NORMALIZED NATURAL REMANENT MAGNETIZATION INTENSITY DURING THE LAST 240000 YEARS IN PISTON CORES FROM THE CENTRAL NORTH-ATLANTIC OCEAN - GEOMAGNETIC-FIELD INTENSITY OR ENVIRONMENTAL SIGNAL

We have examined in detail the stratigraphic variations in magnetic pa rameters of four cores from the North Atlantic Ocean in areas where th e depositional environment has varied with climatic changes. Our objec tive is to assess whether normalisation of the natural remanent magnet isation (NRM) intensity can cancel the effect of climatically induced variations in magnetic mineral content and grain size and whether a re liable record of relative changes in geomagnetic field intensity can b e obtained. After selecting the core sections which meet published cri teria for paleointensity normalisation, we have attempted to obtain a record of the geomagnetic field intensity variations over the past 240 kyear, using both ARM and IRM as normalising factors for the NRM. The two methods yield overall similar results, except for the interval 20 -30 kyear, where IRM normalisation yields a record more consistent wit h previous sedimentary and volcanic results, than does the more freque ntly employed ARM normalisation. The final intensity record documents a picture of the dipole field moment which presents large similarities with profiles obtained from sediments deposited in different environm ental conditions in various regions around the world. We observe a maj or low at about 42 kyear, which could correspond to the Laschamp event , a broad low in the interval 90-130 kyear, possibly connected to the Blake event, and another low at about 190 kyear, which could reflect t he Biwa I event. The processes of acquisition of depositional and post depositional remanent magnetisation and the physics of the normalising methods are still insufficiently understood. However, these results, obtained from a region characterised by complex environmental dynamics , confirm the potential of sedimentary deep sea cores for relative pal eointensity determinations.