Fractionation of rare earth elements in tropical soils from marine mudstone along a toposequence

Marine mudstones are major reservoirs for rare earth elements (REEs), and their topographical positions influence the distribution of soil REEs. This study investigated the influence of pedogenic factors on the concentration, spatial distribution, and fractionation of REEs in the soils from marine mudstone along a toposequence in Taiwan. Soil samples from four pedons were collected along the toposequence and were further analyzed for general properties and elemental composition. The results showed the highest REE concentrations (ranging from 176 to 221 mg kg^-1) in the foot slope, while the summit had the lowest content (ranging from 95.7 to 156 mg kg^-1), revealing the influence of the landscape on the distribution of REEs. Increasing ∑LREEs/∑HREEs ratios (ranged from 1.5 to 3.9), δCe (ranged from 0.3 to 1.0), and δEu (ranged from 1.1 to 1.4) values were observed relative to the decreasing elevation, which was dictated by the solubility and mobility of the elements during the transportation processes, provoking the fractionation of REEs. δCe positively and significantly (p < 0.001) correlated with dithionite-citrate-carbonate extractable iron, indicating the close interactions of iron oxides and the redox chemistry of Ce. The clustering of the samples in the principal components analysis was not observed in the distinguished prevalence of total REEs, LREEs, HREEs, and REEs fractionations, demonstrating the indirect control of the general properties to the behavior of REEs in the soils derived from marine sediments.