YREE scavenging in seawater: A new look at an old model

Abstract

In the ocean, yttrium and the rare earth elements (YREEs) show nutrient-like vertical profiles. Since the YREEs have no manifest biological function, their removal from solution (scavenging) is probably caused by sorption on particles rather than active microbial uptake, yet the exact nature of these particles is uncertain. An existing theoretical model describes scavenging as an equilibrium between complexation with dissolved inorganic ligands and with functional groups on particle surfaces. This model was able to predict input-normalized (i.e., shale-normalized) YREE abundance patterns in seawater without requiring poorly known parameters like particle concentrations or the site densities of functional groups. Employing well-established stabilities of inorganic YREE complexes, while assuming that the sorbent particles are organic with functional groups represented by a mixture of simple monocarboxylic acids, it reproduced some key features of seawater YREE patterns, specifically the characteristic increase of shale-normalized abundance with atomic number and distinctive anomalies of certain trivalent REEs (La and Gd). The familiar negative Ce anomaly of seawater, however, is due to redox reactions that were not explicitly accounted for.In this study, we refined calculations of YREE solution speciation by adding complexation with desferrioxamine B to gauge the influence of strong organic ligands prevalent near the ocean surface. The scavenging model was then inverted by subtracting high-quality YREE abundance patterns, reported for the open ocean, from this solution speciation to yield an average pattern of relative YREE affinities for the sorbent particles. The resulting affinity pattern is compared with patterns of distribution coefficients, derived from laboratory experiments, for YREE sorption on relevant solid phases including hydrated Fe and Mn oxides (HFO/HMO), calcite, and the green macroalga Ulva lactuca as a substitute for marine organic matter. For deep seawater, the best agreement is observed with HMO, which may thus be the dominant carrier of YREEs to the sediment. Since Ce is catalytically oxidized on manganese oxide surfaces, this could have implications for the evolution of the Ce anomaly. Direct comparisons of distribution coefficient patterns with YREE analyses of suspended particles from the Atlantic Ocean also favor HMO, but this may be dictated in one case by the use of a selective leaching method. For shallow seawater, particularly in the Atlantic Ocean, the best agreement is observed with U. lactuca and calcite, although the latter is offset by the low YREE contents of this biogenic mineral. Matches with HFO are generally less convincing. The aforementioned mixture of simple monocarboxylic acids is not a good proxy for the sorptive properties of marine organic matter.

The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans

“…The thermodynamics of desorption become even less favorable as the increasing dissolved concentration at depth would tend to push the equilibrium toward further adsorption (reiterating arguments posed by Elderfield, 1988). Moreover, given free Nd is <16% of the total dissolved Nd (Millero, 1992;Schijf et al, 2015), and surface adsorption coefficients are typically not exceptionally high (in the range of 3 to 5 onto carboxyl and moncarboxylic groups; Smith and Martell, 1989;Byrne and Kim, 1990;Ngwenya et al, 2010), the potential for REE transfer from the shallow to deep ocean via particle surface scavenging should be limited (Elderfield and Greaves, 1982;Stichel et al, 2015).…”

“…Finally, removal of REEs from the ocean is thought to be predominantly via metal-oxides, although the role of organic matter, phosphates, or even carbonates, is likely to be important (Byrne and Kim, 1990,? ;Byrne and Sholkovitz, 1996;Schijf et al, 2015). This model is considered the standard of marine REEs.…”

“…Considering all the arguments and data, a preferred model is that REEs are most likely transferred from the ocean surface to depth through incorporation onto complex organics, such as humic acids, polysaccharides, or other complex organic molecules that have binding coefficients higher than simple carboxyl groups (logK > 9; Byrne and Kim, 1990;Stanley and Byrne, 1990;Davranche et al, 2005;Pourret et al, 2007;Schijf et al, 2015). In this way, scavenging is actually achieved via an organic coating carrier phase that is remineralized, thus offering a mechanism for transfer of REEs from the particulate to dissolved phase without subsequent re-adsorption (i.e., not a desorptive process, but a remineralization; Sholkovitz et al, 1994).…”

Effect of Mg and Ca on the Stability of the MRI Contrast Agent Gd–DTPA in Seawater

“…The contribution of the single-protonated complex is negligible. The reactive Gd fraction displays the usual speciation in standard seawater (Schijf et al, 2015a), dominated by carbonate complexes, where the Gd(CO 3 ) − 2 species makes up nearly 70% and the GdCO + 3 species nearly 30% of the total. The remaining few percent consist of other Gd species, mainly free and hydrolyzed cations and the sulfate complex.…”

“…We will assume here that this fraction consists of Gd-DTPA. Some of the remaining 60 pM may also be from industrial sources but this fraction is present as reactive species, predominantly complexes with dissolved inorganic and less stable organic complexes (Schijf et al, 2015a). Speciation was calculated with MINEQL2.0 (Westall et al, 1986) for the same standard seawater referred to in the previous section.…”