Effects of sediment aggregate size on DRM intensity: a new theory

Previously, the acquisition of depositional remanent magnetism (DRM) in red eposition experiments has been described as a balance between the alignment of magnetic moments by the imposed magnetic field and the misaligning effe cts of Brownian motion, an approach that ignores interactions between sedim ent particles. Inter-particle attractions due to electrostatic or van der W aals forces, or biologically mediated flocculation, lead to the formation o f aggregates, which prevent the settling of individual grains. We propose a new model in which we examine the angular momentum balance on a clay-magne tite aggregate, which can be tens of micrometers in diameter, as opposed to an individual grain of single-domain magnetite, which is usually less than 1 mum in diameter. Viscous drag becomes important for these large aggregat es and larger magnetic fields are necessary to bring their moments into ali gnment. We model the acquisition of DRM by assuming a log-normal size distr ibution of flocs, all of which have the same magnetic moment. The undersatu ration of magnetic intensity in laboratory DRM experiments is explained by the larger viscous drag encountered by the larger aggregates which prevents perfect alignment with the applied magnetic field. (C) 2001 Elsevier Scien ce B.V. All rights reserved.