Distribution of rare earth elements and neodymium isotopes in settling particulate material of the tropical Atlantic Ocean (EUMELI site)
Résumé
We analysed rare earth element (REE) concentrations and Nd isotopic ratios of large sinking particles to investigate chemical scavenging processes. The sinking particles were collected with time-series sediment traps at 2500 m at two sites in the tropical northeastern Atlantic: a mesotrophic site (M-site: 18 degrees N, 21 degrees W) and an oligotrophic site (O-site: 21 degrees N, 31 degrees W). Shale-normalized REE patterns of the trapped material are characterized by a light REE (LREE) to heavy REE (HREE) enrichment and a positive Ce anomaly. The Nd isotopic ratios of the trapped material show intermediate values between Saharan dust and local seawater. Information brought by the REE patterns and Nd isotopic ratios suggests that the LREE enrichment of the trapped material is due to preferential adsorption of LREE onto the particulate matter. Estimated residence times of dissolved Nd and Ce at the O-site are 1000 yr. and 300 yr., respectively. While the M-site trapped material shows REE/Al and LREE/HREE ratios similar to the Saharan dust, the O-site trapped material is 10 to 20% enriched in the LREE compared to the dust. The larger dust flux at the M-site dilutes the authigenic REE contribution. A binary mixing model, using the Nd isotopic ratios, predicts that the M-site trapped material, as well as the O-site material, imprints the seawater derived Nd signal. The Nd isotopic ratios reveal high REE reactivity that we cannot estimate with REE concentration only. These results suggest that the combination of REE pattern and Nd isotopes is powerful for investigating processes in the water column. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.