Short-term cycle of eolian dust (Kosa) recorded in Lake Kawaguchi sediments, central Japan

摘要

The fluctuation during the last 100yr of the eolian dust (Kosa aerosol) originating from arid and semi-arid areas of China has been reconstructed by using the sediments from Lake Kawaguchi, central Japan with high temporal resolution. The quantification of Kosa contribution to the sediments was carried out by a new method using scanning electron microscopy-energy dispersive X-ray microanalysis (SEM-EDX) proposed by us. The correlation plot of (Na_2O + K_2O) contents against SiO_2 was used for individual Si-rich particles having SiO_2 content over 80%. The Kosa fraction of Si-rich particles in Lake Kawaguchi sediments during the last 100yr is approximately 10-30%. The fluctuation of the Kosa fraction during the last 100yr does not coincide with that of the total amount of Si-rich particles, because detrital components from Japanese igneous rocks control the fluctuation of the total number of Si-rich particles. The discrimination method based on single particle analysis is more effective than that of bulk analysis for the lake sediments formed by complex matrix components. We can first show a short-term (approximately 10-20 yr scale) cycle in Kosa aerosol fluctuation. Higher sedimentation rates (5-10yr-cm) of the Lake Kawaguchi sediments and the new analytical method using SEM-EDX revealed a remarkable fluctuation pattern of Kosa aerosol, suggesting climate cycles much shorter than glacial-interglacial. Such short-term cycles may be related to sun-spots. The number of days of Kosa events during the last 30 yr, obtained by visual observation by Meteorological Agency of Japan, also supports the presence of such a short-term cycle.