The estuarine chemistry of rare earth elements: comparison of the Amazon, Fly, Sepik and the Gulf of Papua systems

A comparison of the Amazon, Fly/Gulf of Papua and Sepik River systems has s hown that there are two distinct processes operating on dissolved rare eart h elements (REE) in estuaries, large scale salt-induced coagulation in the low salinity region and small to extensive release in the mid to high salin ity region. Fractionation of the REE occurs during both removal and release . The order of removal follows LREE greater than or equal to MREE > HREE an d the order of release follows HREE > MREE greater than or equal to LREE wh ere L, M and H refer to light, middle and heavy REE, respectively. Hence, e stuarine reactions can modify the relative abundance of dissolved REE reach ing the oceans. Operating in tandem, the removal and release lead to the pr eferential removal of the LREE and the preferential release of HREE during estuarine mixing. This combination, in turn,results in an 'effective' river water flux (river flux after modifications by estuarine chemistry) which i s evolved toward the REE composition of seawater, which has a heavy REE-enr iched composition when normalized against the REE composition of rivers or the Earth's continental crust. The increase of dissolved REE in the mid to high salinity waters of the Amazon and Fly estuaries suggests a sediment an d/or suspended sediment source. The lack of REE increases in the Sepik Rive r estuary strengthens this explanation as the mixing zone of this estuary i s located over a deep shelf where there is little physical contact between bottom sediment and estuarine waters. In marked contrast, the Amazon and Fl y estuaries are dominated by the active resuspension and deposition of bott om sediments. The resupply of REE, accompanied by fractionation, may result from REE-carbonate complexation during the reaction of seawater with suspe nded particles and/or bottom sediment. Estuarine and shelf sediments may be important sources of dissolved Nd to the oceans, thereby helping to mainta in the inter-ocean differences in the Nd isotopic composition of seawater b y reducing the residence time of Nd in the oceans. (C) 2000 Elsevier Scienc e B.V. All rights reserved.