Surface waters, dissolved lanthanides

Fig. 1. (a) The concentration of dissolved lanthanides in the surface waters of the Sargasso Sea. A composite of data measured by TIMS (Sholkovitz and Schneider 1991) and INAA (De Baar et al. 1983). Note classic sawtooth abundance pattern. Pm does not exist in nature, (b) Shale-normalized pattern of the composite seawater shown in (a) using shale concentrations of table 1. Tb, being inconsistent, probably reflects an incorrect concentration of the seawater. 

The Amazon and Fly River (Papua New Guinea) estuaries serve as good examples of the five features outlined above. The salinity distribution of dissolved Nd (<0.22 pm) in fig. 12 is representative of the other bivalent lanthanides. This figure contains data for both surface and deep waters the bottom part B is an expanded scale for S >3 samples. The Fly and Amazon data sets have Nd-salinity distributions for surface waters which are remarkably similar and exhibit the same two major features (1) removal of dissolved lanthanides in the low (0-5) salinity region and (2) desorption in the seaward region. Hence, it appears that the removal process and the release process (desorption) are decoupled. 

The large-scale removal of dissolved lanthanides in the low salinity region is an ubiquitous feature of estuaries (Martin et al. 1976, Goldstein and Jacobsen 1988c, Sholkovitz and Elderfield 1988, Elderfield et al. 1990, Sholkovitz et al. 1992, Sholkovitz 1993, 1995). As illustrated for the Amazon and Fly (fig. 12), the Nd concentrations of surface waters decrease sharply between the river (S 0) and a salinity of 5-7. In the case of the Amazon, Nd removal represents approximately 95% of the river-borne dissolved flux. Of all the trace element distributions reported for estuaries, the lanthanides stand out as being the most similar to dissolved Fe and humic acids, both of which undergo extensive removal in estuaries. Since there is also a strong correlation between... 

The extensive fractionation of trivalent lanthanides between Sargasso Sea water and suspended particles can be qualitatively explained by a solution/surface competition model (fig. 15). The progressive decrease from La to Lu in the extent to which dissolved lanthanides are removed by suspended particles results from an increase in the solution complexation constants as predicted by chemical models (e.g., sect. 4 of this chapter Erel and Morgan 1991, Erel and Stolper 1993, Byrne and Li 1995). 

The relationship between dissolved lanthanides and nutrients has received considerable discussion in the literature. Unlike for many other trace metals, there appears to be no metabolic requirement for lanthanides by plankton. At pmol/kg levels such a requirement is unlikely. Hence, surface water depletion must reflect a general type of scavenging onto newly formed surfaces (plankton). The extremely low concentrations of lanthanides in the calcium carbonate or siliceous skeletal matter of plankton suggest that such... 

The oxyhydroxides of Mn provide important surface sites for the oxidation of dissolved Ce(IIl) to particulate Ce(I V) and the fractionation of trivalent lanthanides during sorption. Oxidative processes become important at water depths between... 

The study by Sholkovitz et al. (1994) emphasized the role of sorption to suspended particles in the marine chemistry of the lanthanides. They concluded that It is difficult to reconcile a geochemical coupling between Si and REEs. Our data point to the uptake and release of REEs from surface coatings ( soft parts) while Si is released from dissolved siliceous plankton ( hard parts) in deep waters. .. . 

---