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Abstract:

More than 25 ephemeral saline lakes are found in La Mancha, an endorheic region with a semi-arid climate in central Spain. The annual rainfall is 400 mm/yr, with an evaporation rate of 1,400 mm/yr. Many of the lakes occupy the small closed or nearly closed hydrological basins, approximately 10 km2 in extent. The area annually flooded by brines is approximately 0.5 km2. The water depth in the lakes during the rainy season (winter) is <0.5 m. In some lakes, precipitation of salts during summer develops a "bull's eye" pattern of concentrically zoned salts, with bischofite crusts in the innermost zone of the lake, then a zone of halite-epsomite crusts, followed by hexahydrite-halite crusts, and finally, hexahydrite-gypsum crusts in the outermost zone.

The thin crust of the outer zone is always overlain by organic muds; some crusts commonly display tepee structures. Salts precipitated in summer are dissolved during winter, and mass-balance solute models show some playas to be nearly closed systems. An experimental pond was built in one of the lakes (Quero). This pond has allowed us to monitor temporal variations in the concentration and temperature (both upper and lower layers) of the brine, and the precipitation of saline minerals. The sequence of minerals precipitated in the experimental pond agrees with the observed natural sequence; most salts grow at the air-brine interface. A direct correlation has been recognized between the vertical temperature patterns, and various parameters of the brine concentration during periods of brine "stagnation."

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