Grain size of loess, palaeosol and Red Clay deposits on the Chinese Loess Plateau: Significance for understanding pedogenic alteration and palaeomonsoon evolution

To assess pedogenic modification to grain size distributions of loess, palaeosol and Red Clay deposits on the Chinese Loess Plateau and to understand long-term evolution of the East Asian palaeomonsoon since the late Miocene, we investigate a continuous loess–palaeosol–Red Clay sequence at Lingtai (south Chinese Loess Plateau) and another parallel Red Clay sequence at Zhaojiachuan (central Chinese Loess Plateau). By analyzing the grain size distributions of bulk samples and chemically isolated quartz samples, the relative intensity of pedogenic alteration of the loess, palaeosol and Red Clay deposits can be quantified. Comparisons of the grain size distributions and contents of different grain size fractions between the bulk and the quartz samples suggest that pedogenic alteration is apparently stronger in the Red Clay sequence than in the overlying loess–palaeosol deposits. Furthermore, grain size parameters derived from the bulk and the quartz samples, such as mean grain size, coarse fraction content and U-ratio, exhibit similar variations in the loess–palaeosol sequence (Quaternary, 0–2.6 Ma), whereas these grain size parameters of quartz samples show different variability from those of bulk samples in the Red Clay sequence (late Miocene to Pliocene, 2.6–7 Ma). Grain size of quartz from the loess–palaeosol and Red Clay deposits exhibits distinct and persistent oscillations from the late Miocene to the Pleistocene, implying that significant fluctuations of the palaeomonsoon climate in East Asia might have occurred at least since late Miocene time.