The periodicity of geomagnetic reversals

Several periodicities have been claimed in the time series of geomagnetic reversals, but none has survived subsequent methodological objections. We apply the established tool of periodogram analysis to the most accurate geomagnetic scale available and obtain the following results: a trend, that is well fitted by an exponential curve, is present in the rate of reversals occurrence and there is a 15 Ma periodicity that is significant at approximately the 0.05 level; no significant period is present in the series of the intervals between reversals. These results indicate that geomagnetic reversals since 85 Ma BP occur according to a generalized Poisson process with a mean which increases exponentially, and which may have a superimposed periodic modulation. In physical terms, this would imply a mechanism of reversal generation which is stochastic and has an energy input increasing in time modulated by a 15 Ma periodicity. This is consistent with reversals originated by cyclones in the outer core, a long-term change in the lower mantle temperature distribution, and a pulsating 8-10 km thick layer at the base of the mantle. © 1992