A review on the potentiality of Rare Earth Elements to trace pedogenetic processes

This review aims at considering the use of REEs to trace pedogenetic processes, in order to quantify the respective contribution of the successive processes on pedogenesis. We first consider the origins and contents of REEs in soils, to assess their natural occurrence, and tackle the common data treatment used to interpret their concentrations. We review subsequently the impact of the different pedogenetic processes on the fate of REEs in soils: from their release into a soil solution by primary mineral (heavy minerals, phosphates, silicates and carbonates) dissolution during weathering, to their fixation on- or into main REE-bearing secondary minerals (clay minerals, Fe–Mn-oxides and carbonates), then their mobilization by plant recycling and organic matter, and finally the impact of argilluviation and redox processes on their fractionation. We conclude that REEs, much more than other trace elements, can interestingly trace the considered processes that are encountered in a large diversity of non-polluted soils arising from diverse parent materials. Weathering generally fractionates REEs and accounts for a significant part of the variation in the initial REE stock of the soils. Plant recycling induces a weak mobilization of REEs and their accumulation in topsoil. Argilluviation can be traced using the MREE-enrichment and the Eu-anomaly of the clay minerals. Redox processes can be traced by the Ce-anomaly caused by the precipitation of cerium on Mn-oxides. To use REEs as tracers of pedogenetic processes, we recommend to: (i) precisely characterize the REE-bearing minerals, (ii) adapt the choice of reference(s) for normalization and (iii) perform mass balance calculations.