干旱区生态环境知识资源中心

Gypsiferous and saline soils are among the major soils of arid and semi-arid regions of the world. Although numerous studies on salic, gypsic and petrogypsic horizons have been carried out, the co-occurrence of gypsum and halite and their morphological expression are still poorly documented. Eight pedons located on a co-alluvial fan (Bam area, southeast Iran) were described, sampled and analyzed for physicochemical and micromorphological characteristics based on standard methods. The highest amounts of gypsum (approximate to 60 %) comprising xenotopic gypsum and/or fibrous bassanite pseudomorph remaining behind after xenotopic gypsum dehydration were determined in the surface crust and in the underlying 2Byz horizon. At a depth of 15 cm, a horizon cemented by gypsum and halite was observed. The highest amount of gypsum was determined at the upper part of this horizon followed by a sharp decreasing trend towards the lower depth. The amount of halite increases with increasing depth towards the bottom of 3Byzm horizon. Deeper, in the 5Byz horizon the quantity of gypsum increases drastically and coarse elongated gypsum pendants dominate. Micromorphological observations demonstrate that the dominant cementing agent is halite rather than gypsum. However, due to inexistence of petrosalic diagnostic horizon in Keys to Soil Taxonomy, these soils are to be classified as Petrogypsic Haplosalids at subgroup level in Soil Taxonomy. In WRB Taxonomy, they are classified as Petrosalic Solonchaks. Co-occurrence of gypsum and halite in the same horizon, their specific layering and vertical distribution patterns in the studied pedons might be considered as indicators for polygenetic soils in this area.