Behavior of rare earth elements in seawater at the ocean margin: a study along the slopes of the Sagami and Nankai troughs near Japan

Abstract

Using a flow injection ICP-MS method, yttrium and all the lanthanides in seawater were determined in the coastal/offshore mixing regime near Japan. Marked enrichments in the trivalent rare earth elements (REEs) in the surface waters are ascribed to the influence of coastal waters from Tokyo Bay in March and perhaps somewhere else in September and October to this region. At less than half of the stations where the detailed vertical profiles were obtained throughout the water column, we found marked excess REE(III) concentrations near the bottom. These bottom concentration anomalies are probably caused by resuspension of underlying sediments. Excluding those surface and bottom anomalous concentrations, the distribution of REE(III)s in the midwater column are very similar to those of the open ocean and are highly correlated with each other (R 2 > 0.95). This suggests that the boundary effects on the chemical fractionation of REE(III)s are small. We find no effect of the boundary scavenging known for 210Pb and 231Pa on the REE distribution, whereas there may be preferential release of light and middle REE(III)s over heavy REEs from slope sediments to the seawater much like 227Ac. From the shale-normalized REE patterns, we obtained up to 90% negative Ce and 10% positive Gd anomalies in the water column.