Lithium distribution and isotopic fractionation during chemical weathering and soil formation in a loess profile

Lithium (Li) is a fluid-mobile element and delta Li-7 in secondary deposits represents an excellent proxy for silicate weathering and authigenic mineral formation. The soil samples from 1205 to 1295 cm in the Weinan profile, one of the best developed loess-paleosol sequences covering the last glacial-interglacial climatic cycle, were collected and chemically separated into detritus and carbonate fractions for subsequent analyses of Li, delta Li-7, major and trace elements. Other desert specimens (i.e., Qaidam Desert, Tengger Desert, Badain Juran Desert and Taklimakan Desert) near the Chinese Loess Plateau (CLP) and various standard clays were analyzed for assisting provenance determination. The Li and delta Li-7 distributions in the detritus are rather homogeneous, 1.4-2.0 mu g/g and +2.5 parts per thousand to +4.7 parts per thousand, respectively, compared with the carbonate fraction. The detrital delta Li-7 varies systematically with magnetic susceptibility and grain size changes, reflecting significant Li isotopic variation associated with sources and mineralogy of detrital material. On the other hand, Li and delta(7) Li in carbonates show large changes, 781-963 ng/g and -4.1 parts per thousand to +10.2 parts per thousand, respectively. These carbonate Ski correlated well with the estimated index of chemical weathering, as a result of Li mobilization and soil formation during chemical weathering.