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 hydratio n water of gypsum may provide very useful information, particularly ab out the paleo-hydrological conditions in soils and sediments. Little i nformation is available on the stable isotope geochemistry of gypsum h ydration water in arid regions. This study was initiated to measure th e delta D and delta(18)O values of the gypsum crystallization water in different landforms to determine their relationship with meteoric wat er and to identify the possible mechanism(s) for the large gypsum accu mulation in the Isfahan region, central Iran. Samples from different h orizons of twelve pedons covering two adjacent soil landscapes were an alyzed. The delta D and delta(18)O values of eight rainwater samples f rom different seasons and water samples from the Zayandehrud river in the area were also determined. The mean and standard deviation of delt a D and delta(18)O values of gypsum hydration water were -68.75 +/- 5. 51 parts per thousand and +1.11 +/- 1.68 parts per thousand for colluv ial fans and -56.5 +/- 7.28 parts per thousand and +4.47 +/- 2.07 part s per thousand for plateaus. The slope of Delta delta D/Delta delta(18 )O = +3 and the significant positive correlation (P < 0.01) between th e isotopic composition of the gypsum crystallization water (both delta D and delta(18)O) and the amount of gypsum in the soils indicated tha t evaporation is the major process for the accumulation of gypsum. The delta D and delta(18)O 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 s till preserved (partly or entirely) in the gypsum hydration water. Dif ferences found in the isotopic composition of gypsum hydration water, with respect to landscape position, may be useful to establish, more a ccurately, segments of a landscape in arid environment.