Relative paleointensity stack during the last 250 kyr in the northwest Pacific

摘要

We have conducted a paleomagnetic study of 10 sediment cores recovered from the northwest Pacific. The purpose is to establish a regional paleointensity stack during the last 250 kyrs (NOPAPIS-250) for global coverage of paleointensity data as well as a regional reference for paleointensity-assisted stratigraphy. In the studied cores, reductive dissolution of magnetite appears to have occurred mainly in glacial periods. Such horizons accompany large decreases in S-ratio (S?0.3T = (1 ? IRM?0.3T/SIRM)/2) as low as 0.90. The sediments are also intercalated by frequent volcanic ash layers. Prerequisites for reliable relative paleointensity estimation are not satisfied at these horizons. We rejected unsuitable data by quantitative criteria based on S-ratio and saturation isothermal remanent magnetization (SIRM) and constructed the NOPAPIS-250 curve from the remaining data. In the construction of the curve, we chose IRM as a normalizer because acquisitions of anhysteretic remanent magnetization (ARM) are considered to be influenced by magnetic interactions among magnetic grains in the northwest Pacific sediments. Relative intensity variation of the NOPAPIS-250 gives a standard deviation that is 33% of the average, if the normal distribution is assumed. The NOPAPIS-250 curve generally shows similar variation patterns compared with other high-resolution records reported from other regions: many paleointensity lows are recognized in common. Six out of the nine lows can be correlated with reported excursions in literature.