Behaviors of REEs during pedogenetic processes in the karst areas of Southwest China

Abstract

To understand the behavior of rare earth elements (REEs) during pedogenetic processes in karst area, SW China, we investigated the leaching characteristics in order to better understand the mobility and fractionation of REEs in two soil profiles developed on dolomite. The two soil profiles, namely, Qingzhen (QZ) and Pingba (PB), were sampled from Guizhou province. Hydrochloric acid leaching method was used, and the soil samples from the profiles were compared. REE contents decreased from the bottom of the profiles (1360.39 ppm of QZ and 2236.61 ppm of PB) to the topsoil (247.32 ppm of QZ and 168 ppm of PB). From the soil samples to leachates, (La/Yb)N (N means chondrite-normalized values) increased from 3–8.92 to 2.36–25.02 in QZ and 6.34–9.06 to 5.28–14.83 in PB, and (Gd/Yb)N increased from 1.01–1.80 to 1.40–2.71 in QZ and 1.16–2.03 to 2.07–3.29 in PB. These results indicate that the degree of REE fractionation increases with continuous weathering. Obvious differences in LREE/HREE and ΣREE/Th ratios between the soil samples and leachates indicated strong weathering and pedogenesis under a simulated acid leaching environment. Negative and positive cerium (Ce) anomalies were observed in the lower and upper parts, respectively, of the profiles. Ce anomalies in the soil samples and leachates may be due to the sensitivity of Ce to redox conditions. Given that the decrease of Y/Ho ratio and the negative correlation with soil pH, incipient pedogenetic processes likely play an important role in Y-Ho fractionation. On the basis of the characteristics of REE behavior, the studied profiles have the characteristics of in situ weathering.