Rare earth elements in French stream waters - Revisiting the geochemical continental cycle using FOREGS dataset

The geochemical behavior of rare earth elements (REEs) has been investigated mainly in geological systems where these elements represent the best proxies of processes involving the occurrence of an interface between different media. This behavior is assessed according to REE concentrations recorded along the REE series normalized with respect to upper continental crust. In this study based on a field approach, the geochemical behavior of REE was investigated in French stream waters. This study is based on FOREGS (Forum of European Geological Surveys) Geochemical dataset that consists on a sampling at regular mesh on all Europe. In France, 119 stream water samples were extracted in drainage basins <100km2. The aim of the study is that of describing the spatial variation of REEs and finding the hydro-topo-geochemical factors that affect their distribution by means of a Multivariate Factorial Kriging. On the basis of their atomic number and of the results of a preliminary Principal Component Analysis three REEs have been selected (La, Eu and Lu) and five physicochemical properties (pH, organic carbon, carbonates, Fe, Mn). A cokriging has been applied that shows a similar spatial organization of REEs: higher values are especially observed in the Aquitaine basin. In order to investigate more deeply on the different sources of variation acting in the study area, a factorial cokriging is applied. The first 2 regionalized factors have been estimated to give a synthetic description of the studied process at the different selected spatial scales. At higher spatial scales (250. km) environmental parameters like Fe, carbonates, and pH, supposed to be ascribed to the rock's nature or to other geological larger scale processes (i.e., hydrographic network and topography), have shown to affect REE distribution. At short range, only Eu and Mn weigh more, which are ascribed to the process of liberation of Mn oxides in rivers that also release the REEs sorbed onto these oxides.