Loess deposits of the upper Hanjiang River valley, south of Qinling Mountains, China: Implication for the pedogenic dynamics controlled by paleomonsoon climate evolution

Aeolian deposits in the Chinese Loess Plateau (CLP) provide a detailed archive for reconstructing the pedogenic intensity as well as the East Asian monsoon climate change. However, study on the loess in the upper Hanjiang River valley, south of Qinling Mountains has seldom been comprehensively reported. Located at the transition zone between temperate and subtropical monsoon climate, the study area is more sensitive to the climate change. In this paper, three loess-paleosol profiles at the first terrace of the upper Hanjiang River were studied in detail. High-resolution investigations, including field observations, optically stimulated luminescence (OSL) dating, and measurements of magnetic susceptibility (MS), grain-size (GS), color variation, loss-on-ignition (LOI) and chemical elements were carried out. The results show that the stratigraphic sequences, in order from the top to the bottom, are topsoil (TS), recent loess (L0), paleosol (S0), transitional loess (Lt), Malan loess (L1) and fluvial deposits (T1-al). The pedogenic intensity varies significantly in different layers and presents such a tendency of S0 > L0 > Lt > L1. This indicates four distinct stages in the paleoclimate evolution: a cold-dry period (55.0-11.5 ka B.P.); a phase of gradual transition to warm-wet (11.5-8.5 ka B.P.); the maximum warm-wet period (8.5-3.0 ka B.P.); and a phase of gradually shifting to cool-dry (3.0-0.0 ka B.P.). The climate change trends are similar with the loess records from the CLP and the stalagmite and peat records in southern China. But the paleosol development in the study is probably a better indicator of the strength of summer monsoon climate change during the mid-Holocene Climatic Optimum. This study also provides basic data for exploring the pedogenesis and climate differences in the East Asian monsoon climate zones.