Dependence of the duration of geomagnetic polarity reversals on site latitude

Abstract

An important constraint on the processes governing the geodynamo—the flow in the outer core responsible for generating Earth's magnetic field—is the duration of geomagnetic polarity reversals; that is, how long it takes for Earth's magnetic field to reverse1. It is generally accepted that Earth's magnetic field strength drops to low levels during polarity reversals, and the field direction progresses through a 180° change while the field is weak1. The time it takes for this process to happen, however, remains uncertain, with estimates ranging from a few thousand up to 28,000 years. Here I present an analysis of the available sediment records of the four most recent polarity reversals. These records yield an average estimate of about 7,000 years for the time it takes for the directional change to occur. The variation about this mean duration is not random, but instead varies with site latitude, with shorter durations observed at low-latitude sites, and longer durations observed at mid- to high-latitude sites. Such variation of duration with site latitude is predicted by simple geometrical reversal models, in which non-dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction, and provides a constraint on numerical dynamo models.

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Figure 1: Comparison of the transition-zone thickness in the lower Jaramillo record from ODP Hole 758A (ref.
Figure 2: The duration of the directional changes during polarity transitions varies as a function of site latitude.
Figure 3: The durations do not correlate with sedimentation rates.
Figure 4: Simple geometrical models in which standing non-dipole fields are allowed to persist while the axial dipole field decays to zero and then builds up in the opposite direction produce transition durations that vary systematically with latitude.

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Acknowledgements

R. Coe, D. V. Kent, J. Dauphin & B. Midson provided comments that improved the manuscript. J. E. T. Channell and T. Yamazaki provided their data for this work.

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Correspondence to Bradford M. Clement.

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