GEOCHEMICAL CONTROLS ON MAJOR AND MINOR ALKALINE ELEMENTS IN CLOSED-BASIN LAKES, TAYLOR VALLEY, ANTARCTICA

Taylor Valley is one of the McMurdo Dry Valleys, the largest ice-free area in Antarctica. The valley is classified as a polar desert with a mean annual temperature of -20° C and low annual precipitation (<10 cm). Despite these extreme hydrologic conditions, three perennially ice covered lakes exist in the valley. Because these are closed-basin systems, they can serve as important observatories on the evolution of natural waters under extremely cold conditions. The lakes range from fresh to hypersaline and are chemically stratified with the hypersaline lakes the oldest. Although the sources of solutes to the lakes have been constrained, little is known about solute behavior in these closed-basin lakes and what processes control their distributions. We have used major ion and minor alkali element concentrations to model the saturation indices of numerous minerals in the fresh and brackish lakes (Lake Hoare and Lake Fryxell, respectively). These results are in accordance with the findings of Green and others (1988) and Neumann (1999) that these lakes are not in equilibrium with respect to carbonate minerals. In Lake Fryxell, both calcite and aragonite are supersaturated throughout the water column. In Lake Hoare, saturation generally decreases with depth and only the surface waters are capable of precipitating calcite, aragonite, and dolomite. The saturation index pattern for amorphous silica increases with depth and salinity. The lack of diatoms in the water columns of these lakes suggests that silica is controlled by inorganic processes. Below 8 meters, the minor elements, barium, lithium, rubidium, and strontium, normalized to chloride, display conservative behavior in Lake Hoare. Barium appears to be conservative in Lake Fryxell, but lithium, rubidium, and strontium are enriched in the surface waters. The higher ratio of Sr:Ca in the lakes compared to the streams entering these lakes implies that calcium is preferentially incorporated into carbonate minerals and lost, while the majority of strontium remains in solution. These results provide insight into the processes controlling alkaline earth element and alkali metal behavior during lake evolution in a polar desert environment.