Plant-controlled supratidal anhydrite from Al-Khiran, Kuwait
We report here that in the supratidal zone of the Al-Khiran sabkha, diagenetic anhydrite occurs exclusively in association with the living halophyte Halocnemum strobilaceum (Pallas) M .B. This anhydrite has a hair cream-like consistency and the bulk of it has formed as pseudomorphs after gypsum crystals. On the death of the plant at the end of its life cycle, the anhydrite is hydrated back to gypsum. This two-way transformation results in a series of characteristic textures, whose preservation could be due to the comparative ‘youth’ of the sabkha, which is in the very earliest stages of evaporite diagenesis.
Primary gypsum nodules in a modern sabkha on the Mediterranean coast of Egypt
Nodules of anhydrite in Holocene sabkhas of the Arabian Gulf and Baja California have been used as analogues to interpret calcium sulfate nodules in ancient rocks to be of sabkha origin. Nodules and incipient enterolithic veins of gypsum occur in a modern sabkha in Eqypt about halfway between Alexandria and El Alamein, in a depression between a modern and a Pleistocene beach ridge. The displacive gypsum is apparently being precipitated from hypersaline calcium sulfate-saturated interstitial water that increases in salinity as it rises by capillarity from the water table to the surface. Calcium and sulfate ions seem to be derived mainly from dissolution of pre-existing lagoonal gypsum beneath the water table. The nodules occur within a supratidal sand unit of a sabkha sequence capped by a gray, saline soil on which grow clumps of halophytes, separated by salt-encrusted flats. This discovery shows that calcium sulfate nodules can develop (1) within sediments of a region where the climate is almost semiarid rather than very arid, (2) as primary gypsum rather than as anhydrite, and (3) as a consequence of redistribution of calcium sulfate.
Sulphur-Isotope Geochemistry of an Arid, Supratidal Evaporite Environment, Trucial Coast
In the interpretation of sulphur isotope data from ancient sulphates the effect of sulphur isotope fractionation by sulphate crystallization has been neglected. In the Abu Dhabi supratidal flat, dolomitization and concomitant sulphate precipitation are responsible for depleting > 90% of the original SO 4 -2 available in marine-derived pore brines at, or close to, halite saturation. This results in a fractionation of Δ34 Sc of between -3‰ and -4‰ in precipitated sulphates relative to the source marine waters, which have δS34 ≃ + 23‰.