Geochemical records from loess deposits in Japan over the last 210 kyr: Lithogenic source changes and paleoclimatic indications

Abstract

The generation, transport, and accumulation of tropospheric dust have changed with the paleoclimatic changes of the Quaternary period. Such dust has accumulated in Japan ∼3000 km leeward of the source deserts in China. We analyzed the fractions of windblown fine quartz and bulk major and trace elements, and Sr-Nd-Pb isotopic compositions of loess in SW Japan deposited over the past 210 kyr. The results indicated extensive accumulation of tropospheric dust mixed with tephra fragments derived from the nearby Daisen volcano. The accumulation rate of fine quartz and selected elemental/isotopic compositions can be used as climatic proxies that reflect greater accumulation of dust in times of colder climate. Chemical indices for weathering show enhanced effects of weathering during times of warmer climate. The trace element compositions of the loess deposits are surprisingly similar to those of the Chinese loess, hemipelagic sediments in the Sea of Japan and the western Pacific Ocean, and the distal Chinese dust found in Canada. This similarity indicates that the loess dust shares major fractions of these fine-grained sediments and that geochemical fractionation during the transport was limited. The Sr-Nd-Pb isotope compositions of the SW Japan loess indicate an origin predominantly in the Gobi Desert. The high-latitude Pacific sediments and high-latitude dust in Canada also show the same signature. However, isotopic compositions of samples from the southern Chinese loess plateau and mid to low-latitude Pacific sediments are largely derived from the Taklimakan desert, which indicates different delivery pathways of the tropospheric dusts.