Isotopic composition of gypsum hydration water in selected landforms from central Iran

During its formation, hydration water of gypsum may be preserved, due to lack of sufficient contact with soil moisture in arid environments. If this hypothesis is valid, the isotopic composition of the hydration water of gypsum may provide very useful information, particularly about the paleo-hydrological conditions in soils and sediments. Little information is available on the stable isotope geochemistry of gypsum hydration water in arid regions. This study was initiated to measure the δD and δ18O values of the gypsum crystallization water in different landforms to determine their relationship with meteoric water and to identify the possible mechanism(s) for the large gypsum accumulation in the Isfahan region, central Iran. Samples from different horizons of twelve pedons covering two adjacent soil landscapes were analyzed. The δD and δ18O values of eight rainwater samples from different seasons and water samples from the Zayandehrud river in the area were also determined. The mean and standard deviation of δD and δ18O values of gypsum hydration water were −68.75±5.51‰ and +1.11±1.68‰ for colluvial fans and −56.5±7.28‰ and +4.47±2.07‰ for plateaus. The slope of ΔδD/Δδ18O=+3 and the significant positive correlation (P<0.01) between the isotopic composition of the gypsum crystallization water (both δD and δ18O) and the amount of gypsum in the soils indicated that evaporation is the major process for the accumulation of gypsum. The δD and δ18O values confirmed that gypsum was formed likely before the Quaternary era and paleowater, from a slightly more moist and/or colder climate, or different climatic pattern in the past, is still preserved (partly or entirely) in the gypsum hydration water. Differences found in the isotopic composition of gypsum hydration water, with respect to landscape position, may be useful to establish, more accurately, segments of a landscape in arid environment.