East Asian summer monsoon intensity inferred from iron oxide mineralogy in the Xiashu Loess in southern China

The Xiashu Loess, in comparison to the well-studied loess sequences in the Chinese Loess Plateau (CLP), provides a good opportunity for studying East Asian monsoon variations from a southern China perspective. Here we present a study of the iron oxide mineralogy of the Xiashu Loess using integrated geochemical and diffuse reflectance spectroscopy (DRS) measurements as well as magnetic data. Our results show that the free iron oxide (Fe_d) to total iron (Fe_t) ratio (Fe_d/Fe_t), hematite (Hm) to goethite (Gt) ratio (Hm/Gt) and saturation isothermal remanent magnetization (SIRM) to magnetic susceptibility (χ) ratio (SIRM/χ) all indicate particularly strong summer monsoons during the formation of paleosols PS5 and PS4 (equivalent to Marine Isotope Stage 13 and 11, respectively). However, magnetic susceptibility and Fe_d/Fe_t are not consistently reliable indicators of summer monsoon intensity for the whole section. Our results indicate that a multi-proxy approach can give a more reliable summer monsoon intensity reconstruction. The summer monsoon shows a cooling trend and a declining of precipitation from 0.5 to ∼0.3 Ma, after which it becomes warmer and wetter towards the top of paleosol PS1 (equivalent to MIS 5). However, PS1 was formed under a relatively cooler temperature and wetter soil conditions in comparison to PS5 and PS4. Such supra-orbital variations in the East Asian summer monsoon superimposed on the effects of glacial-interglacial cycles in southern China are also reflected in the 0.4-0.5 Ma cycle of marine carbon isotopes in the global ocean, possibly indicating a strong link between terrestrial weathering and the global carbon cycle.