Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling

Abstract

Concentrations of dissolved Y and rare-earth elements (REE) are reported for oxic eastern Mediterranean seawater from the western Levantine Basin, and for anoxic hypersaline brine and overlying oxic seawater from the Tyro sub-basin, the latter data allowing a comparative study of Y and REE behaviours during redox-cycling. All oxic waters show shale-normalized Rare-Earths and Yttrium (REYSN; Y inserted between Dy and Ho) patterns with HREE enrichment, negative CeSN anomalies, and positive anomalies of LaSN, EuSN, and, most pronounced, YSN. The anoxic brine in the Tyro sub-basin displays the highest REY concentrations and the least HREE enrichment of all samples, and positive anomalies of LaSN, EuSN, GdSN, YSN, and, in marked contrast to the oxic samples, CeSN. Compared to overlying oxic water, the anoxic brine shows enrichment factors that decrease from La (12.1) to Yb (2.7), with a pronounced positive deviation for Ce (51.2) and a strong negative deviation for Y (2.4, compared to 4.3 for Dy and 3.7 for Ho). The YHo molar ratio decreases from 102 above to only 67 below the seawater-brine interface, due to preferential sorption of Ho with respect to Y on Fe- and Mn-oxyhydroxide particles that eventually dissolve under anoxic conditions. The pronounced Y-Ho fractionation during redox-cycling of Mn and Fe is further evidence for the considerably lower marine particle reactivity of Y compared to Ho, resulting from lower stabilities of surface complexes of Y relative to those of its REE neighbours. Hence, the YHo ratio appears to be a sensitive indicator of the impact of particles on the distribution of dissolved trace elements in natural waters.