Geomagnetic reversal spurts and episodes of extraterrestrial catastrophism

Abstract

The possibility that the observed 26–32-Myr periodicity in the records of extinction events¹ and astrobleme ages² was caused by periodic episodes of physical bombardment of the planetary surface has raised the likelihood that such an astronomical clock also affects geomagnetic reversals^3,4. However, the detection⁵ of a comparable 30-Myr harmonic in the geomagnetic reversal record has now been found⁶ to be an accident of data length while the earlier-reported⁷ 32–34-Myr harmonic of the reversal record may well be an artefact of the data base. Here we examine the temporal relation between geomagnetic reversals and those catastrophic episodes which occurred during periods of frequent reversals. As geomagnetic history comprises long intervals of frequent reversals, alternating with intervals of fixed polarity, our rationale is that the geomagnetic field should be most susceptible to catastrophism during its relatively unstable periods of frequent reversals. We find spurts in the frequency of geomagnetic reversals at approximately 30-Myr intervals, correlated with catastrophic episodes. These spurts can be seen as the geomagnetic signatures of terrestrial catastrophism, and we tentatively ascribe them to enhanced core turbulence during episodes of bombardment. The absence of reversal spurts during long intervals of fixed polarity is attributed to the failure during these intervals of the geomagnetic dynamo's bi-stable oscillations.