The Bocana de Virrila, a relict estuary on the coast of northwestern Peru, exhibits unique lateral zonation of evaporite minerals (calcite, gypsum, halite) along its 20-km length. The salinity gradient ranges from normal marine at the mouth to approximately 330,000 ppm TDS in the innermost waters. Gypsum and halite precipitate in 2.5-km zones characterized by water salinities greater than 160,000 ppm and 320,000 ppm TDS, respectively. Calcite precipitation is not as extensive as expected, and very high values of alkalinity (6.0 mM 1 (super -1) as CaCO 3 ) are attained. We support a model of the Bocana as a barred lagoon operating in a flow evaporative mode. Limited diurnal data show significant variation in brine composition as well as thermal and density stratification. We did not determine the patterns of brine circulation. However, the lack of bittern salts in the saline brine zones suggests removal of K- and Mg-rich brines.--Modified journal abstract.

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