Sulfur isotope geochemistry of gypsiferous Aridisols from central Iran

Abstract

Gypsum accumulation is one of the prominent pedogenic processes occurring in many and regions of the world. Gypsiferous soils occur in large areas of the Iranian central plateau. The origin of gypsum in the Aridisols of central Iran and its distribution in different landscapes were studied using sulfur and oxygen isotopic composition of both solid and dissolved sulfates. The results strongly support the hypothesis that the area was cut off from the Tethys seaway at the end of the Mesozoic era and, as a result, the Lower Cretaceous sulfate has controlled the S geochemistry of the younger sediments, including the soils studied. The mean values of δ 34S and δ 18O of the soil gypsum and 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 SO 4 and SO 4 in gypsum have not undergone any reduction and re-oxidation reactions. Dissolution and reprecipitation appear to be the possible mechanisms responsible for the observed minimal isotope fractionation. Both δ 34S and δ 18O values of the soil gypsum decrease slightly with decreasing elevation in the study area. These findings are important for understanding the landform evolution in this arid region.