The Marine Geochemistry of the Rare Earth Elements

Novel methods were developed for the determination of 12 of the 14 Rare Earth Elements (REE) in seawater. Initial extractions of the REE by chelating ion exchange chromatography is followed by cation exchange for removal of co-extracted U and remaining traces of major ions. Finally traces of U are removed by anion exchange before irradiation for 8 hours at a flux of 5 x 1013 neutrons.cm-2 .sec~-. After post-irradiation separation of 24Na, the gamma spectra are recorded over four different time intervals with a Ge(Li) detector. An internal standard (144Ce) is carried all along the procedure for improved precision by avoidance of counting geometry errors. Vertical profiles are reported for three stations in respectively the Northwest Atlantic Ocean, the Eastern Equatorial Pacific Ocean and the Cariaco Trench, an anoxic basin. This data set represents the first detailed profiles of Pr, Tb, Ho, Tm and Lu in seawater, together with profiles of La, Ce, Nd, Sm, Eu, Gd and Yb. The first observations of positive Ce anomalies in seawater are ascribed to regeneration of Ce under reducing conditions. The first reported positive Gd anomalies are ascribed to the unique chemical properties of the Gd(III)-cation, which has an exactly half-filled 4f electron shell. Concentrations of the REE range from 0.3 pmol.kg~I (Lu) to 86 pmol.kg~I (Ce) and are among the lowest reported so far for trace elements in seawater. The REE as a group typically exhibit a quasi-linear increase with depth. In the deep water there appears to be some degree of correlation with silicate. Concentration levels in the deep Pacific Ocean are 2-4 times those in deep Atlantic waters. Ce has an opposite behaviour, with very strong depletions in deep Pacific waters. In the Cariaco Trench all REE, but especially Ce, are strongly affected by the chemical changes across the oxic/anoxic interface.