Major element and REE compositions of Pliocene sediments in southwest Japan: Implications for paleoweathering and paleoclimate

Abstract

The history of the East Asian monsoon and the relationship between the development of the monsoon climate, tectonics and global climate are complicated and controversial. Earlier studies have indicated that the East Asian monsoon fluctuated in the Asian interior during the Pliocene. Thus, the chemical weathering record in coastal regions of East Asia can be the key to elucidating its evolution as well as the Kuroshio Current variability. The present study clarifies the chemical weathering conditions in southwest Japan based on the concentrations of major elements and rare earth elements (REEs) in lacustrine muddy sediments. The chemical weathering was moderate between 4.4 and 3.8 Ma. Between 3.8 and 3.4 Ma, chemical weathering significantly intensified, as indicated by the high values of the chemical index of alteration that reached a maximum of 98, as well as the high concentrations of REEs and light REEs against heavy REEs and kaolinite-rich clay mineral compositions, despite the wide variation in source rock compositions and grain sizes. The degree of chemical weathering remained moderate during the early Pleistocene. The intense chemical weathering on land in southwest Japan from 3.8 to 3.4 Ma may have been regulated by alternating periods of warm and humid climate brought by the invasion of the Kuroshio Current and the intensification of the East Asian summer monsoon.