Pedochemical behaviors of rare earth elements in soil profiles along a lithosequence in eastern Taiwan

Abstract

The composition and concentration of rare earth elements (REEs) in soil are primarily dependent on the lithology of parent materials. The different association preferences among individual REE and soil solid phases also cause REEs fractionation during pedogenetic processes. However, studies on the interaction between lithology and pedogenesis, affecting the geochemical behaviors of REEs, are lacking. Thus, this study elucidated the influence of parent materials on REEs concentration and distribution during pedogenesis in four pedons along a lithosequence covering schist, andesite, shale, and mafic rocks identified in eastern Taiwan. The highest ΣREEs was observed in the schist pedon (253–319 mg kg−1), followed by the andesite pedon (239–277 mg kg−1), mafic pedon (243–254 mg kg−1), and shale pedon (195–222 mg kg−1). Moreover, REE patterns and ratios of light REEs (LREEs) to heavy REEs (HREEs) indicated that soils derived from felsic rocks (schist and shale) tended to be rich in LREEs; conversely, soils derived from mafic rocks were rich in HREEs. HREEs decreased with an increase of Si, but values of REEs fractionation proxies (LaN/YbN and LaN/SmN) increased with increasing Si. The mass transfer coefficients (τ) of REEs, LREEs, and HREEs exhibited a significant correlation with LaN/SmN. Compared with LREEs, HREEs exhibited a greater depletion and contribution in REEs fractionation in the studied pedons, as evaluated based on τHREEs, w and its correlation with LaN/SmN and GdN/YbN. The relatively high mobility of HREEs, coupled with the clear illuviation, resulted in decreased values of LaN/SmN and GdN/YbN along with the increasing degree of soil weathering. This study elucidated the shift in the factors controlling the REEs fractionation from the lithologic characteristics of parent materials to the pedogenesis, along with the progressed soil development. Additionally, the increasing degree of soil weathering can be quantified using the ratios of LaN/SmN and GdN/YbN.