Enhanced seasonality of the East Asian monsoon indicated by prevailing hydroxy-interlayered vermiculite formation since 1.8 Ma

Abstract

The seasonality of the East Asian monsoon is characterized by the coexistence of warm and humid summers and cold and dry winters. However, the current understanding of when and why the seasonality of the East Asian monsoon occurs is poor. Hydroxy-interlayered vermiculite (HIV), which is formed under conditions of low soil organic matter content, pH values between 4.6 and 5.8, and oxidizing states with frequent wetting‒drying cycles, may serve as a potential indicator of monsoon seasonality variation. In this study, we present detailed mineralogical and micromorphological investigations of HIV collected from an eolian sequence on the Chinese Loess Plateau (CLP) to explore the evolution of East Asian monsoon seasonality. The mineralogical results deduced from X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy show that a transition from chlorite to HIV occurred at ∼1.8 Ma. Moreover, the authigenic origin of HIV, as indicated by the presence of many neatly arranged flakes in the SEM images and the presence of irregularly interstratified chlorite-HIV observed via transmission electron microscopy (TEM), collectively imply that HIV is the weathering product of chlorite. This transition from chlorite to HIV generally reflects an increase in monsoon seasonality on the CLP since ∼1.8 Ma, which may be caused by the contemporaneous uplift of the northeastern Tibetan Plateau, or global cooling in combination with La Niña-like sea-air interactions. Our study suggests that the HIV serves as an effective index for monsoon seasonality reconstruction, thus providing a new insight into Asian monsoon dynamics.