The Role of Minor Alkali Elements in the Evolution of Closed-Basin Lakes

The ultimate chemistry of what is known as a closed basin lake is determined at a very early stage by the interaction of dilute inflow waters with the surrounding geology. The evapoconcentration of these waters results in a "geochemical divide". That is, as a lake evaporates the precipitation of minerals results in the enrichment of more soluble ions. Alkaline earth elements and alkali metals play an important role in these processes as the final brine type is often defined by the abundance of these elements. The role of so-called major ions in brine evolution has been studied in great detail, but little has been done on the involvement of minor alkali elements despite their similar behavior. We have examined three major anionic brine types, chloride, sulfate, and carbonate, in fifteen lakes in North America and Antarctica for their chemical compositions as they pertain to lithium, rubidium, strontium, and barium. Lithium and rubidium are largely conservative in all lake types, and their concentrations are the result of long-term solute input. In practically all lakes, strontium behaves much like calcium. In carbonate and sulfate lakes, strontium may be removed from solution from the precipitation of strontianite (SrCO₃) or celestite (SrSO₄), and removal is most evident in sulfate lakes. Removal is apparent in highly evolved, more concentrated carbonate lakes with the exception of Walker Lake. In the carbonate and sulfate lakes examined in this study, barium is readily removed from the solution, and in some instances, it is removed from the chloride lakes. Removal may be a result of biological uptake or the inorganic precipitation of witherite (BaCO₃) or barite (BaSO₄). This research shows that minor elements play a crucial role in determining the ultimate composition of saline lakes due to their interactions with elements of higher concentration.