Dissolved rare earth elements in the Black Sea

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 300–400 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 σθ = 15.65 (~75 m depth) in the intermediate suboxic zone. The dissolved REE minima at 107 m exist at ambient sulfide levels of about 1 μM and coincide with a strong particle maximum. At the very same σθ = 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(III) to Eu(II).