VERTICAL DISTRIBUTIONS AND SPECIATION OF DISSOLVED RARE-EARTH ELEMENTS IN THE ANOXIC BRINES OF BANNOCK BASIN, EASTERN MEDITERRANEAN-SEA

Vertical distributions of dissolved rare earth elements (REEs) are pre sented for the anoxic, highly sulfidic brines of Bannock Basin in the eastern Mediterranean Sea. REE concentrations at the seawater-brine in terface are the highest ever recorded in the water column of an anoxic basin and comparable only to REE concentrations at depth in the porew aters of reducing sediments. Below the seawater-brine interface, throu ghout the upper of two chemically distinct brine layers, REE concentra tions decrease with depth as a result of scavenging, while shale-norma lized REE patterns are characterized by little or no fractionation bet ween the light REEs (LREEs) and the heavy REEs (HREEs) and by a positi ve Ce anomaly. Throughout the lower of the two brine layers REE concen trations are constant, while REE patterns are characterized by the sam e lack of LREE or HREE enrichment, as well as by the absence of any Ce anomaly. A pronounced REE minimum at the interface between the two br ine layers is apparently caused by uptake into dolomite that is formed through in situ recrystallization of Ca carbonate, a process that was previously reported to cause similar minima for Mn and Fe. Calculatio ns of REE speciation, incorporated into a model of REE scavenging (dev eloped by Byrne and Kim, 1990), which uses REE complexation by monocar boxylic acids to represent REE adsorption onto particulate matter, are surprisingly successful at reproducing the unusual shape of REE patte rns in the brine. This suggests that this shape, although very differe nt from that of REE patterns in the overlying seawater due to the very high ionic strength of the brine, is the result of much the same proc esses that constitute REE scavenging in the open ocean.