A study of strontium isotopes in lakes and surficial deposits of the ice-free valleys, southern Victoria Land, Antarctica

Abstract

The ice-free valleys of southern Victoria Land, Antarctica, contain saline lakes including Lake Vanda in Wright Valley and Lake Bonney in Taylor Valley. The source of the salts dissolved in the brines has not yet been identified. Strontium in water-soluble salts of soil samples in Wright Valley has 87 Sr 86 Sr ratios ranging from 0.7119 to 0.7157 with an average of 0.7144 ± 0.0008 (1 σ). This value is very similar to the 87 Sr 86 Sr ratio of brines in Lake Vanda ( 87 Sr 86 Sr = 0.7149 ± 0.00017 ), but differs significantly from the 87 Sr 86 Sr ratios of seawater (0.7094 ± 0.00012) and basaltic rocks of the McMurdo Volcanics (0.7044 ± 0.00046). The Sr in Lake Vanda therefore could not have originated from seawater or from volcanic rocks of the area, but may have been derived by chemical weathering of the igneous and metamorphic rocks exposed in Wright Valley. The 87 Sr 86 Sr ratios of Lake Bonney are invariant with depth and average 0.7130 ± 0.00014 (1 σ), whereas the Sr concentrations increase from 0.7345 ppm near the surface to a maximum of 39.92 ppm at a depth of 25 m. The Sr concentrations at greater depth decrease slightly to 35.88 ppm at 30 m. The Sr in water-soluble soil salts near Lake Bonney and of the Taylor Red Cone is similar isotopically to the Sr in the lake. The 87 Sr 86 Sr ratios of soil salts and of meltwater in Taylor Valley decrease systematically from Lake Bonney toward the coast. The water in Lake Fryxell, which is closest to the coast, has an 87 Sr 86 Sr ratio of 0.7090 and is identical to that of seawater. The evidence from this and similar studies in other parts of the world indicates that the 87 Sr 86 Sr ratios of saline brines in closed continental basins are representative of the Sr in the rocks underlying the basin. The Sr in carbonate and sulfate minerals precipitated from such brines preserves the 87 Sr 86 Sr ratios that existed at the time of deposition. Because of these relationships we suggest that stratigraphic variations of the 87 Sr 86 Sr ratios of non-marine carbonate or sulfate rocks reflect changes in the geology of the drainage basin. The isotopic composition of Sr of such rocks may therefore provide useful information about the geologic histories of continental basins.