The rare earth elements in rivers, estuaries, and coastal seas and their significance to the composition of ocean waters

The concentrations of the rare earth elements (REEs) in samples from 15 rivers, from 6 estuarine transects, and of 5 coastal seawaters are reported and have been used with literature data to examine the continuity in average REE pattern between average continental crust and the dissolved input of REE to the oceans via estuaries. Concentrations in river waters span a wide range (e.g., 120–3300 pmol kg −1 Nd). REE patterns, normalised to average shale, are of two main types: all show heavy REE enrichment between La and Gd but either heavy REE enrichment or depletion between Gd and Lu. Some patterns show negative cerium anomalies, and these can be related to pH in a small suite of carbonate rivers. Different patterns were found in rivers sampled at different times, and rivers draining different rock types had the same patterns, suggesting that the catchment rocks play a minor role in defining the REE chemistry of river waters. The results may be explained by the presence of two main pools for the REEs in rivers: a colloidal pool of REE-rich particles < 0.4 μm having a shale-like pattern, and a dissolved pool characterised by heavy REE enrichment. Significant REE removal during the mixing of river water and sea water was measured for the Connecticut, Delaware, Mullica, and, on two occasions, Tamar estuaries. This was confirmed by mixing experiments using radiotracers. Removal varied from about 30% to near-quantitative. In some cases, preferential removal of the light REE was observed, but this may be caused by coagulation of colloidally associated REEs, changing the proportions of the two REE pools. The REE concentrations and patterns of the coastal seawaters are intermediate between those for rivers and for ocean waters, reflecting the influence of continental drainage. The dissolved input of REEs to the oceans is characterised by REE patterns in which the evolution of the characteristic oceanic REE pattern has started to develop. The same fundamental processes that define the oceanic pattern appear to operate for continental waters also, but the REE patterns are not as “evolved,” i.e., fractionated relative to continental crust, because of lower pH and shorter water residence times.