Geochemical characterization of the forest loess soils along a precipitation gradient in Northern Iran

Introduction: The loess-paleosol sequences in Northern Iran are important archives represent several cycles of Quaternary climate change and can be used to complete the information gap on loess between Europe and central Asia. Loess geochemistry reflects paleoweathering conditions and it can be used to determine the nature and provenance of loess deposits. In the Caspian Lowlands, a pronounced precipitation gradient is reflected in mean annual precipitation rates decreasing from about 1850 mm at Bandar Anzali in the West to about 435 mm at Gonbad- e Kavoos in the East. The results of the along a loess climosequence in Northern Iran showed that with increasing precipitation, soil pH and calcium carbonate contents decreases, whereas soil organic carbon, clay content, and cation exchange capacity increases. Many studies have been carried out on the loess-paleosol sequences and modern loess soils in Northern Iran but more investigation is needed with emphasis on the forest soils, their geochemical properties and provenance. Materials and methods: The study area is located in the northern slopes of Alborz Mountain Ranges covered with Caspian or Hyrcanian deciduous forests. Field sampling started in summer 2015. More than ten soil pedons with loess parent material were investigated based on former studies. Finally, six representatives modern pedons were selected and dug in an east-west direction on loess deposits. The climate data shows that precipitation varies from 500 mm in Qapan to up to 800 mm in Neka. Physiochemical properties of soils were studied using standard methods. Element concentrations measured on the fine-grained fractions. These extracted by sieving out material <63 µm and drying it at 105 °C for 12 hours. Eight g of the sieved material mixed with 2 g Fluxana Cereox, homogenized with a vibratory disk mill and pressed to a pellet with a pressure of 19.2 MPa for 120 s. Powdered press pills were analyzed for trace elements (V, Cr, Co, Ni, Zn, Rb, Sr, Zr, Nb, Ba, Nd, Pb) and fused discs for major (Na2O, MgO, Al2O3, P2O5, K2O, CaO, TiO2, MnO, Fe2O3) elements using energy dispersive polarized Spectro Xepos X-ray fluorescence (XRF) device. Every sample measured twice, rotating the pellet in between. Subsequently, mean values calculated from the two measurements. Results and discussion: The results showed that the downward decalcification and the subsequent clay illuviation were the main criteria influencing assessment of soil development in this study. So, all of the soils host argillic and calcic horizons and are classified as Alfisols and Mollisols. All studied pedons had nearly uniform chemical composition in their parent materials, suggesting similar alteration history of these sediments. In general, all parent materials showed strong depletions of mobile element (Na) and weak depletion of Sr and Mg in UCC-normalized spider diagrams. Ca was distinctly enriched in loess compared to UCC. Variation trend of major and trace elements concentration in the studied regions was more similar to average of world loess composition (AWL), but a little different from PAAS (post-Archaean average Australian shale). The significant depletion in Ca and Sr in Bt horizon compared to their parent materials, evidenced in all pedons that was in line with present precipitation gradient and attributed to similar geochemical behavior of Ca and Sr suggesting the alteration of plagioclase. Also, slightly depletion trend of Na and P was in accordance with precipitation gradient. It seems that the absolute concentration of the major elements, especially Si in the studied soils, was influenced by dilution effect of carbonates and consequently amount of this element was significantly lower than AWL and PAAS chemical composition. Chemical Index of Alteration (CIA) values of the samples vary from 62 to 75, higher than those of the UCC (CIA~48) suggesting moderate degree of weathering in studied pedons. The spread of data in A–CN–K ternary diagram space parallel to A–CN line close to the plagioclase weathering trend, revealing comparatively stable K-feldspar/plagioclase ratios in loess.   Conclusion: In general, these results support the important influence of parent material on soil geochemistry in studied areas. Additionally, calcification as prevailing pedogenic processes in these areas, can effectively affect elemental distribution within soils. So, the variation of elemental composition and distributions within soil profiles are influenced by parent materials and pedogenic processes. In addition, chemical index (CIA), elemental ratios (Ba/Sr, Rb/Sr) and the A-CN-K ternary diagram indicate a moderate degree of chemical alteration for these soils, compatible with the weathering regime prevailing in the region.