Numerical modelling of remanence carriers and remanence acquisition in marine sediments

Abstract

Sediments act as recording media for changes in the earth´s magnetic field. Highly resolved paleomagnetic records from these sediments give detailed insight into secular variations and excursions of the earth´s magnetic field which in turn provide information about the geodynamo. The parameter on which paleomagnetic studies are based is the natural remanent magnetization (NRM). The interpretation of NRMs from sediments requires a knowledge of recording mechanisms, magnetization structure and composition of the magnetic minerals carrying the paleomagnetic signal.The knowledge of the size range of uniform magnetization structure is fundamental to the interpretation of NRMs. Former micormagnetic models have proved to be an effective tool in investigating magnetization structures but were based on rectangular particles. Naturally occurring magnetite is usually not rectangular in shape thus the first part of this thesis focusses on a new micromagnetic program for calculating magnetization structures of irregularly shaped particles. This work showed that shape irregularities allow for bigger stable SD particles than formerly predicted by rectangular models and helped to explain discrepancies between earlier models and experimentally observed behavior.In reconstructing the earth´s paleofield from marine records it is assumed that NRM is acquired parallel to the magnetic field and that normalized NRM and paleofield are proportional. Thus, each sedimentary factor influencing the degree of particle alignment alters the magnetic signal and therefore the reconstruction of the past geomagnetic field. The advantage of numerical sediment models is that they can be used to investigate the influence of a single process upon remanence acquisition. The second part of this thesis deals with numerical models of sedimentary remanence acquisition. It was possible to successfully develop a first model sediment which was used to investigate magnetic sediment properties.