Seasonal dynamics of evaporite mineral precipitation, dissolution, and back-reactions in shallow and deep hypersaline lakes

Abstract

Seasonal water temperature variations exhibit a first-order control on the mineralogy, texture, and timing of formation of modern, temperate zone 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 during the drying up of the shallow (2.4 m) Owens Lake, California (ca. 1969). Additional parameters were then added to simulate a deep (>10 m), perennial lake with the same chemical composition (e.g., ancient Searles Lake). Variations in temperature drive saline mineral precipitation and syndepositional alteration in both models, but in different ways. In shallow, well-mixed temperate lakes, minerals crystallize and transform in rhythm with seasonal temperature fluctuations. In deeper stratified lakes, alteration occurs when 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 that precipitate at low temperatures later dissolve or back-react in warmer bottom waters. Closed basin evaporite deposits formed in temperate lakes thus preferentially preserve high temperature salts. This means that midlatitude lacustrine evaporites contain more climatic and limnological information than previously recognized, and can be interpreted in terms of seasonal temperatures, mean annual temperatures, and lake depths.