EXTENSION OF THE LAC-DU-BOUCHET PALEOMAGNETIC RECORD OVER THE LAST 120,000 YEARS

A high-resolution profile of the palaeomagnetic variations recorded in the upper 20 m of the Lac du Bouchet sedimentary sequence has been reconstructed from several parallel Mackereth and "Livingstone" type cores. Inter-core correlations were established by comparing individual core profiles of magnetic susceptibility. Individual records of AF cleaned directions were then "stretched" to a common depth scale and merged; the mean record was then filtered and smoothed in order to reduce the background noise. Accelerator radiocarbon determinations and pollen analyses show that the sequence covers the last interglacial/glacial cycle (back to ca. 120 kyr B.P.), and allow to perform a depth to time transformation. There is a noted absence of widely divergent directions back to 90 kyr B.P. It is estimated that the recorded signal (a PDRM signal) represents the geomagnetic input signal, attenuated by about 19%. It follows that the excursions reported for the Laschamp and Olby lavas, if they existed at all, must have been of rather short duration. Two durable excursions appear in narrow diameter sections at the base of three independent cores (B, C and D), while a reverse to normal transition is possibly documented in the lowermost meter (core D). This part of the sequence is dated from the recognition of major climatic events related to the 18O isotopic stages defined in the oceanic record between 116 and 95 kyr. This introduces the possibility of new excursions at ca. 104 and 95 kyr, i.e, immediately after the recovery of normal polarity documenting the end of the Blake event. © 1990.