Dissolved rare earth elements in the South China Sea: Geochemical characterization of the water masses

We have measured the vertical profiles of dissolved rare earth elements (REEs) and yttrium in the South China Sea together with conductivity-temperature-depth and hydrographic measurements to compare with those in the western North Pacific and the Sulu Sea. Although the South China Sea is rapidly flushed by the Pacific through the Luzon Strait with a sill depth of similar to 2500 m [Broecker et al., 1986], a unique REE pattern is developed within the sea. The most striking difference exists in the dissolved Ce profiles. Dissolved Ce generally decreases from high values (6-9 pmol/kg) at the surface to a minimum of similar to3 pmol/kg at around 300-500 m where the North Pacific Intermediate Water penetrates. In deepwaters of the North Pacific and the Sulu Sea it remains at a relatively low and nearly constant concentration level of similar to5 pmol/kg throughout the water column, whereas in the South China Sea, it gradually increases with depth to a maximum of 12.9 pmol/kg at similar to 2500 m, resembling the "nutrient-like" profiles of other strictly trivalent REEs, and then sharply drops to a constant value of similar to6 pmol/kg in the bottom water below 2900 m. Some lighter REEs like Pr, Nd, and Gd, though to a much lesser extent, also show similar concentration breaks at the sill depth, but the other hydrographic properties like dissolved oxygen, nutrients, pH, and alkalinity do not. Therefore dissolved REEs may best be utilized to characterize the water masses. Two major sources for dissolved REEs in the South China Sea are fluvial and coastal input to the surface ocean and a bottom release into the deep water during the passage over the-luzon Strait. Redox chemistry including reduction of Ce(IV) to Ce(III) in the pore water of hemipelagic sediments and subsequent release of dissolved Ce(III) to the overlying deep water may be involved in the latter. The middle REE-enriched patterns with a significant Gd depression relative to that of the North Pacific Deep Water are characteristic of the South China Sea and prevail throughout the water column by physical circulation. Since the bottom water of the basin is also fed by the North Pacific, the dissolved Ce(III) must be oxidized through bacterial mediation and removed from the bottom water presumably by scavenging near/at the sediment-water interface along the slopes of the basin.