Rare earth elements in the NW Indian Ocean

Abstract

Dissolved rare earth element (REE) distributions have been determined at three stations in the high productivity NW Indian Ocean (NWIO) upwelling zone. Dissolved REE distributions within the well-oxygenated surface waters and deep portions of the water column are similar to those reported previously from other open-ocean locations. At depth, dissolved REE concentrations are intermediate between those of the Atlantic and Pacific Oceans, consistent with the continuous regeneration and enrichment of the strictly trivalent REEs in and the progressive oxidative removal of Ce from, the deep ocean. Within the mid-depth oxygen minimum zone dissolved REE concentrations exhibit a notable enrichment superimposed upon these “typical” oceanic profiles. Further, dissolved Ce concentrations are preferentially enriched within the suboxic horizon compared to the rest of the series, indicating the reduction of insoluble Ce(IV) to dissolved Ce(III) at these depths. By analogy with our previous studies of REE distributions in anoxic basins we propose that the more modest enrichments of the strictly trivalent REEs observed in the NWIO suboxic zone must derive from uptake through scavenging within the well-oxygenated portions of the water column and regeneration from sediments and settling particles within the more reducing conditions of the oxygen minimum zone.