Behavioral differences between weathering and pedogenesis in a subtropical humid granitic terrain: Implications for chemical weathering intensity evaluation

Continental weathering plays a crucial role in the evolution of the Earth’s surface by linking Earth's spheres, shaping landscapes and regulating chemical cycles and global climate. Regolith weathering studies in some cases ideally assume successive and progressive bottom-up physicochemical variation trends. To validate this assumption, we target a granitic regolith profile in a subtropical monsoon climate-dominated region (southeast China). We present mineralogical, petrographic and geochemical data from soil, saprolite and bedrock samples to characterize physical and chemical alterations. The petrographic and mineralogical results indicate that both plagioclase and K-feldspar are depleted and kaolinite is the major neoformed mineral phase in top-soil samples, revealing intensive chemical weathering on the regolith. However, these top-soil samples have much lower chemical index of alteration (CIA, ca. 64–67) values than the underlying oxidized-soil samples (CIA values ranging 65–80). Some chemically immobile elements, such as Al, Ti, Zr and Hf, are depleted in top-soil samples but are comparatively enriched in oxidized-soil samples. This is attributed to vertical leaching and translocation of fine-grained minerals (e.g., kaolinite, illite and zircon) through the soils. Furthermore, the analyzed top-soil samples are comparatively rich in elements K and P, which are most likely due to the evident biological fixation process. These physical and biological processes in the unconsolidated, porous and biotic top-soil layer can be responsible for the unexpected low CIA values. Our findings demonstrate the complex physical, chemical and biological alterations on soils and the differential behaviors between weathering and pedogenic processes, especially for those tropical–subtropical high rainfall regions. This case study also highlights the importance of petrographic and mineralogical proxies to chemical weathering intensity evaluation for regolith profiles and siliciclastic sediments.