Balancing Rare Earth Element distributions in the Northwestern Mediterranean Sea

Abstract

Elemental concentrations of Rare Earth Elements (REE) and isotopic compositions of Neodymium (εNd) have been measured in three water column profiles in the North Western Mediterranean Sea. Clear enrichments of REE are observed when comparing to adjacent Atlantic waters suggesting REE inputs along the circulation in this area. For the first time, relative proportions of external sources including submarine groundwater discharges (SGD) have been quantified for the studied area. Atmospheric deposition is estimated to be the most important external source for all the REE with an average contribution of 44%, followed by diffusion from porewaters, which provide a 30%. Dissolved riverine fluxes account for 11%, SGD for 10% and dissolution of remobilized surface sediments the remaining 6%. Mass balances accounting for seawater transport and identified external sources have been delineated for the three main water masses (Modified Atlantic Waters, Levantine Intermediate Waters and Western Mediterranean Deep Waters). They show that the balances of REE in this area are dominated by seawater mass mixing. Superimposed on this hydrography, REE vertical profiles are affected by external sources and biogeochemical cycling. Dissolved REE are correctly balanced in deep waters whereas substantial missing fluxes are identified in the surface and intermediate water masses. Additional net LREE outputs and HREE inputs are required in the surface waters while net output fluxes for all the REE are missing at intermediate waters. The most likely process suggested here is an active reversible scavenging, consistent with a stronger adsorption of LREE compared to HREE. In the particular case of the redox-sensitive cerium, the most plausible mechanism to explain the net output missing fluxes is Ce+3 removal by particle scavenging via oxidation to insoluble Ce+4. Estimated Ce oxidation rates of 0.33% d−1 in surface waters agree well with previously published values. Exchange fluxes derived from the isotopic Nd mass balance indicate higher Nd scavenging in surface compared to intermediate waters.