Unraveling controls on rare earth elements in global rivers: a data compilation and modelling approach

Dissolved rare earth elements (REEs) in river waters are potential tracers for various physical and chemical processes within river corridors, because their concentrations and relative distributions (i.e., REE patterns) are sensitive to changes in solution chemistry and physicochemical interactions with particulate material in the water. The utility of REEs in the marine realm has been demonstrated by numerous (paleo-)oceanographic studies, but controls on REE distributions in rivers remain ambiguous. Multiple factors have been proposed to influence REE patterns and concentrations based on local rivers, including pH, water chemical compositions, and drainage basin lithology. However, increasing REE data warrants a broader perspective for regional or global comparisons, promoting a coherent interpretation of REE variations in rivers and marine REE cycles. To better investigate the behavior and distribution of river REEs, we compiled a comprehensive database of dissolved (<0.45 m) and truly dissolved (1-300 kDa) REE concentrations in rivers. This database comprises 1620 data from more than 300 rivers and tributaries in 27 countries. We also include information about river locations, major and minor elements, pH, DOC concentrations, and discharges. Based on this database, riverine REEs display obvious fractionation relative to the upper continental crust. Compared with seawater, REE patterns in rivers are more enriched in middle REEs with lower heavy to light REE ratios. For the same river, different filter sizes could influence the measured REEs concentrations but hardly change the overall REEs patterns. Negative Ce anomalies (Ce/Ce*<1) are prevalent, except for rivers with high DOC content, low pH, or weathered nanoparticles. Rivers contaminated by medical wastes are characterized by extremely positive Gd anomalies (Gd/Gd*>10). We also improve estimation of the global riverine REE flux to the ocean considering REE concentrations and river discharges. Since the river REE data exhibit large variability, more measurements in space and time are still needed to accurately constrain the riverine REE input into the ocean.