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 presented for the anoxic, highly sulfidic brines of Bannock Basin in the eastern Mediterranean Sea. REE concentrations at the seawater-brine interface are the highest ever recorded in the water column of an anoxic basin and comparable only to REE concentrations at depth in the porewaters of reducing sediments. Below the seawater-brine interface, throughout the upper of two chemically distinct brine layers, REE concentrations decrease with depth as a result of scavenging, while shale-normalized REE patterns are characterized by little or no fractionation between the light REEs (LREEs) and the heavy REEs (HREEs) and by a positive Ce anomaly. Throughout the lower of the two brine layers REE concentrations are constant, while REE patterns are characterized by the same 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 brine 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. Calculations of REE speciation, incorporated into a model of REE scavenging (developed by Byrne and Kim, 1990), which uses REE complexation by monocarboxylic acids to represent REE adsorption onto particulate matter, are surprisingly successful at reproducing the unusual shape of REE patterns in the brine. This suggests that this shape, although very different 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 processes that constitute REE scavenging in the open ocean.