Micro-morphological analysis and estimation of paleoprecipitation in the Loess-Paleosol sequence, eastern Golestan Province

The study of loess sediments and loess-derived soils, which cover approximately 10% of the Earth's surface, is of considerable importance because of their potential to provide information on the paleoenvironment, particularly during the Quaternary. The northeastern region of Iran, with its unique climatic archive, is of significant importance as a repository of Quaternary period deposits, attracting much attention from Iranian and international scientists in recent decades. Considering that soil forms in response to interactions among the lithosphere, hydrosphere, and atmosphere, it potentially contains physical, biological, and chemical environmental information at the time of its formation. The process of becoming loess sediments into paleosol depends on various factors such as vegetation, weather, topography, microbial activity, and time, so paleosols have the potential to restore the paleoenvironment by maintaining the effect of these factors. Therefore, studying soils derived from loess can enhance our knowledge and understanding of past climatic conditions in the region and the world. Studying undistributed soils using microscopic techniques can provide valuable information for understanding the environmental and climatic conditions during the formation of derived soils from loess (paleosols). One of the quantitative methods for studying micromorphology samples is the use of indices such as MISECA, which indicate the degree of paleosol evolution. By using this index, we can have an approximate estimation of paleoprecipetation. The main aim of this research is to reconstruct the precipitation amount during the formation of late Pleistocene paleosols in the eastern part of Golestan province, using micromorphological studies.

In order to conduct this research, after field visits, a profile of approximately 55 meters was excavated to examine the identified loess-palaeosol sequences. Then, undisturbed samples were obtained from each horizon to prepare thin sections, and the samples were impregnated under vacuum conditions using a combination of resin, acetone, and catalyst. After the samples hardened, a thin section was cut from them and affixed to a glass slide. The thin sections were then polished using sandpaper until a thickness of 30 microns was achieved. The prepared thin sections were studied using a polarizing microscope.

Micromorphological analysis of the samples revealed that the presence of horizons rich in calcium carbonate and clay in well-developed paleosols has a significant relationship with increased precipitation.  Micromorphological studies showed that the coarse to fine (C/F) relative distributed results showed that the amount of it in loess horizons and weakly developed paleosols are 4/6 or 3/7 but this ratio in moderately and well-developed paleosols become smaller in comparison with loess horizons, indicating the weathering and leaching of primary minerals in loess sediments and their transformation into secondary minerals in paleosols. Furthermore, with increasing paleosol evolution, the microstructure of the samples changed from massive to subangular blocky and then to angular blocky. Therefore, the results of examined microstructure in the samples showed that it can be one of the important factors for identifying paleosols from loess horizons and paleoenvironment conditions. Based on the data obtained from the MISECA index, the degree of paleosol development ranged from weakly developed (MISECA index was smaller than 8) to well developed (MISECA index was bigger than 16). The estimated paleoprecipitation amount using this index showed that paleosols with weak development formed under climatic conditions with precipitation less than 500 mm per year, while paleosols with moderate and well developed formed under climatic conditions with average precipitation of 500 to 800 mm per year and more than 800 mm per year, respectively.

Considering that soils are formed as a result of interactions between the lithosphere, hydrosphere, and atmosphere, they potentially contain physical, biological, and chemical information about past environmental conditions. Therefore, by studying the Late Pleistocene loess-paleosol sequences in Golestan province, it is possible to reconstruct past climatic changes in northeastern Iran. The micromorphological investigation of the loess-paleosol sequence under study showed that the processes of transfer and deposition of clay and secondary calcium carbonate and the subsequent creation of horizons rich in clay and secondary calcium carbonate are among the most important pedogenic processes in well-developed soils. Micromorphology can be a useful tool for investigating the paleoclimate and paleoenvironments conditions during the formation time of paleosols. The results showed that the use of the MISECA index can be a valid and logical method for evaluating the degree of paleosol development.