The marine geochemistry of the rare earth elements

Abstract

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 10 13 neutrons.cm~2.sec-l. 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-l (Lu) to 86 pmol.kg-l (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. Distributions of the dissolved REE in ocean waters seem to be dominated by their internal cycling within the ocean basins. With a few notable exceptions, the ultimate external sources (riverine, aeolian, hydrothermal) and sinks (authigenic minerals) appear to have little impact on the spatial distribution of the REE in oceanic water masses. Analogies with distributions of other properties within the oceans suggest that the REE as a group are controlled by two simultaneous processes: A) cycling like or identical to opal and calcium-carbonate, with circumstantial evidence in suport of the latter as a possible carrier. B) adsorptive scavenging, possibly by manganese-oxide phases on settling particles. The latter mechanism is strongly supported by the parallels between REE(III) speciation in seawater and the 'typical 1 seawater REE pattern. This general correspondence is highlighted by the very distinct excursions of Gd in both Gd(III) speciation and the observed seawater REE patterns.Combination of both apparent mechanisms, for instance scavenging of REE by adsorptive coatings (Mn oxides) on settling skeletal material, is very well conceivable. Upon dissolution of t...

Dissolved rare earth elements in the Black Sea

“…The REE were first extracted and separated from the major alkaline (Na, K) and earth alkaline (Ca, Mg, Sr, Ba) metals in seawater with a Chelex-lOO column (DE BAAR, 1983). Then a second AG50W-X8 cation exchange column was used to remove final traces of these major elements.…”

“…Early studies of artificial radionuclides ofthe REE (mostly 141Ce, 144Ce and 147Pm) from bomb fallout have not dealt with reducing marine environments (review in DE BAAR, 1983). The 144Ce released from nuclear fuel reprocessing plants is apparently strongly retained by the finest size fraction of marine sediments (HETHERINGTON and JEFFERIES, 1974;GUEGUENIAT et al, 1979).…”

Hydrography and local sources of dissolved trace metals Mn, Ni, Cu, and Cd in the northeast Atlantic Ocean

“…Trace metal samples of JGOFS-2 were preconcentrated in the shore-lab using Chelex-100 ion-exchange chromatography (Kingston et al, 1979;De (4 2o 16…”

On cerium anomalies in the Sargasso Sea

“…Despite different geographical locations and some nine years sampling interval all rare earths appear to agree nicely with our original dataset, except for Ce being considerably lower, exhibiting the negative Ce anomaly which now had been observed all around the world. Attention was drawn to one of our unpublished 198 1 trial runs of six replicate surface water samples, with rather poor reproducibility since no internal ""'Ce tracer had been used then (DE BAAR, 1983). Otherwise those six samples by and large had the same rare earth concentrations as published in 1983 (except Ce!)…”

“…Other than the now apparently very peculiar positive Ce anomaly, we still have no reason to doubt the Ce data, unless some unnoticed systematic interference in the INAA routines has betrayed us. Here the overlap of short-lived (4.2 d) '75Yb with longerlived (32.6 d) 14'Ce comes to mind, but this pitfall had been well recognized and avoided by estimating the Ce (i.e., 14'Ce) many weeks post-irradiation in order to allow '75Yb to decay away completely (DE BAAR, 1983, 1984. Nevertheless, the Ce values in the first set of nine samples of the upper 1000 m now appear to be very high.…”