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Stothers, R.B., 1986: Periodicity of the Earth's magnetic reversals. Nature, 322, 444-446, doi:10.1038/322444a0.
Reversals of the Earth's magnetic field may occur with a certain regularity. Using observed percentages of normal and reversed polarity during different time intervals, Negi and Tiwari detected several significant periodicities, notably one of 32-34 Myr. Raup used the dates of individual reversals to propose a 30-Myr period in the frequency of reversals, although Mazaud et al. obtained a periodicity of only 15 Myr. Like Negi and Tiwari other detected short periods, the 15-Myr period may be harmonically related to a basic 30-Myr period. It has also been suggested that these are accidental periodicities arising in a short record, or are harmonics of the record length itself. In fact, all of the cited studies have used different data, different record lengths and different methods of time-series analysis. Raup has consequently retracted his original claim. Here I present a much fuller analysis of the reversal record and show that a statistically significant period of ~30 Myr does formally exist, in spite of the cited differences.
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