Anomalies in rare earth distributions in seawater: Gd and Tb

Abstract

We have measured profiles of the rare earth elements (REE) in Atlantic and Pacific Ocean waters. The data, normalized versus shales, exhibit a pronounced anomaly in Gd relative to its neighbors Eu and Tb in the REE series such that the Gd concentrations are high by 30–50%. Closer inspection reveals that the anomaly is made up of both elevated Gd and depressed Tb concentrations, likely associated with solution chemistry shifts in the transition from an exactly half filled 4f electron shell. Anomalies in Gd and Tb solution complexation are also indicated by the Turner-Whitfield-Dickson speciation model. The overall trend of heavy REE(III) enrichment in seawater and the Gd Tb anomaly described here tend to support scavenging as an important removal mechanism for the REE from seawater.