A normalised intensity record from Lac du Bouchet: geomagnetic palaeointensity for the last 300 kyr?

A normalised intensity record has been obtained from 2500 sediment samples taken from three cores of length 50 m from Lac du Bouchet, a small maar crater lake in the Massif Central of France. The sediment sequence has been dated by 14C in the top 40 kyr, by correlation of the pollen-derived climatic stratigraphy with the marine δ 18O stratigraphy, and by Ar/Ar ages (275 ± 7 kyr) for a tephra interbedded at 43-m depth. The sediments are mainly detrital, and fall into two broad sediment types: silty clays (83% of the total) deposited under glacial climatic conditions with high titanomagnetite concentrations (0.5-1% by weight); and compacted gyttjas (organic-rich clays) deposited under warmer conditions, with a more variable magnetic mineralogy of lower concentration. Normalisation of the NRM by susceptibility, ARM and SIRM yields palaeointensity records that are similar to each other, but bear little resemblance to the normalisor records. SIRM gives the best results of the three. However, the compacted gyttjas give consistently low palaeointensity values, and high palaeointensities correspond to high SIRM/ARM ratios, which occur during the full glacials of δ 18O stages 2 and 6 —normalisation has incompletely removed lithological and climatic factors from the NRM. To overcome this, we have attempted a secondary normalisation by weighting the palaeointensity values according to a least-squares fit to the SIRM/ARM, resulting in an improved palaeointensity estimate. The average accumulation rate of 15 cm/kyr, together with the close sampling interval, gives a palaeointensity record of much greater resolution than most published palaeointensity records.