YTTRIUM AND LANTHANIDES IN EASTERN MEDITERRANEAN SEAWATER AND THEIR FRACTIONATION DURING REDOX-CYCLING

Concentrations of dissolved Y and rare-earth elements (REE) are report ed for oxic eastern Mediterranean seawater from the western Levantine Basin, and for anoxic hypersaline brine and overlying oxic seawater fr om 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 an d Ho) patterns with HREE enrichment, negative Ce-SN anomalies, and pos itive anomalies of La-SN, Eu-SN, and, most pronounced, Y-SN. The anoxi c brine in the Tyro sub-basin displays the highest REY concentrations and the least HREE enrichment of all samples, and positive anomalies o f La-SN, Eu-SN, Gd-SN, Y-SN, and, in marked contrast to the oxic sampl es, Ce-SN.Compared to overlying oxic water, the anoxic brine shows enr ichment factors that decrease from La (12.1) to Yb (2.7), with a prono unced 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 ra tio decreases from 102 above to only 67 below the seawater-brine inter face, due to preferential sorption of Ho with respect to Y on Fe- and Mn-oxyhydroxide particles that eventually dissolve under anoxic condit ions. The pronounced Y-Ho fractionation during redox-cycling of Mn and Fe is further evidence for the considerably lower marine particle rea ctivity of Y compared to Ho, resulting from lower stabilities of surfa ce 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 partic les on the distribution of dissolved trace elements in natural waters.