Saturday, 15 July 2006

Micromorphology, SEM and Genesis of Saline and Gypseous Soils of Bam Area , Southeastern Iran.

Ebrahim Moghiseh, Tehran Univ, Emam Zadeh Hasan Agriculture College , Soil Div, Karaj, Iran

Gypseous soils are mostly widespread soils of arid and semiarid regions of the World. However, there is still limited information on the mechanism of genesis, morphology and micromorphology of gypsum in those soils. Our investigation has been carried out to characterize the existed forms of gypsum crystals and more soluble salt minerals in some soils from southeastern Iran. Eight profiles were described according to the Soil Survey Manual (1993). A saline gypsic horizon (2Byz) with 60% gypsum and 130 dS/m EC was observed at a depth of 5-15 cm and a saline petrogypsic horizon (3Byzm) with 20% gypsum and a higher EC (180dS/m) was found below the 2Byz horizon. Beneath the 3Byzm horizon in all studied pedons, lies the 4Byz horizon with lower contents of gypsum and reduced EC values. The soils were classified as Petrogypsic Haplosalids according to the Soil Survey Staff Manual (2003). Physico-chemical and micromorphological characteristics were determined using standard methods, whereas SEM coupled with EDS were used for detailed determination of morphology and mineral compositions. We observed common pedogenic acicular and elongated fibrous crystals of gypsum in the 2Byz horizon, lenticular gypsum in the 3Byzm and elongated gypsum pendants with layered structure in thin sections of the 4Byz horizons. Cubic Halite crystals as infillings of voids and engulfed by gypsum crystals were observed in thin sections in the 3Byzm horizon as also determined in the studies of Hanna and Stoops (1976) and Amit and Yaalon (1996). However, its high concentration and the engulfment by the surrounding gypsum crystals may have facilitated the preservation and so the detection in the thin sections. Taking into account the variability of neoformed gypsum in different soil horizons and their occasional co-existence along with more soluble salt minerals, we may suggest that, (1) gypsum pendants, interlocked fibrous gypsum crystals at the 4Byz horizon formed under the past more humid climate, (2) while the 2Byz and 3Byzm horizons form under the present climate. The solubility of gypsum in the 3Byzm horizon is controlled by low annual precipitation and high temperature, as well as by gypsum and halite enriched parent materials and the ionic effect of NaCl. A capillary rise of dissolved gypsum along with halite and their following redistribution due to evaporation causes the 2Byz horizon to form near the soil surface. SEM/ EDS analysis supports the hypothesis of an upward movement of gypsum and soluble salts from the upper parts of the 3Byzm horizon. Alternately, rapid leaching of more soluble salts into 3Byzm horizon causes a higher concentration of gypsum at the overlying 2Byz horizon. Keywords: Saline gypseous soils, Micromorphology, SEM, EDS, Halite, Gypsum, Petrogypsic

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