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

Realignment of magnetic grains below the sediment-water interface is though t to impart a post-depositional remanent magnetization (pDRM). However, the re is little convincing evidence in the published literature that pDRM is t he dominant mechanism by which sediments become magnetized. We report here preliminary results from two kinds of laboratory experiments designed to in vestigate 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 o f magnetic particles. In a second experiment, we put live worms in a multi- core tube with the original sediment-water interface intact. Remagnetizatio n was only observed in samples taken from a mound of fecal pellets formed a t 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. O ur preliminary results do not support the hypothesis that post-depositional reorientation occurs in natural, undisturbed sediments below the sediment- water interface. (C) 2000 Elsevier Science B.V. All rights reserved.