Paleoclimatic and orbital modulation of the geomagnetic field

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1. We investigated the strength of magnetostatic interaction among magnetic particles in marine sediments. On relative paleointensity estimation, IRM is preferable to ARM for normalizing difference in NRM acquisition efficiency of sediments because ARM acquisition is strongly affected by the magnetostatic interaction.2. We constructed a paleointensity stack during the last 250 kyr in the Northwest Pacific. To obtain reliable relative paleointensity records, it is necessary to exclude horizons that suffer from dissolution of magnetic minerals owing to early diagenesis. The S-ratio, which represents a proportion of low coercivity magnetic minerals, is sensitive to magnetite dissolution, and thus can be used for detecting such horizons.3. We obtained a relative paleointensity record during the last 1.6 m.y. from a sediment core in the North Pacific. The paleointensity and magnetic properties of this core were compared with those from the western equatorial Pacific. Relative paleointensiti … More es of the two areas coincide well, whereas magnetic properties differ greatly. A time-series analysis using a wavelet transform showed that paleointensity records contain variations with a time scale of 100 kyr. Variations of magnetic properties are not synchronous with those of paleointensity, and there is no significant correlation between the two even if they have a common time scale of 100 kyr. We thus conclude that the 100-kyr variations in paleointensity records are of geomagnetic origin, and not caused by magnetic property variations of sediments induced by paleoclimate changes.4. Paleointensity and inclination variations during the last 800 kyr were recovered from cores in the eastern equatorial Indian Ocean (EEI). The area of a large inclination anomaly in the western equatorial Pacific (WEP) does not extend to the Indian Ocean. The model of long-term inclination variations that a weaker dipole field causes a larger relative contribution of persistent non-dipole components is supported. Less