Mineralogical and geochemical characteristics of vesicular horizons of desert soils in the Rayen region-Kerman

Desert soils are very different from the soils of other regions. Extreme temperatures, little water, and limitedplant cover create an unusual environment. Desert soils show a variety of distinctive morphologicalfeatures such as the presence of gravel surfaces, the development of organic and inorganic surface crusts,and the widespread occurrence of vesicular horizons beneath the gravel layer (Dixon, ۱۹۹۴). Vesicularhorizons are mainly associated with surface rock layer and, are formed in aeolian sediments accumulatedbeneath desert pavement and texture classes that are enriched in silt to fine sand size particles (Turk andGraham, ۲۰۲۰). Common features of vesicular horizons include the presence of numerous nearly sphericalvesicular pore throughout the horizon, blocky, prismatic or platy structure, fine and mostly silty texture,and accumulation of calcium carbonate and clay (Turk and Graham, ۲۰۲۰). In previous scientific references,the vesicular horizon is indicated by the symbol Av, which is used as a suffix v after the name of the masterA horizon (Sweeney et al., ۲۰۱۳). However, in new references (۲۰۱۷ onwards) these horizons are shown asa master horizon and marked with a V symbol due to their importance in the soils of arid regions (SoilScience Division Staff., ۲۰۱۷).The study area is near the Rayen region in the Kerman province located onan alluvial fan, which is covered with desert pavement. The soil moisture and temperature regimes arearidic and thermic, respectively (Soil Survey Staff, ۲۰۱۴). To take sample from vesicular horizon, first thedesert pavement was removed and then the vesicular horizon was sampled (see Figure ۱). The soil of thelower horizon of the vesicular horizon was also sampled to compare the measured parameters. X-raydiffraction analysis was performed on the powder sample of soil fraction smaller than ۲ mm to identify themineral components of the samples. From each clay sample, four treatments including saturation with Mg,saturation with Mg and ethylene glycol, saturation with K and saturation with K plus heat at ۵۵۰ °C wereprepared and analyzed using X-ray diffraction. In addition, the total concentration of major elements insoil samples was determined by EDXRF method using soil samples prepared in the form of Press pellets.X-ray diffraction of powder samples of particle sizes smaller than ۲ mm in the vesicular horizon and itslower horizon shows that there are minerals quartz, calcium carbonate (calcite), calcium and sodiumfeldspars, cristobalite and clay minerals such as smectite, illite and kaolinite in vesicular horizon. There isno significant difference between the dominant minerals of vesicular horizon and the underlying horizon.The intensity of quartz peak (۳.۳۴ angstrom) of the vesicular horizon is higher than of the underlyinghorizon. Since vesicular horizons are formed in soils rich in silt and fine sand, it is expected that quartz hasbeen added to this horizon by aeolian processes of particle transport (especially saltation) (Sweeney et al.۲۰۱۳). X-ray diffraction pattern of oriented samples of clay fraction of the vesicular horizon and underlyinghorizon showed that the main clay minerals include smectite, chlorite, illite and kaolinite (see Figure ۳). Bycomparing the X-ray diffraction patterns of the vesicular horizon and its lower horizon, it is observed thatthere is no difference between them and, both have similar minerals. According to the above mentioned, itcan be concluded that the source of wind sediments, from which the vesicular horizon is formed, are closeto each other and, are actually local sources. Sweeney et al. (۲۰۱۳) stated that the fine to medium sand inthe vesicular horizons is likely to be transported from the distal alluvial fan deposits near the vesicularhorizons through the saltation process. The total concentrations of silicon, calcium and sodium in thevesicular horizon are higher than in the underlying horizon (see Figure ۴). Higher concentrations of siliconin the vesicular horizon are probably related to higher amounts of quartz in this horizon. The results of Xraydiffraction in powder samples confirm the presence of higher amounts of quartz in the vesicularhorizon. The percentage of calcium in the vesicular horizon and its lower horizon are ۸.۳۷% and ۶.۳۶%,respectively; which is most likely related to higher amounts of calcite (CaCO۳) in the vesicular horizon.The results of chemical analysis showed that the amount of calcium carbonate equivalent of the vesicularhorizon and its lower horizon are ۱۸.۸% and ۱۳.۲%, respectively. Therefore, it is possible that calciumcarbonate was added to the vesicular horizon by aeolian processes but was not transferred to the lowerhorizons by using leaching process due to limited rainfall in the study region. In general, the sedimentsforming vesicular horizon appear to have originated from local sources and most likely from alluvial fansnear the vesicular horizon.

کلیدواژه ها:

Aeolian sediments ، Desert soils ، Rayen region ، Vesicular horizons