Rare earth elements and zinc as tracers of pedogenetic processes : the case of soils developed from mineralized limestones

This project aims at considering the potential of zinc and rare earth element (REEs) at quantifying pedogenetic processes along a soil sequence developed from a limestone formation consisting in a succession of marl and limestone strata which underwent carbonate dissolution, redox cycles related to soil hydromorphy and eluviation. The chosen approach relies on (i) the speciation of rare earth elements - by a method combining sequential extractions and physical separations - in the pedological features resulting from the different processes and in the materials from which they developed; (ii) the quantification of the REE and major element fluxes by mass balance calculation. To do so, two methodologies were developed: a methodology of normalization of REEs based on the theory of transformation fronts to quantify the impact of the processes on REE fractionations; an innovative approach of the reconstruction of the former parent material for each soil horizon as mass balance calculation requires the quantification of initial stocks in elements. Two processes were then further studied: carbonate dissolution and redox processes. We thus point out the necessity to compare max fluxes as computed by mass balance to quantities released by carbonate dissolution in order to forecast the fate of elements released into the soil solution. We also evidenced the potential of REEs to quantify the dissolution/precipitation cycles of iron and manganese oxides.