Concentrations of rare earth elements in sediments and fossils, in particular cerium that, as an exception, adopts both the +III and +IV oxidation states depending on redox conditions, lie at the heart of many studies aiming at assessing past environments chemistry. New analytical developments, carried out in the frame of long-term collaboration between IPANEMA, the French National Museum of Natural History and the DIFFABS and LUCIA beamlines of SOLEIL, allowed a direct micrometric mapping of the oxidation state of cerium in well-preserved 95-million-year-old fossils.

The new approach set up by IPANEMA is based on the collection of X-ray absorption spectra and on the production of speciation maps through the collection of successive micro-X-ray fluorescence maps at excitation energies across the Ce L3-edge. It reveals a remarkably homogeneous oxidation state of cerium, partly under the +IV form, within and between different fossil fishes and shrimps from the locality. The presence of cerium under the +IV form suggests slightly oxidative local conditions of burial, and agrees well with the indirect data obtained from the usual point concentrations.

This study provides a complementary approach to better describe and constrain palaeo-environmental indicators up to now essentially studied using invasive and destructive laboratory techniques without imaging capacities, and shows a high potential in other rare earths fields such as the mining industry, metallurgy, or medicine.