Iron geochemistry of loess and red clay deposits in the Chinese Loess Plateau and implications for long-term Asian monsoon evolution in the last 7.0 Ma

Abstract

Recently, some groups of authors have demonstrated that the Tertiary red clay deposits underlying the Pleistocene loess–paleosol sequence in the Chinese Loess Plateau are wind-blown in origin, thus extending the eolian record in the Plateau from 2.6 Ma back to about 7.0 Ma, and providing a good opportunity to reconstruct long-term monsoon changes. As magnetic susceptibility, the widely used paleomonsoon proxy of loess deposits, is problematic in reconstructing the monsoon history recorded in the red clay, the development of other proxies is urgently needed. In this study, we analyzed the ratios of citrate–bicarbonate–dithionite (CBD)-extractable Fe₂O₃ to total Fe₂O₃ concentrations in four loess sections along a north–south transect of the Loess Plateau, and in the Lingtai loess–red clay sequence of ∼305 m thick. The Fe₂O₃ ratio in the loess sections clearly shows higher values in soil units than in loess horizons, and a southward increase in this ratio is also remarkable along the transect. These are in good agreement with the temporal–spatial variations in the East Asia summer monsoon intensity identified by other monsoon proxies and soil development observations. The Fe₂O₃ ratio thus attests to being a good proxy indicator of the summer monsoon evolution. The Lingtai Fe₂O₃ ratio record shows high values over three time intervals: ∼4.8–4.1 Ma, ∼3.4–2.6 Ma, and during the interglacial periods of the past 0.8 Ma. The increase in summer monsoon intensity over the three intervals also coincides with the well-developed soil characteristics. It is therefore concluded that the East Asia summer monsoon has experienced a non-linear evolution since the late Miocene. The occurrence of a monsoon prime at about 4.8–4.1 Ma implies that the Tibetan Plateau, one of the most important boundary conditions in maintaining the monsoon circulation, must have been uplifted to a critical height at that time.