Paleointensity of the geomagnetic field during the last 2.5 million years

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Since the establishment of a stacked geomagnetic paleointensity record during the Brunhes Chron (Sint-800), researches on relative paleointensity have been directed to obtaining high-resolution records during the last ca. 100 kyr such as the NAPIS-75,and to extending records towards ages older than the Brunhes Chron. This study was aimed to clarify relative paleointensity variations since 3 Ma. A paleomagnetic study was conducted on six sediment cores obtained from the West Caroline Basin in the western equatorial Pacific and the Manihiki Plateau in the South Pacific. These cores cover the last 1.2 to 3 m. y. The measurements include magnetic susceptibility, natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM). Deconvolution was applied for samples taken by u-channels to increase time resolution. Relative paleointensity of the geomagnetic field was estimated from NRM intensity normalized by ARM. Age control of the sediment cores was performed mainly by correlating magnetic concentration changes (magnetic susceptibility and ARM) to the oxygen isotope curve of ODP Site 1143 in the South China Sea. Partly color reflectance was used for the correlation. For one core, oxygen isotope ratios were measured and used for the age estimation. Relative paleointensity variations of all cores agree well within uncertainty of age (a few tens of thousand years). By stacking these records with minor adjustments of ages, we have established a paleointensity curve during the last 3 m. y. The variations are characterized by quasi-periodic occurrence of intensity lows throughout the record, and many of them seem to correspond to previously reported geomagnetic excursions or polarity boundaries. These features are similar to those observed in the Sint-800 record during the Brunhes Chron. The "asymmetric sawtooth pattern" was not observed.