A reassessment of post-depositional remanent magnetism: preliminary experiments with natural sediments

Realignment of magnetic grains below the sediment–water interface is thought to impart a post-depositional remanent magnetization (pDRM). However, there is little convincing evidence in the published literature that pDRM is the dominant mechanism by which sediments become magnetized. We report here preliminary results from two kinds of laboratory experiments designed to investigate whether post-depositional reorientation of magnetic particles is likely to occur in nature. In the first experiment, we monitored changes in the magnetization of natural sediments in response to changing laboratory fields. Our results are inconsistent with post-depositional reorientation of magnetic particles. In a second experiment, we put live worms in a multi-core tube with the original sediment–water interface intact. Remagnetization was only observed in samples taken from a mound of fecal pellets formed at the surface. These pellets had been suspended by a worm, and redeposited in the laboratory field. The other samples, which were not resuspended, but nonetheless bioturbated by the worms, showed no change in magnetization. Our preliminary results do not support the hypothesis that post-depositional reorientation occurs in natural, undisturbed sediments below the sediment–water interface.