Characterization of the major ion chemistry of the saline lakes of the Vestfold Hills, Antarctica
Creators: name: Stark, Scott Charles
Publisher: name: University Of Tasmania
Year: 2023

description: The Vestfold Hills is a predominantly ice-free 'oasis' lying on the coast of Princess Elizabeth Land, East Antarctica. The region was formed as a consequence of isostatic readjustment of the coastline following retreat of the Pleistocene continental ice sheet. During this event, sea water was constrained and eventually trapped by the rising landmass. Over a period of ca. 8-10 000 yr, pockets of relict sea water have evolved to produce several hundred lakes, scattered throughout the Vestfold Hills. While some of the lakes have been flushed of their sea salt by glacial meltwaters, others have become hypersaline brines with total dissolved salt contents in excess of 200 g kg- 1. An accurate and precise characterization of the major ion chemistry of a natural brine is fundamental to an understanding of its geochemical evolution, the current state of mineral equilibria in solution and in the sediment, the prediction of physicochemical properties such as density and activity coefficients, and the accurate determination of absolute salinity. However, the accurate and reproducible analysis of the major ions in concentrated, multicomponent brines is often problematic, and obtaining a satisfactory ion balance between anions and cations is not a trivial task. Methods for the determination of the major ionic components of sea water and marine-type brines (sodium, potassium, magnesium, calcium, strontium, chloride, sulphate, bromide, and total alkalinity) were investigated and a set of suitable methods was adopted for the characterization of the brines of the Vestfold Hills. The methods employed were mainly classical 'wet chemistry' techniques and include potentiometric and photometric titration along with conventional titrimetry, gravimetry, colorimetry, and flame atomic emission and absorption spectrometry. The reliability of the set of methods was demonstrated by determining precise major ion composition profiles for 36 brine samples collected from 10 hypersaline lakes, with a mean absolute ion balance error of only 0.09 ¬¨¬± 0.07 %. Analysis of a sample of secondary standard sea water, and in some cases analyte recovery tests, provided a measure of the accuracy of the methods. Empirical composition relationships for the saline lakes of the Vestfold Hills were derived using the major ion data obtained for the set of brine samples. Furthermore, measurements of the density of brine solutions at 20 ¬¨‚àûC were combined with estimates of their TDS content, calculated from the composition data, to derive relationships between density and absolute salinity (and chlorinity). These complement existing relationships correlating the conductivity, temperature and density of Vestfold Hills brines. Interpretation of the major ion data for the brines suggests that in general, they conform to a simple closed-basin brine evolution model in which the freezing of sea water was the main process directing evolution. This was likely to have begun before the complete isolation of the relict water from the sea, in stratified marine basins within bays and fjords (open systems with restricted circulation). Biological sulphate reduction and the input of solutes from non-marine sources appear to have had little influence on the present-day composition of the brines examined in this study. All of the brines were saturated with sodium sulphate, precipitated as mirabilite, and the most saline were saturated with sodium chloride (hydrohalite and/or halite). The observed fractionation of magnesium, potassium, chloride and bromide, however, suggests that at least some of the brines were considerably more saline in the past, probably concentrated to saturation with the chlorides of potassium and magnesium (sylvite, magnesium chloride dodecahydrate, carnallite). This would have required climatic conditions that were more frigid and/or arid than exist today. There is also evidence for the precipitation of calcium sulphate (gypsum) in the brines, as well as other sulphate salts such as strontium sulphate (celestite) and possibly potassium-sulphate phases. This is attributed to diagenetic reactions and also to the mixing of lake waters with sulphate-rich brines, derived from mirabilite dissolution occurring within the lake or in deposits located in the catchment. The best evidence for the latter mechanism, favoured by a net positive water balance, was uncovered by examining major ion depth profiles for Deep Lake, one of the most saline brines in the Vestfold Hills.