Dissolved rare earth elements in the Black Sea

Abstract

Abstract-Concentrations of rare earths in the deep anoxic Black Sea are about one order of magnitude higher than in normal open ocean waters. From a minimum at the suboxic-anoxic interface at about 107 m depth, concentrations increase strongly to a maximum at about 3()()-4()() m depth. Concentrations of Ce range from 3 pM at 107 m to 205 pM at 300 m depth, partly as a result of its oxidation-reduction chemistry. The other, strictly trivalent, rare earths exhibit similar trends albeit not as dramatic as for Ce. Minima and maxima are dominating features throughout the rare earth elements (REE) series, yet with increasing atomic number the gradients become more subtle. No distinct trends are observed for the REE at the primary maximum of particulate Mn at Go = 15.65 (-75 m depth) in the intermediate suboxic zone. The dissolved REE minima at 107 m exist at ambient sulfide levels of about l,uM and coincide with a strong particle maximum. At the very same Go = 16.10 density horizon a distinct maximum of phototrophic bacteria (with concomitant levels of chlorophyll e) has been reported as well as the secondary particulate Mn maximum. From thermodynamic considerations the latter was not expected to occur within sulfidic waters. Apparently all REE are involved in the redox-cycling across the suboxic-anoxic interface, where the abundant bacterial particles most likely serve as carriers, possibly through Mn-oxide surface coatings. While Ce exhibits active oxidation-reduction chemistry, there is little or no evidence suggesting the reduction of dissolved Eu(lIl) to Eu(II).

Vertical distributions and speciation of dissolved rare earth elements in the anoxic brines of Bannock Basin, eastern Mediterranean Sea

“…4. REE concentrations at the seawater-brine interface are the highest ever recorded in the water column of a marine anoxic basin German and Elderfield, 1989;Schijf et al, 1991) and comparable only to REE concentrations in the porewaters of reducing sediments at depths of 30-40 cm below the sedimentseawater interface (Elderfield and Sholkovitz, 1987;Sholkovitz et al, 1989). REE enrichment across the seawater-brine interface, determined by dividing the concentration of each REE at 3323 dbar by its concentration at 3306 dbar, ranges from 9 to 40 for the trivalent REEs and is as high as 370 for Ce as a result of its unique redox chemistry.…”

“…Unlike elevated dissolved REE concentrations in the anoxic water of some other marine anoxic basins, which are due entirely to the redox cycling of Mn and Fe at the oxic-anoxic interface German and Elderfield, 1989;Schijf et al, 1991 ), elevated dissolved REE concentrations in the Bannock brine may be due in part to the dissolution of Messinian evaporites. Nevertheless, most aspects of the verwas proposed by de Lange et al (1990b) to account for several conspicuous features at the second interface, such as maxima of phosphate (Fig.…”

Petrography and the REE-composition of apatite in the Paleoproterozoic Pilgujärvi Sedimentary Formation, Pechenga Greenstone Belt, Russia

“…In stratified water-bodies with well-developed redoxclines (e.g. the Black Sea), negative Ce anomalies become increasingly negative with depth until reaching the redoxcline, where it sharply diminishes due to reduction of Ce 4+ to a soluble Ce 3+ as the environment becomes anoxic and euxinic (Schijf et al, 1991). Ce anomaly reflecting the redox status of the depositional and/or diagenetic environment is recorded in early diagenetic authigenic minerals such as Ca-phosphate and calcite/aragonite precipitating in equilibrium with the seawater or pore water, which has been shown in modern settings (Wright et al, 1987, Arning et al, 2009and Piper and Bau, 2013.…”

Rare earth element abundances in high phosphorus and low iron groundwaters from the Nishinomiya district, Japan: Variations in Ce anomaly, redox state and heavy rare earth enrichment.

“…Ce anomalies seen in REE patterns are generally ascribed to the oxidation of Ce(III) to Ce(IV) under oxidizing conditions (e.g., Schreiber et al, 1980;Elderfield and Greaves, 1981;German and Elderfield, 1989;De Baar et al, 1988;Moffett, 1990Moffett, , 1994aMoffett, , 1994bGerman et al, 1991German et al, , 1995Schijf et al, 1991Schijf et al, , 1995Sholkovitz and Schneider, 1991;Kagi and Takahashi, 1998;DeCarlo et al, 1998;Akagi and Masuda, 1998;Takahashi et al, 2000). De Baar et al (1988) reported drastic changes in Ce anomalies at the oxic/anoxic tran-sition in the seawater column at the Cariaco Trench, and interpreted these data by using Ce speciation calculations for the seawater and the CeO 2 stability field on the pe(Eh)-pH diagram.…”