T. Yamazaki $^{a}$ and H. Oda $^{a,b}
$^{a}$ Institute for Marine Resources and Environment, Geological Survey of Japan, AIST $^{b}$ Paleomagnetic Laboratory 'Fort Hoofddijk', Utrecht University, The Netherlands
A continuous record of the inclination and intensity of Earth's magnetic field, during the past 2.25 million years, was obtained from a marine sediment core of 42 meters in length. The core MD982185 was taken in West Caroline Basin (3^\circ05.1'N, 134^\circ59.8'E) at a water depth of 4415 m during the IMAGES IV cruise. The sediment comprises of hemiperagic clay with cyclic variations in carbonate content. Paleomagnetic measurements were conducted with a cryogenic magnetometer on 1746 discrete samples of 7cm3 taken from half sections of the core. Paleomagnetic directions were determined by fitting linear regression lines to the magnetization vectors measured at some AF demagnetization steps. Relative geomagnetic paleointensity was estimated by normalizing the natural remanent magnetization with the artificial magnetization reflecting the concentration of magnetic mineral. The magnetic polarity reversal sequence down to the Reunion Subchron (2.14 to 2.15 Ma) could be identified from the declination. Age model was constracted by the magnetic polarity boundaries at and before the Brunhes/Matuyama boundary, and the tuning of the relative paleointensity variations with the paleointensity master curve Sint-800 (Guyodo and Valet, 1999) within the Brunhes Chron.The ages based on the relative paleointensity are consistent with those estimated from the correlation of the susceptibility variations with the standard marine oxygen isotope curve. This record reveals the presence of 100,000-year periodicity in inclination and intensity, which suggests that the magnetic field is modulated by orbital eccentricity. The important point is that the 100,000-year periodicity is persistent through the measured interval including the transition of the climate modulation from 40,000-year to 100,000-year periodicity. The expected inclination for the axial dipole field at the site is 6^\circ, however, an inclination anomaly dI of -5~-6^\circ is known to exist in this region, which is thought to be caused by persistent nondipole components. The correlation between inclination and intensity shifted from antiphase to in-phase, corresponding to a magnetic polarity change from reversed to normal. We proposed a model, in which the strength of the geocentric axial dipole field varies with 100,000-year periodicity, whereas persistent nondipole components do not (Yamazaki and Oda, 2002). Further analysis suggests that the vertical component of the paleomagnetic field is dominated by 100,000-year periodicity than in inclination. The implication is that the non-dipole component has 100,000-year periodicity, which is directly introduced from the orbital eccentricity but not from the indirect climate modulation.