GEOCHEMICAL MODELING OF MINERAL STABILITY DURING EVAPORATION AND MIXING OF WATERS IN THE MONO BASIN, CALIFORNIA, USA

Closed basin lakes in arid environments are highly sensitive to hydro-climate change, making the Mono Basin of eastern California an important locality for evaluating the geologic record of climate change in the western Great Basin. During the last glacial period the lake was much higher, and the physical stratigraphic evidence for lake level fluctuations (Lajoie, 1968, Berkeley Ph.D. thesis) has been linked to changes in carbonate flux in the late Pleistocene Wilson Creek Formation (Zimmerman et al., in press, GSA Bull.). In addition to changes in carbonate, the lake sediments show large variations in leachable Li, Mg, Sr and Ca that correspond to the variations in carbonate. Mono Lake has an unusual chemistry. It is highly alkaline and highly oversaturated with respect to calcite and Mg-smectite (the host mineral for Li), but the aqueous Ca and Mg concentrations are very low, leading to a situation where the amount of precipitation of calcite and Mg-smectite is related to the amount of input of fresh stream and/or spring water into the lake. In order to better understand the controls on precipitation of these authigenic phases, with the goal of developing a highly resolved record of paleo-hydroclimate for the Mono Basin, we are making simple models of the predicted lake chemistry changes based on evaporation and mixing of saline and fresh waters in proportions that approximate the changes in lake level through the last glacial cycle. We have used the PHREEQC model to calculate the stability of mineral phases precipitating out of Mono Lake waters, thus providing insight into the chemical evolution of Mono Lake through the last glacial cycle. The results support our observational evidence, initially counterintuitive, that glacial-age Mono Lake sediments have high carbonate and leachable Li concentrations when lake level is high.