Geochemical characteristics and behaviour of elements during weathering and pedogenesis over karst area in Yunnan–Guizhou Plateau, southwestern China

Abstract

A study of two typical red soil profiles, Gumu (GM-limestone) and Sanjiacun (SJC-dolomite) was conducted at Yunnan–Guizhou Plateau, China. The study employed mineralogy, geochemical and elemental ratios to assess the mobilisation and distribution of elements during weathering and pedogenesis. The mineralogy results revealed similarity in distribution of secondary minerals in both profiles, with exception at the ferricrete layer. The results revealed inheritance relationship between red soils and their underlying bedrocks indicating that clay minerals were likely derived from in situ weathering products or metasomatic process rather than detrital process. During weathering and pedogenesis process, alkaline elements such as K and Rb behave similarly and were closely related. All lanthanide elements except for Ce shared similar transport trend with Y and Be; however, Be exhibited significant enrichment at ferricrete layer of the dolomite profile. Transitional elements such as Co and Mn behave similarly; while, the High Field Strength Elements (HFSE) were stable, except for Th and Sc which indicate depletion in upper soil layer of the dolomite profile. Our findings further revealed that both profiles have experienced intense chemical weathering with high Chemical Index of Alteration values (CIA: 83–93) and a constant Ba/Sr ratio > 2. Overall, both profiles experienced similar elemental behaviour with the accumulation of major elements, the leaching of alkaline earth elements and the fluctuation of trace elements including Rare earth elements (REEs). The elemental behaviour was primarily controlled by redox reaction, pH, water availability, ionic radii and the stability of secondary oxides/hydroxides. Our study further highlights the use of newly proposed geochemical proxy Fe/Na ratio, which revealed high correlation (R2 ≈ 0.90) with U, Ba, Zn, Cu, Mo, Ni, Sb and REEs of power functions. These findings indicate that Fe/Na ratio may be used to investigate the behaviour of trace elements including REEs in highly weathered soils.