New perspectives on lateritic weathering process over karst area – Geochemistry and Si-Li isotopic evidence

Abstract

Silicon and lithium isotopes are increasingly used to trace current and past weathering processes. Herein, four soil profiles (SL1, SL2, XNC, and DP) in the karst critical zone in southwest China were studied. The bedrock of the SL1, SL2, and XNC profiles are limestone, while the bedrock of the DP profile is shale. By analyzing their major elements, trace elements, and mineral compositions as well as using Si and Li isotope methods, we were able to identify the material source of the laterite in southwest China and study its element behavior and isotope fractionation characteristics during the weathering process. The results show that the δ30Si of the profiles overlying limestone (SL1, SL2, and XNC) range from −0.5‰ to 0.7‰, and those of the δ7Li range from 0.28‰ to 3.54‰, while the profile overlying shale (DP) has a large range of δ30Si variation (from −1.2‰ to 1.1‰). The results reveal a clear inheritance from the underlying bedrock, indicating that the profiles reflect in-situ weathering. Although no obvious relationship exists among the chemical index of alteration (CIA), the δ30Si, and the δ7Li, a significant correlation between δ30Si and δ7Li indicates that the isotope fractionation of the studied profiles is mainly affected by the dissolution and formation of minerals during the weathering process, and the biological effect is relatively small. Further, a diagram of Li/Al versus δ7Li shows a decrease in both Li/Al and δ7Li during weathering.