Preliminary 50m palaeomagnetic records from Lac du Bouchet, Haute Loire, France

Abstract

Following on from earlier studies carried out in successive phases, four much longer cores were collected from Lac du Bouchet, a maar lake in the Velay region of France (44·9°N; 3·8°E). Preliminary palaeomagnetic records from core H to a depth of 46m are presented here. Measurements of natural remanant magnetization (NRM), both raw and after demagnetization in alternating fields of up to 60mT were carried out on 1382 subsamples. Also anhysteretic [ARM] and isothermal [IRM] remanent magnetizations, and low field magnetic susceptibility were measured to give a picture of the changes in magnetic mineralogy recorded downcore. These laboratory implanted magnetizations are useful as proxy indicators of climate and for assessing the suitability of the sediment for making estimates of relative geomagnetic palaeointensity. We have, at this stage of the work, normalized NRM intensity only with susceptibility but argue that this provides a valid record of palaeointensity, except for where the sediment is highly organic. Preliminary pollen analyses suggest that Holsteinian interglacial sediments are present at a depth of about 42m, so it is though that the records cover at least the last 250000 years. Long wavelength, high amplitude oscillations in palaeointensity are found which are different from the oscillations in climate over this period.