High-resolution mineralogical record of soil genesis and dust influx in a relict palaeosol

Evidence of profound climatic changes and wetter conditions during the late Quaternary are mentioned by several authors for the deserts of central Iran (e.g. Khademi and Mermut, 1999; Jalilian et al., 2022). The region today is strongly influenced by aeolian and desertification processes which are mainly attributed to human activities. To examine the role of chemical weathering (under moist conditions) and long-term dust influx (under dry conditions) on soil genesis, we studied the mineralogical composition of soil materials in a relict paleosol of an arid region of eastern Isfahan, central Iran. A high-resolution sampling strategy (10 cm interval) and qualitative and quantitative X-ray diffraction method were applied. The paleosol is located on an upper terrace with a flat surface having a gravelly structure and neither groundwater influence nor input of materials from adjust landforms. The results showed that quartz, calcite, Na-plagioclase and chlorite are dominant minerals in the clast fraction of the paleosol. The comparison of the mineralogical composition of soil materials and gravels revealed that K-feldspar, gypsum, smectite and palygorskite in the soil matrix were not inherited from the gravels but were provided by dust influx and/or pedogenesis processes. K-feldspar was absent in the gravels and was added by dust influx as its neoformation in the soil environment is unlikely. This hypothesis is supported by the exponential increase of its amount towards the soil surface and the maximum accumulation of the mineral in the surface dust-derived (vesicular) horizon. Smectite is also absent in both the clast fraction and the vesicular horizon and showed a maximum abundance in the middle and lower parts of the pedon where pedogenic calcite deposition occurred ~29 ka, suggesting a pedogenic origin of the mineral under the semiarid and seasonal climate. Palygorskite is absent in gravels but occur in the surface vesicular horizon and was relatively uniformly distributed throughout the pedon. It seems that palygorskite has both exogenic (from dust) and endogenic (by pedogenic processes) sources in the paleosol. Scanning electron microscopy (SEM) images support this postulation. SEM images exhibited dense fibers of palygorskite in the soil matrix and broken and small pieces of palygorskite in the dust-derived horizon. The investigated paleosol provided evidence of environmental changes from a semi-arid and seasonal climate during the time of smectite pedogenesis to an arid and dust deposition environment. Consequently, the palaeosol exhibited a mostly natural trend of aridification and desertification in this region during the late Quaternary.

Jalilian, T., Lak, R., Taghian A. and J. Darvishi Khatooni, 2022, Evolution of sedimentary environments and geography of the Gavkhouni Playa during the Late Quaternary, International Journal of Environmental Science and Technology, 19, 1555–1572.

Khademi, H. and A. R. Mermut, 1999, Submicroscopy and stable isotope geochemistry of carbonates and associated palygorskite in Iranian Aridisols, European Journal of Soil Science, 50 (2), 207-216.