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

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 direction. 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 field. 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 evolutionary or climatic processes. For example, the statistically significant coincidence in levels of some marine faunal extinctions and reversal boundaries in ocean sediments 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 remanent magnetism is acquired in marine sediments

Palaeomagnetic field intensity variation recorded in a Brunhes epoch deep-sea sediment core

Deep-sea sediments have been shown to possess a natural remanent magnetisation (NRM) that often can be attributed to the statistical alignment of detrital magnetic grains in the Earth's magnetic field at or shortly after the time of their deposition. In favourable circumstances this remanence can be interpreted as a record of palaeomagnetic field behaviour. In the study reported here we have attempted to describe relative variations in palaeomagnetic field intensity on a time scale of 10^4-10^5 yr, during the past 700,000 yr, using the palaeomagnetic record of deep-sea sediment piston core RC10-167 (33°2′N, 150°23′E), which has an exceptionally thick section of sediment deposited during the Brunhes normal polarity epoch (Fig. 1). After subtracting the stratigraphic contribution of several distinct volcanic ash layers interspersed with the otherwise uniform pelagic sediment, we calculate an average deposition rate of 2.1cm kyr^-1 between the adjusted level (1,470cm) of the Brunhes-Matuyama boundary (t=700,000 yr) and the top of the core, assumed t=0 yr. A 2-cm thick sample, representing about 1,000 yr of deposition, was taken at an average interval of 3.3 cm (representing about 1,600 yr). This sampling placed a theoretical limit of 3,000-4,000 yr on the period of a resolvable sinusoidal variation