Normalised natural remanent magnetisation intensity during the last 240 000 years in piston cores from the central North Atlantic Ocean: geomagnetic field intensity or environmental signal?

Abstract

We have examined in detail the stratigraphic variations in magnetic parameters of four cores from the North Atlantic Ocean in areas where the depositional environment has varied with climatic changes. Our objective is to assess whether normalisation of the natural remanent magnetisation (NRM) intensity can cancel the effect of climatically induced variations in magnetic mineral content and grain size and whether a reliable record of relative changes in geomagnetic field intensity can be obtained. After selecting the core sections which meet published criteria 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 with previous sedimentary and volcanic results, than does the more frequently employed ARM normalisation. The final intensity record docmments a picture of the dipole field moment which presents large similarities with profiles obtained from sediments deposited in different environmental conditions in various regions around the world. We observe a major 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 the Biwa I event. The processes of acquisition of depositional and postdepositional 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 paleointensity determinations.