Evolution of the geomagnetic reversal rate since 160 Ma: Is the process continuous?

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Abstract
Two main interpretations have been advanced for the evolution of the geomagnetic reversal rate λ since about 160 Ma. These interpretations have quite different implications for Earth history and associated events and processes. Our own interpretation is that λ gradually and continuously decreased from its value of about 41/2 per million years at about 160 Ma to zero at about 118 Ma, then at about 83 Ma λ began to recover, again gradually and continuously, until reaching its present value of about 41/2 per million years. The Cretaceous Superchron occurred from about 118 to about 83 Ma, when the reversal rate was zero. Recently, Gallet and Hulot [1997] suggested an alternative interpretation where μ (= 1/λ) was constant from 160 Ma to about 130 Ma, at which time the process was rapidly and strongly perturbed. This perturbation led to a discontinuous increase in the mean length μ of intervals and caused the Cretaceous Superchron. The process then gradually recovered from the strong perturbation with the mean length μ gradually decreasing until the reversal process again became stationary at about 25 Ma. This interpretation is predicated on μ being constant from 160 to 130 Ma, whereas our interpretation requires a gradual increase in μ over this time interval. Consequently, a new statistic is suggested to distinguish between the null hypothesis that the reversal process is stationary against the specific alternative that there is a trend in the mean length μ. Application of this statistic shows that the data require a trend toward larger values of μ in the reversal process going from 160 to 130 Ma and that this trend continues strongly to the beginning of the superchron. Thus the addition of a discontinuity at 130 Ma is neither required nor supported by the data. Furthermore, application of this statistic shows that the trend toward smaller values of μ from 83 to 25 Ma appears to continue from 83 Ma to at least 12 Ma. Certainly there is no requirement for a change in the style of process at 25 Ma. Overall, the Gallet and Hulot interpretation seems untenable with the available data. Despite this, the Gallet and Hulot interpretation has great value in that it is inherently testable and our ability to reject it has increased our knowledge about the reversal process.

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