Evidence for a geomagnetic excursion recorded in the sediments of Lac St Front, France: A link with the Laschamp excursion?

Anomalous paleomagnetic directions, accompanied by low intensities of natural remanent magnetization are recorded at 17 m depth in the sediments cored from Lac St. Front (Massif Central, France). The rock magnetic properties and the parameters characterizing anisotropy of susceptibility of samples taken from the sediment horizons carrying these anomalous directions, spanning some 30-40 cm, are identical to those of samples taken from the other stratigraphic intervals, carrying a record of normal geomagnetic secular variation. Therefore the sequence of anomalous directions is interpreted as a geomagnetic excursion. Furthermore, it is concluded that this excursion occurred at a time when the paleointensity field was low. The excursion is dated at about 33 C-14 (similar to 37 ka B.P.) by correlating the susceptibility and paleomagnetic logs from Lac St. Front with the corresponding logs from the nearby Lac du Bouchet sedimentary sequence. The excursional directions in Lac St. Front can be averaged out with a triangular window of 20 cm. These smoothed excursional directions resemble the directional variations observed in the corresponding Lac du Bouchet interval. This finding indicates that the absence of this excursion in Lac du Bouchet may be due to a larger smoothing of the geomagnetic signal.