CHEMICAL WEATHERING OF CHLORITE IN THE CHINESE LOESS - PALEOSOL STRATIGRAPHY AND CLIMATE CHANGE

Clay mineral assemblages dominated by illite and chlorite characterize the Quaternary and Tertiary loess sediments in the Chinese loess plateau. The variations of illite and chlorite in clay fractions of five selected loess-paleosol sequences are quantified by the illite/chlorite ratio (I/C), corresponding to the ratio of the height of the (002) peak of illite at 0.500 nm and of the (003) peak of chlorite at 0.474 nm in XRD diagrams. The I/C ratios record loess-paleosol alternations, and are positively related to magnetic susceptibility values. That is, the I/C ratio was consistently low for loess layers with low susceptibilities, and slightly higher in paleosol samples with higher susceptibilities. Furthermore, a good linear correlation between I/C ratios and latitude indicates a spatial gradient of chemical weathering intensity of chlorite closely related to the summer monsoon circulation. Transmission electron microscopy and analytical electron microscopy observations indicate that Fe release from Fe-bearing chlorite forms nanometer scale hematite, goethite and magnetite crystals coating on clay minerals, especially on smectite minerals. We suggest a dominant climatic control of chlorite distribution in loess sections, and weathering of chlorite during the interglacial times results in an increase of pedogenetic iron oxide minerals (especially nanometer scale and nanoporous magnetite) and thus enhanced magnetic susceptibility signal in paleosol. The I/C ratio can be used as a new proxy indicator to reveal a long climatic history from Chinese loess-paleosol sequences back to at least 8 Ma ago.