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

Abstract

A comparison of the Amazon, Fly/Gulf of Papua and Sepik River systems has shown that there are two distinct processes operating on dissolved rare earth elements (REE) in estuaries, large scale salt-induced coagulation in the low salinity region and small to extensive release in the mid to high salinity region. Fractionation of the REE occurs during both removal and release. The order of removal follows LREE≥MREE>HREE and the order of release follows HREE>MREE≥LREE where L, M and H refer to light, middle and heavy REE, respectively. Hence, estuarine reactions can modify the relative abundance of dissolved REE reaching the oceans. Operating in tandem, the removal and release lead to the preferential 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 is evolved toward the REE composition of seawater, which has a heavy REE-enriched 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 and/or suspended sediment source. The lack of REE increases in the Sepik River estuary strengthens this explanation as the mixing zone of this estuary is located over a deep shelf where there is little physical contact between bottom sediment and estuarine waters. In marked contrast, the Amazon and Fly estuaries are dominated by the active resuspension and deposition of bottom sediments. The resupply of REE, accompanied by fractionation, may result from REE–carbonate complexation during the reaction of seawater with suspended particles and/or bottom sediment. Estuarine and shelf sediments may be important sources of dissolved Nd to the oceans, thereby helping to maintain the inter-ocean differences in the Nd isotopic composition of seawater by reducing the residence time of Nd in the oceans.