SEASONAL DYNAMICS OF SALINE-ALKALINE LAKES: MODEL SOLUTIONS FOR SHALLOW (MODERN) OWENS LAKE AND DEEP (LATE PLEISTOCENE) SEARLES LAKE, CALIFORNIA (USA)

Seasonal temperature variations exhibit a first-order control on the mineralogy, texture, and timing of formation of modern lacustrine evaporites. However, seasonality is generally absent from the thermochemical models used to interpret ancient nonmarine evaporites. Here, a seasonal model is presented which correctly reproduced the complex history of evaporite precipitation, dissolution, and back-reactions observed in the shallow (2.4 m), ephemeral Owens Lake (ca. 1969). Additional parameters were then added to simulate a deep (>10 m), perennial system of the same chemistry (e.g., Late Pleistocene Searles Lake). Temperature is the key driver of saline mineral precipitation and syndepositional alteration in both models, but in different ways. In shallow, well-mixed lakes, minerals crystallize and transform in rhythm with seasonal temperature fluctuations. In deeper stratified brines, alteration occurs as salts settle from surface waters (variable seasonal temperatures) to the bottom waters (constant temperatures). The deep lake model shows that more than half of the salts precipitated at low temperatures either dissolve or back-react in warmer bottom waters. Closed basin evaporite deposits may thus preferentially preserve high temperature salts, while the rare preservation of low temperature salts records anomalous depressions in mean annual air temperature. Seasonal models are applied to the Late Pleistocene Searles Lake deposits to demonstrate how evaporites can be re-interpreted in terms of seasonal temperatures, mean annual temperatures, and lake depth.