Anhydrite formation on the coastal sabkha of Abu Dhabi, United Arab Emirates

Abstract

A fluvial marine sabkha along the coastal area of Abu Dhabi, United Arab Emirates, is hypersaline from evaporative losses of groundwater originating from rain, seawater intrusion, lagoons that border the sabkha, and inland groundwater sources. Anhydrite (CaSO4) is present in these soils and is regarded to be both a neoformed mineral and a product of gypsum transformation. Six pedons (designated 1—6) were described, sampled, and characterized from a 13-km transect across the sabkha in order to better understand the distribution of anhydrite across the sabkha, determine suitable laboratory methods for detection and quantification of this mineral, and evaluate soil genesis and mineral formation. Soils were highly saline with electrical conductivity (EC1:2) ranging from 11 to 167 dS m -1. Evaporative minerals identified by x-ray diffraction include calcite, gypsum, halite, aragonite, and anhydrite. Together, salts, gypsum, and anhydrite composed 5—100% of the mineral matter of sabkha soils. Quantification of anhydrite was achieved by the difference in the acetone method (gypsum + anhydrite quantification) and low-temperature weight loss (for gypsum quantification). Both a thermal gravimetric analyzer (TGA) and an oven were tested for the latter procedure. The TGA method was found to provide the most reliable data, while the oven method yielded inconsistent results. Anhydrite was identified in the two sites (pedons 5 and 6) most distant from the coast, ranging up to 43% of the <2-mm fraction and occurring in thicknesses of 70 and 55 cm, respectively.