Plastically deforming clay-rich sediment to help measure the average remanence anisotropy of its individual magnetic particles, and correct for paleomagnetic inclination shallowing

摘要

The inclination of remanence in fine-grained sediment may be shallower than the inclination of the field in which it was acquired. This paleomagnetic inclination shallowing can lead to an underestimation of paleolatitude. Fortunately, compaction also induces a magnetic anisotropy in the sediment that can help correct for the inclination shallowing. Such correction generally requires measuring the average remanence anisotropy of the individual magnetic particles in the sediment, which can be difficult. For example, various authors have tried diluting the sediment with a glue, placing it in a strong magnetic field to align the long axes of the magnetic particles while the glue hardens, and then measuring the anisotropy of the resulting sample. This often overestimates particle anisotropy, probably because the magnetic particles tend to form chains along the magnetic field lines. We describe a method that avoids this problem, in the case of clay-rich soft sediment; we align the magnetic particles, without using a magnetic field, by plastically deforming the sediment with an axial compression. This method was applied to suite of clay-rich deep-sea turbidite samples that bear pseudo-single-domain magnetite particles. Measuring the remanence anisotropy induced by compressing a composite sample yielded an estimate of the average remanence anisotropy of the individual magnetite particles in the sediment. Although this proved to be an underestimate, it can be useful in conjunction with the overestimate yielded by the magnetic field alignment method. We also experimented with a modification of our method that involves measuring the change in remanence inclination as well as the remanence anisotropy induced by compressing the sample. This yielded a particle remanence anisotropy estimate that allowed us to successfully correct for the inclination shallowing observed in the turbidites. Our method should be applicable to any clay-rich soft sediment with magnetic particles of any domain state if the particles are dominated by uniaxial shape anisotropy.