Trace-metal behaviour in oxic and anoxic CaCl brines of the Wright Valley drainage, Antarctica

Abstract

Geochemical speciation and reaction path models have been used to identify the major factors controlling Mn, Fe, Cu, Pb, Zn, Ni and Co concentrations in meltwaters and CaCl brines of the closed-basin Wright Valley drainage system in Antarctica. The major aqueous features of this drainage system are: (a) the Onyx River, a meltwater stream; (b) Lake Vanda, an ice-covered stratified lake in which CaCl brines become more concentrated and ultimately anoxic with depth; and (c) Don Juan Pond, a small, highly saline CaCl brine pond. Trace-metal concentrations in the groundwater have been estimated and used to derive hypothetical concentrations for trace metals in the ancient brine pool from which Lake Vanda was formed. The results of this calculation support the premise that the trace-metal distribution profiles observed in Lake Vanda are attributable to the influx of trace metals from the Onyx River and to modern, ongoing lake processes. In the Onyx River and in the oxic CaCl brines of Lake Vanda and Don Juan Pond, trace-metal concentrations appear to be consistently controlled by adsorption and desorption from the surface of particulate Fe-oxides or Fe-hydroxides. Variations in the trace-metal concentrations measured in the oxic brines can be directly related to the effect of changes in pH and major-ion chemistry on the adsorption of each metal at the Fe-oxide surface. For the Onyx River and Lake Vanda, this hypothesis is presented as modification to the currently accepted model (Green et al., 1989) which postulates Cu and Ni adsorption onto particulate Mn-oxide. In the anoxic, H 2S-bearing CaCl brines at the base of Lake Vanda, Cu, Pb, Zn, Ni, Co and Fe concentrations appear to be limited by precipitation of metal-sulphide mineral phases and by the stability of metal-bisulphide and -polysulphide complex species. Mn is relatively mobile in the anoxic brine, but Mn concentrations may be limited by precipitation of a Mn-carbonate phase.