The continent-to-ocean transfer of rare earth elements in a mediterranean setting: natural processes and anthropogenic emissions

Despite the ecological and geochemical importance of coastal and estuarine ecosystems, the continental inputs and anthropogenic emissions of trace elements in their global marine budgets are not well constrained due to a lack of comprehensive and inclusive assessment of diverse sources. Here, we investigated two small but representative rivers (Las and Eygoutier) of the Mediterranean Sea to determine the contributions of rare earth elements (REEs) from terrestrial loadings, atmospheric depositions, and anthropogenic emissions within the watersheds and Toulon Bay (France). Both the dissolved and particulate loadings of the rivers significantly increased during intermittent flood conditions relative to base flow. The flow-weighted mean concentrations of dissolved Nd (as a representative REE) ranged from 29 ± 6 and 41 ± 16 ng L^-1 for the two rivers repectively, while the time-weighted mean particulate concentrations (TWMC) were 8.0 ± 3.4 and 18.8 ± 6.3 mg kg^-1. Similarly, TWMCs of atmospheric depositions were 13.3 ± 1.8 and 23.7 ± 4.0 mg kg^-1 for dry and wet conditions. Atmospheric depositions and fluvial particulate loadings are the primary input and output within the watershed, while river dissolved fluxes, porewater diffusion, and atmospheric depositions are the primary external sources of Nd to the water column. Furthermore, we observed significant La anomalies in the dissolved and atmospheric fractions while the discharge of treated wastewater is a significant REE input to the bay, marked by Tb anomalies. Overall, these results suggest considering small but typical rivers, rather than focusing solely on major fluvial systems, to gain a more comprehensive understanding of the transport and fate of REEs at the continent-to-ocean interface.