SULFUR ISOTOPE GEOCHEMISTRY OF GYPSIFEROUS ARIDISOLS FROM CENTRAL IRAN

Gypsum accumulation is one of the prominent pedogenic processes occurr ing in many arid regions of the world. Gypsiferous soils occur in larg e areas of the iranian central plateau. The origin of gypsum in the Ar idisols of central Iran and its distribution in different landscapes w ere studied using sulfur and oxygen isotopic composition of both solid and dissolved sulfates. The results strongly support the hypothesis t hat the area was cut off from the Tethys seaway at the end of the Meso zoic era and, as a result, the Lower Cretaceous sulfate has controlled the S geochemistry of the younger sediments, including the soils stud ied. The mean values of delta(34)S and delta(18)O of the soil gypsum a nd of dissolved sulfate match with the average isotopic values of the marine sediments which are the most common soil parent materials. The results also confirm that S and O in dissolved SO4 and SO4 in gypsum h ave not undergone any reduction and re-oxidation reactions. Dissolutio n and reprecipitation appear to be the possible mechanisms responsible for the observed minimal isotope fractionation. Both delta(34)S and d elta(18)O values of the soil gypsum decrease slightly with decreasing elevation in the study area. These findings are important for understa nding the landform evolution in this arid region. (C) 1997 Elsevier Sc ience B.V.