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

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 (REY(SN); Y inserted between Dy and Ho) patterns with HREE enrichment, negative Ce-SN anomalies, and positive anomalies of La-SN, Eu-SN, and, most pronounced, Y-SN. 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 La-SN, Eu-SN, Gd-SN, Y-SN, and, in marked contrast to the oxic samples, Ce-SN. 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 Y/Ho 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 Y/Ho ratio appears to be a sensitive indicator of the impact of particles on the distribution of dissolved trace elements in natural waters.