Geochemistry of Carbon, Oxygen and Sulfur Isotopes in Soils along a Climotoposequence in Kerman Province, Central Iran

Carbon and sulfur isotopic data are helpful in understanding environmental conditions under which soils are formed. Soils from 18 sampling sites along a climotoposequence from higher Laleh Zar Mountains (4,351 m above sea level with xeric-mesic soil moisture-temperature regimes) to Shahdad depression (250 m above sea level with arid-hyperthermic soil moisture-temperature regimes) in Kerman Province, central Iran, were studied. δ 13C values of the soil organic carbon varied between -20.6 ‰ and -26.9 ‰ PDB. δ 13C values of pedogenic carbonates varied between -2.2 ‰ and +2.2 ‰ PDB and a decreasing trend was observed downward in the climotoposequence. A positive correlation (r= 0.6) was found between δ 13C and δ 18O values in the lower arid landscape, but a different relationship (r= 0.37) in the upper Mediterranean climatic positions was observed. It seems that the presence of more C3 plants under stress and more decomposition of organic matter in lower positions of the landscape has taken place. δ 13C values of soil organic carbon and pedogenic carbonates and their trend along the gradient showed that higher proportion of C3 plants at lower elevations and presence of more C4 plants in the past history of upper landscape positions may have been experienced. Mean δ34S value of 11.3‰ showed that lower Cretaceous sea sulfate may have been the source of sulfur in the area. Mean δ D and δ 18O values of -51.2‰ and +6.0‰ in gypsum crystallization water showed that direct deposition from a solution saturated with gypsum is the only probable mechanism of gypsum formation in the area.