Soils and Brine Geochemistry and Mineralogy of Hyperarid Desert Playa, Ouargla Basin, Algerian Sahara

The Ouargla basin is located in hyperarid north Africa and characterized by an endoreic landscape (playa) with Solonchak (accumulated soluble salts) and Gypsisol (accumulated gypsum) soils, and subsurface groundwater. The chemical and mineralogical specificity of this hyperarid ecosystem has been compared to other areas under arid environment (Tunisian sub-Sahara and Algerian steep-lands). Chemical data on the major and minor elements, such as Sr2+, were obtained, and geochemical studies of groundwater, soil solution samples and brines have been performed. In total, 84 samples (42 groundwater, 39 soil solutions, 3 brines) were collected. X-ray diffraction and micromorphological observations (SEM with microprobe) were also performed in order to analyze soils and efflorescence. The concentrations of chemical components showed a strong variability (standard deviation of 1.15 for Cl− and 1.08 for Na+) which was reduced by an increasing ratio between concentrated and diluted samples. The Principal Component Analyses showed that an increase in the concentration of soluble ions was responsible for 56% of the total variance. When the soluble salt concentration increased, the chemical composition changed from Ca-sulfate dominance to Na-chloride dominance. Thermodynamic equilibrium data indicated that these salt solutions were controlled by the following precipitation sequence: calcite > gypsum > mirabilite > halite. This sequence was confirmed by X-ray diffraction and micromorphology. Genesis of dehydrated forms of CaSO4 · 2H2O were recognized by: (1) partial dehydration of gypsum by sun radiation on the soil surface, and (2) a decrease in water activity (<1) relating to an increase in ionic strength (>3 molc kg−1 of H2O) of concentrated solutions. The presence of mixed sulfates (glauberite, bloedite, and others) appears to be a distinctive feature of the hyperdesert soil. In this environment they are more stable than simple sulfates. A discrete mineral involving a minor element was observed, and celestite (SrSO4) was thermodynamically predicted. This mineral results from the coprecipitation of Sr2+ with gypsum. Micromorphologically, celestite crystals occur as inclusions and within the periphery of gypsum crystals. The seasonal and daily thermal amplitudes in the desert environment determine the precipitation pattern into the Na–Ca–SO4–H2O system (mirabilite-thenardite-halite paragenesis), controlling the process of dissolution/precipitation of gypsum. The hyperarid environment has a remarkable chemical and mineralogical reactivity because of the high solubility and hydration status of minerals, and its sensitive reactivity to climate variation. Based on these results, dehydrated CaSO4 is proposed as a diagnostic pedological criterion for delineating hyperarid and arid ecosystems. © 2004 Taylor & Francis Group, LLC.