Distribution of rare earth elements and neodymium isotopes in suspended particles of the tropical Atlantic Ocean (EUMELI site)

We analyzed the REE, Mn and Al concentrations and Nd isotopic ratios in mar ine suspensions collected on filters (0.65 mu m porosity) with in situ pump ing systems in the tropical northeastern Atlantic (20 degrees N, 18-31 degr ees W). Previously we reported the same parameters on large sinking particl es collected with moored sediment traps at the sites. Shale-normalized REE patterns of the filtered suspensions are characterized by a larger light RE E (LREE) to heavy REE (HREE) enrichment compared to the trapped material an d a Ce anomaly that evolves positively with depth. Depth profiles of REE/Al show maximum values at 50-100 m, where the Mn/Al ratio also reaches a maxi mum. The profile of the Nd isotopic ratios of the filtered suspensions show s variations similar to those of the seawater. These results suggest that t he filtered suspensions preferentially scavenge the LREE, especially Ce, an d that the particulate Mn oxides are potential REE carriers. The relationsh ip between the Ce anomaly and the Ce/Al ratio demonstrates that the particu late Ce anomaly is formed by (1) the LREE adsorption onto the particulate M n oxides in the surface water, (2) Ce(III) oxidation to insoluble Ce(IV)O-2 and (3) preferential desorption of strict trivalent REE from the Mn oxides in deep water. Estimated authigenic Nd contents, using Nd isotopic ratios, decrease with depth. This is consistent with the adsorption of the REE in surface water and their desorption in deep water, suggested by the Ce anoma ly formation. All the results show that the suspended particles record more clearly the authigenic REE contribution than the trapped material does. Th e suspended matter plays a key role in the scavenging of particle-reactive elements. (C) 1999 Elsevier Science Ltd. All rights reserved.