Surface crusts on soils/sediments of the southern Aral Sea basin, Uzbekistan

Abstract

Soils and sediments of the southern Aral Sea basin of Central Asia are commonly covered by salt-bearing surface crusts. These crusts represent a potential source of salt-rich dust that can contribute to salinisation of soils in a large area. Crust types include takyric crusts, surface crusts on Solonchaks, and salt crusts covering dry parts of the lake bed. The takyric crusts are structural crusts, showing features related to flooding and shrinkage. Gypsum, formed within the sediment matrix, is the only evaporite mineral they contain. One studied Solonchak crust is fine-grained, with a thin thenardite and bloedite coating along the irregular surface and halite coatings along the sides of pores. All salts in this crust formed by evaporation of solutions contained in the groundmass. A Solonchak crust with a sandy texture contains prismatic gypsum crystals that formed in a water-saturated surface layer, as well as bloedite, eugsterite and halite, formed by evaporation of interstitial brines. The lake bed crust is a pure accumulation of salt minerals (bloedite, hexahydrite, halite, kainite), with synsedimentary and diagenetic features. Salt mineral assemblages in the three crust types are those that form during successive stages of evaporation of Aral Sea waters, in agreement with published theoretical predictions.