资源环境科技发展态势分析平台

Halite precipitates in the Dead Sea during winter but re-dissolves above the thermocline upon summer warming, "focusing" halite deposition below the thermocline (Sirota et al., 2016, 2017, 2018). Here we develop an "evaporite focusing" model for evaporites (nahcolite + halite) preserved in a restricted area of the Eocene Green River Formation in the Piceance Creek Basin of Colorado, USA. Nahcolite solubility is dependent on partial pressure of carbon dioxide (pCO(2)) as well as temperature (T), so these models covary with both T and pCO(2). In the lake that filled the Piceance Creek Basin, halite, nahcolite or mixtures of both could have precipitated during winter cooling, depending on the CO2 content in different parts of the lake. Preservation of these minerals occurs below the thermocline (>similar to 25 m) in deeper portions of the basin. Our modeling addresses both: (1) the restriction of evaporites in the Piceance Creek Basin to the center of the basin without recourse to later dissolution and (2) the variable mineralogy of the evaporites without recourse to changes in lake water chemistry. T from 20 to 30 degrees C and pCO(2) between 1800 and 2800 ppm are reasonable estimates for the conditions in the Piceance Creek Basin paleolake. Other evaporites occur in the center of basins but do not extend out to the edges of the basin. Evaporite focusing caused by summer-winter T changes in the solubility of the minerals should be considered for such deposits and variable pCO(2) within the evaporating brines also needs to be considered if pCO(2) sensitive minerals are found.