PALEOMAGNETIC AND AUTHIGENIC 10BE/9BE RECORDS IN SEDIMENTS: IMPLICATIONS FOR THE USE OF GEOMAGNETIC MOMENT LOWS AND EXCURSIONS AS GLOBAL CORRELATION MARKERS

Paleomagnetic and authigenic 10Be/9Be studies have been performed along sedimentary sequences deposited over the 0-400 and the 600-1300 ka BP in high accumulation rate areas of the North-East Atlantic and West-Equatorial Pacific. Paleomagnetic records documented numerous episodes of weak relative paleointensity generally accompanied by strong directional deviations, assignable to geomagnetic excursions (such as Laschamps, Blake, Jamaica/Pringle falls, Icelandic basin, Levantine, Delta, Kamikatsura, Santa Rosa) or to reversals (Brunhes/Matuyama, upper and lower Jaramillo, Cobb Mountain). Phases of significant enhancement of the authigenic 10Be/9Be ration (proxy of cosmonuclide production variation, primarily driven, through magnetospheric modulation, by the geomagnetic dipole moment) confirm the geomagnetic moment drops associated with excursions and reversals. The intensity and directional signatures of reversals and excursions are generally recorded significantly beneath the sharp increases of the 10Be/9Be ratio linked to low intensity phases, indicating that the magnetization is acquired with significant delays. The width of the lock-in window (lock-in depth) is strongly variable along one given sequence and often exceeds 10 cm (exceptionally reaching 30-40 cm). This suggests that the evaluation of lock-in delays differences between sites is a prerequisite of the use of paleomagnetic of excursions or reversals as high resolution tools to correlate sub-millenial oscillations along paleoclimatic records obtained from distinct sedimentary sequences. When available, the cosmogenic isotope signatures represents an efficient global correlation tool between sedimentary sequences or with even with ice core records. Beyond such methodological considerations, more fundamental conclusions are: 1) authigenic 10Be/9Be and paleomagnetic data sets agree on the fact that excursions and reversals generally occurred –in the studies time intervals- at the time of interglacials or interstadials; 2) A 100 ka periodicity of the dipole moment variation does not result from a paleoclimatic influence on the sediment magnetization, since it is also present in the 10Be/9Be proxy record.