Uranium paleo-speciation in lake sediments as analogues for long-term near-surface repository sites

This proposal aims to directly determine, for the first time, the evolution of uranium (U) speciation during the last 8700 yrs in naturally U-rich (up to 815 mg/kg) lake sediments in the French Alps. Preliminary TEM-EDX, XAS data and selective chemical extractions suggest that a fraction of non-crystalline U could have evolved towards crystalline phases at depth in the sediments. Demonstrating such a diagenetic evolution will require to identify and quantitatively determine U species as a function of depth in sediment cores that have been already sampled and carefully preserved from oxidation. This goal can only be achieved by using EXAFS spectroscopy at the U LIII-edge analyzed using linear combination of reference compound spectra. Measuring the paleo-speciation of U will yield unique information on the long-term fate of U(IV) in anoxic conditions representative of near/sub-surface contaminated media and repository sites.