoic deviate substantially from random times. For 954 Phanerozoic reversals, ω exceeds the value expected for uniformly distributed random times by many standard deviations, due to three constant polarity superchrons and clustering of reversals in the Cenozoic C-sequence. Reversals are nearly periodic in several portions of the Mesozoic M-sequence, and during these times ω falls below random by several standard deviations, according to some chronologies. Polarity reversals in a convection-driven numerical dynamo with fixed control parameters have an overall ω-value that is slightly lower than uniformly random due to weak periodicity, whereas in a numerical dynamo with time-variable control parameters the combination of superchrons and reversal clusters dominates, yielding a large ω-value that is comparable to the GPTS. Shermanʼs test applied to shorter Phanerozoic reversal sequences reveals two geodynamo time scales: hundreds of millions of years represented by superchrons and reversal clusters that we attribute to time-dependent core-mantle thermal interaction, plus unexplained variations lasting tens of millions of years characterized by alternation between random and nearly periodic reversals.