Fractionation of rare earth elements in soil profiles along an elevation gradient in central Taiwan

Abstract

The distribution of rare earth elements (REEs) in soils is affected by climate, which is one of the main factors controlling soil formation. However, variations in the geochemical behavior of soil REEs with pedoclimatic changes have rarely been investigated. This study determined the mobilization/retention and fractionation of REEs in nine soil profiles derived from sandy shale along an elevation gradient from 49 m to 2394 m above sea level in the Central Mountain Range of Taiwan. The experimental results indicated a gradually decreased ΣREEs trend with increasing elevation/rainfall. Clay content significantly (p < 0.05) correlated with REEs, which accumulated in argillic horizons. Moreover, a decreasing GdN/YbN ratio with increasing secondary minerals (i.e., clay and pedogenic Fe and Al oxides) implied their preferential sequestration tendency for heavy REEs over light REEs. This preferential association coupled with intensifying illuviation along the climatic gradient, resulted in increasing LaN/SmN and decreasing GdN/YbN ratios with increasing weathering degree and elevation (p < 0.01). Our results demonstrate that in the studied humid tropical region, increasing moisture availability caused by higher rainfall and lower air temperatures with rising elevation has led to stronger illuviation and more advanced soil development, which is reflected in REE fractionation and can be quantified by the ratios GdN/YbN and LaN/SmN.