The Isotopic Composition and Concentration of Strontium of the Brine From Tuborg Lake, Ellesmere Island

Abstract

Tuborg Lake is at 81°N, 76°W at the head of Antoinette Bay in northern Ellesmere Island, Northwest Territories. It trends in an east-west direction and is separated from the fjord by a glacier at its western margin. The lake is 20 km long and about 3 km at its widest. An active glacier at the eastern end calves occasional small icebergs into the lake. In June 1963, the level of water in the lake was 10 to 12 m above sea level. The lake is markedly density-stratified; the salinity is less than 0.5 per mil depth of more than 46 m below its surface. The salinity rapidly increases below this depth, and at a depth of 57 m it is 25.594 per mil. Hattersley-Smith and Serson attribute the saline water at the bottom of the lake to sea water trapped by the advance of the glacier across the fjord. The depth of the halocline at 50 to 55 m (thus 40 to 45 m below sea level) and the fact that the present level of the lake is about 10 m above sea level both suggest a complex history of the lake. Recently, the isotopic composition of strontium, conveniently expressed as the 87Sr/86Sr ratio, has been used to indicate the source of dissolved salts .... The 87Sr/86Sr ratio of surface water depends on the Rb/Sr ratios and ages of the rocks exposed in the drainage basin. ... The isotopic composition of strontium in sea water is constant (87Sr/86Sr = 0.7093) and can therefore be useful in identifying marine strontium. The isotopic composition of strontium was determined on a sample of the brine from Lake Tuborg. This sample was collected 22 June 1967 at a depth of 90 m below the surface of the lake. At the sampling site the maximum depth of the lake was 130 m and the depth of the halocline was 60 m; salinity of the sample is 26 per mil. ... The 87Sr/86Sr ratio for the brine at 90 m depth of Lake Tuborg has a value of 0.7096 ±0.0005 (1 sigma), which is in satisfactory agreement with the accepted value for modern sea water. This suggests that the brine at the bottom of the lake could be sea water. However, this is not conclusive, because the dominant bedrock in the region consists mainly of marine carbonates of Early to Middle Cambrian age. The 87Sr/86Sr ratio of these rocks probably does not differ greatly from this value. We analyzed one specimen of limestone of Middle Cambrian age from the Nelson Formation of the Neptune Range, Pensacola Mountains, Antarctica, and obtained an 87Sr/86Sr ratio of 0.7093. The concentration of strontium, determined by isotope dilution using a spike enriched in 86Sr, was 6.239 ppm and is somewhat less than that of normal sea water, which has a strontium content of approximately 8 ppm. Using the established relationship between salinity and chlorinity in sea water and a salinity of 26 per mil, we find a chlorinity of 14.4 per mil for the brine sample. Accordingly, the Sr/Cl ratio of this brine is 0.43. Riley and Tongudai obtained an average value of 0.42 ±0.02 for this ratio for a large suite of sea water samples. The Sr/Cl ratio of the brine from Lake Tuborg is similar to this value, which also suggests that the brine could be sea water.