The effects of inter-particle forces on the acquisition of depositional remanent magnetism

Sediments and sedimentary rocks are invaluable in reconstructing changes in the earth's magnetic field through geologic time. As sediments are deposited, a small fraction of the ferromagnetic grains in the sediment are aligned with the geomagnetic field. The magnetic signature thus recorded and preserved is known as Depositional Remanent Magnetization (DRM).;Previous research in paleomagnetism had assumed the magnitude of DRM resulted from two competing phenomena: the aligning effects of the geomagnetic field on the ferromagnetic grains, and the misaligning effects of Brownian motion on the sub-micron ferromagnetic grains. This approach neglected the effects of interactions between the ferromagnetic grains and the non-magnetic matrix. Theoretical and experimental research demonstrates that magnetite does not exist in isolation, but is always incorporated into a clay-magnetite aggregate due to flocculation. These aggregates encounter a significant viscous drag which hinders perfect alignment of magnetite with the earth's magnetic field. Using a model based on particle aggregation, I can fit experimental results of magnetization of sediments.;The dominant paradigm in paleomagnetism has been the idea of post depositional remanent magnetization (PDRM), which supposes that magnetite grains rotate freely in pore spaces and can be realigned after deposition, before becoming immobilized at some depth after the sediment has been compacted and the pore spaces reduced. Simple calculations show that the energy due to inter-particle attractions are one to two orders of magnitude higher than the magnetic energy of a magnetite grain in the geomagnetic field. Thereby, magnetite grains once incorporated into a floc will not be reoriented subsequent to deposition. We verified our hypothesis with experiments on natural sediments which did not record a PDRM during remagnetization in the laboratory.;Bioturbation, which can break inter-particle bonds in sediments, could contribute to a PDRM. We remagnetized sediments after introducing worms in them, but failed to observe a permanent remagnetization in sub-surface sediments. We conclude that bioturbation does not enhance PDRM below the sediment surface, and propose a new model of sediment magnetism, where bioturbation can cause a lag in the recording of the geomagnetic field.