Evidence for an increase in cosmogenic ¹⁰Be during a geomagnetic reversal

Abstract

Reversals in the geomagnetic field, which occur every few hundred thousand years, represent a dramatic change in the Earth's environment. Although there is no satisfactory theory for such reversals, it is generally accepted that the dipole field intensity decreases to <20% of its ‘normal’ value for a few thousand years during the change in direction¹. Because the galactic and solar cosmic rays which impinge on the Earth's atmosphere are charged, a significant fraction (about half) of them are deflected by the geomagnetic field². At the time of a reversal, this magnetic shielding is greatly reduced, and it has been suggested that the increased flux of high-energy particles could have effects on evolutionary³ or climatic⁴ processes. For example, the statistically significant coincidence in levels of some marine faunal extinctions and reversal boundaries in ocean sediments⁵ could be caused, directly or indirectly, by the decreased geomagnetic intensity during the reversal. We report here evidence in marine sediments for an increase in cosmogenic ¹⁰Be production in the Earth's atmosphere during the Brunhes–Matuyama reversal 730,000 yr ago. In addition to confirming an increase in cosmogenic isotope production, the results provide information on the magnitude and duration of the geomagnetic intensity decrease during such an event, and the depth at which rmanent magnetism is acquired in marine sediments.