The fate of minor alkali elements in the chemical evolution of salt lakes

The ultimate chemistry of a saline, closed-basin lake is determined by the initial composition of precipitation, the weathering reactions between dilute inflow water and lithology, and evapoconcentration (or sublimation) of the lake water [1]. The chemical pathways of evolving brines, termed geochemical divides, are determined very early by the initial chemical composition [1]. During mineral precipitation from a brine, the less abundant ion of the mineral pair of ions will become drastically depleted compared to the other [1].Modelling of closed-basin lakes has focused on the major elements of most natural waters: Ca, Na, K, Mg, Cl, SO4, and carbonate alkalinity (HCO3 + CO3) and the simple salts that these ions produced during evaporation [1-3]. This research was fundamental in illustrating how the geochemistry of a lake can result from dilute inflow water with a composition unlike the final brine. It is clear from these models that the abundance of major alkali elements, Ca, Na, K, and Mg, is key in determining the geochemical pathways involved in brine formation, yet there has been little work to extend the modelling efforts to aid in the prediction of minor and trace metal behaviour during brine formation. In this paper, we have made an attempt to understand minor alkali metal and alkaline earth behaviour in different brine types in order to extend the earlier efforts on major cations. We have analysed water samples from brackish and saline lakes in the Great Basin of the United States, Saskatchewan, and the McMurdo Dry Valleys of Antarctica (Figure 1) and used these data in the geochemical model, PHREEQ, to illustrate the potential removal mechanisms that are occurring in closed-basin lakes.The McMurdo Dry Valleys (MCM) are the largest ice-free areas in Antarctica (approximately 4800 km2) (Figure 1) [4]. The MCM (≈78° S) are a polar desert classified by extremely low average annual temperatures (-16°C to -21°C) and high aridity (< 10 cm snowfall per year) [5].