Micromorphology and development of loess-derived surface and buried soils along a precipitation gradient in Northern Iran

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The northern footslopes of the Alborz Mountains and an extensive hill area in eastern Golestan Province are covered by thick (∼70 m) loess deposits, divided by different types of interstadial and interglacials paleosols. The area shows a precipitation gradient of about 800 mm over 80 km north-south. Along this gradient, eight pedons of modern soils and six paleosols were selected in order to investigate the types of modern and past soil forming processes and to compare the degree of soil development of surface and buried soils. The soils were described and sampled using standard sedimentological/pedological laboratory analysis. The soil pH and calcium carbonate contents show a decreasing trend with precipitation, while soil organic carbon, clay content and cation exchange capacity increase with rainfall. The silt content, however, shows a decreasing trend with rainfall. Clay mineralogy of parental loess is illite > chlorite > smectite > kaolinite. From north to south, the relative proportion of smectite in soil horizons increases, reaching almost dominance in regions with typic xeric soil moisture regime (SMR; ∼600 mm of rainfall). In the areas with udic SMR, vermiculite increases and sometimes dominates. Micromorphology provides evidence for the formation of Bt horizons and intensified decalcification and increase of clay mobilisation as indicated by b-fabrics with increasing rainfall. Soils within the xeric and udic SMRs are either Alfisols or Mollisols, both showing clay illuviation features, whereas clay mobilisation is very limited in the arid part of the area. The paleosols show differential weathering degrees indicated by trends of increasing carbonate depletion, clay mobilisation, and clay enrichment with assumed paleo-precipitation. Pedogenesis, micromorphological properties and clay minerals in the last interglacial paleosols suggest similar climate controlled trends to those reflected in the modern soils.