Modern pedogenesis on loess along a climate gradient in Northern Iran

Climate, topography, parent material, organism and time as soil forming factors, determine the dominantsoil forming processes and consequently the type of soils in a region. The south-eastern Caspian Sea regioni.e., Golestan Province, is characterised by a pronounced precipitation gradient of about ۸۰۰ mm year-۱over about ۸۰ Km distance from north (arid) to south (humid), while there is a negligible variation intemperature along this gradient. The extensive loess deposits in the area reach a thickness of about ۷۰ m inthe loess plateau of Golestan Province. The loess deposits of the study area contain several paleosolsshowing different degrees of weathering probably related of paleo-climatic conditions and duration of soildevelopment (Kehl et al., ۲۰۰۵). As based on pedostratigraphy and luminescence datings, the paleosolshave been correlated with interglacial and interstadial phases of the Middle to Upper Quaternary (Frechenet al., ۲۰۰۹). Knowledge on the pedogenesis of the modern soils along this precipitation gradient couldprovide insight on the formation of loess-paleosols sequences and the paleoclimate of the area. Pedogenesisof the surface soils along the south-north transect have been studied. The soils are all formed on loessdeposits. The soils were classified and the major physico-chemical properties, micromorphology, claymineralogy, magnetic susceptibility and C stable isotope geochemistry of the different horizons wereinvestigated (Khormali and Kehl, ۲۰۱۱; Sharifigarmdareh et al., ۲۰۲۰; Khormali et al., ۲۰۲۰) and correlationof the pedogenetic proxies with precipitation were carried out. In the northern regions showing more aridicclimate, Aridisols with gypsum and carbonate accumulations are formed. Organic matter accumulationand the formation of mollic epipedon are the dominant processes occurring in the steppe (rainfall of ۴۰۰-۶۰۰ mm). Calcification and formation of calcic horizons are typical for loess derived soils. In the northfacingslopes of the Alborz Mountain ranges with the forest vegetation and higher rainfall, deeper leachingof calcite and formation of the argillic horizon are dominant. Soils are mainly Alfisols. The soil pH andcalcium carbonate contents show a decreasing trend with precipitation, while soil organic carbon, claycontent and cation exchange capacity increase with rainfall. The silt content, however, shows a decreasingtrend with rainfall. Clay mineralogy of parental loess is illite> chlorite> smectite> kaolinite. From north tosouth, the relative proportion of smectite in soil horizons increases. Micromorphology provides evidencefor the formation of Bt horizons and intensified decalcification and increase of clay mobilization asindicated by b-fabrics with increasing rainfall. There is a significant positive correlation of MISECA indexwith precipitation which is suggested to be applied in the reconstruction of the paleoclimate in the area.Susceptibility measurements (χlf) are shown to be an efficient proxy for identification of soil formingprocesses (Sharifigarmdareh et al., ۲۰۲۰). Magnetic susceptibility values decrease within the differentpedons with depth and increase along the pedon in regions with higher rainfall. The calculation of thepedogenic susceptibility reveal an almost linear correlation with the amount of precipitation in the studiedregion. Based on the carbon isotope evidence, an increase in the proportion of C۴ biomasses at the expenseof C۳ shrubs and C۳ woody vegetation may be the result of increasingly arid conditions from south tonorth along the ecological gradient through northern Iran. Along a C۳-dominated climatic gradient innorthern Iran, δ۱۳C and δ۱۸O of the bulk soil values showed a significant relationship with mean annualprecipitation over the range from ۲۰۰ to approximately ۷۵۰ mm/yr (Khormali et al., ۲۰۲۰). It is concludedthat the pedogenic proxies all reveal the role of precipitation on the soil distribution along the transect. Thiscorrelation is also seen in the underlying paleosols and it appears very likely that pedogenesis ofinterglacial paleosols was governed by similar climate-controlled trends to those reflected in the modernsoils.

کلیدواژه ها:

Pedogenesis ، Loess ، Climate gradient ، Northern Iran