Pristine magnetic moments of magnetic grains in natural sediments; results of laboratory experiments with lacustrine and marine sediments

The acquisition of detrital remanent magnetization (DRM) during consolidation of lacustrine and marine sediments has been extensively studied for half a century. Controlled experiments with mostly synthetic sediments have resulted in the present-day consensus that a genuine DRM may exhibit significant inclination error while a post-depositional DRM (pDRM) will truthfully record the ambient field some time after deposition. The experimentally established time-delay of pDRM has recently been challenged based on observations of natural systems as well as experiments with flocculating sediments. Realistic models for DRM-acquisition require information of the magnetic moments of grains contributing to the remanent magnetization. Presently, the pristine magnetic moments of magnetic grains in natural sediments is unknown. Controlled re-deposition experiments have been performed in order to assess the relative fraction of pristine magnetic moments in marine and lacustrine sediments. Sediment-suspensions (<38μ) have been deposited in different ambient magnetic fields. The suspensions were exposed to increasing magnetic DC-fields (max 500mT) prior to deposition. Results reveal a) gradual and substantial increases of DRM-intensity (up to 10 times pristine material) and inclination errors with increasing magnetization of the suspensions, b) up to 10 times higher DRM-intensities in fresh water compared to sea-water, c) MDF of DRM (partly dried samples in zero magnetic field) decreases significantly with increasing magnetization and d) relative paleointensity proxies (NRM/ARM, NRM/SIRM) increase with magnetization of suspensions and are 8-30 times higher in sediments deposited in fresh-water. Results are not readily explained by a simple model of gradual magnetization of grains in suspension.