Introduction: The Earth's surface is directly influenced by climate and environmental conditions, resulting in the preservation of diverse information during soil formation and transformation processes. Paleosols, like modern soils, exhibit characteristics that reflect the environmental conditions prevailing during their formation, making them powerful tools for reconstructing past environmental conditions. In recent years, paleosols have become recognized as reliable indicators enabling scientists to reconstruct environmental and climatic changes. The Quaternary period, characterized by alternating cold glacial and warm interglacial cycles, represents one of the most significant periods of climatic fluctuation in Earth's history. These changes are well-preserved in various archives including marine sediments, lacustrine deposits, and continental loess-paleosol sequences. Loess-paleosol sequences, in particular, are considered invaluable archives for reconstructing Pleistocene-Holocene climate variability. Loess deposits, which cover approximately 10% of the Earth's land surface, consist primarily of fine-grained wind-transported sediments that accumulated during cold, dry glacial periods. These deposits frequently appear as alternating sequences with paleosols that developed during warmer, more humid interglacial and interstadial periods. The loess-paleosol sequences of northern Iran, particularly in the Golestan Province along the southern Caspian Sea coast, represent a critical part of the Eurasian loess belt extending from northwestern Europe to Central Asia and China. These sequences, reaching thicknesses of up to 30 meters along the Alborz Mountains and approximately 60 meters in the northern Iranian loess plateau, provide exceptional archives for Quaternary studies. Approximately 17% (3,200 km²) of the Golestan region is covered by loess deposits. These sediments not only provide detailed information on past climatic changes but also serve as a crucial link between Central Asian and European archives. Soil structural changes can be investigated using various proxies influenced by climate, with soil micromorphology being one of the most precise approaches. In recent decades, micromorphology has gained significant importance in paleoclimate and geological studies as an effective tool for elucidating pedogenic processes and reconstructing past environmental conditions. This science focuses on microscopic examination of pedological features, providing valuable genetic information about soil structure including the analysis of relationships between mineral components, particles, and voids, and their changes over time. The MISECA index (Micromorphological Soil Development Index), originally developed by Khormali et al. (2003) for evaluating argillic horizon development in calcareous soils of arid and semi-arid regions, serves as a semi-quantitative tool for assessing soil development degree. This index incorporates micromorphological criteria including microstructure, b-fabric, clay coatings, decarbonated zones, iron and manganese oxides, and mineral weathering degree, with scores ranging from 0 to 24. The Sa'adabad 3 loess-paleosol sequence, located in the northern slopes of the Alborz Mountains near Gorgan, represents a particularly promising archive for investigating Pleistocene-Holocene climatic fluctuations. This study aims to reconstruct paleoclimatic and paleoenvironmental conditions of the Sa'adabad 3 sequence using integrated micromorphological analysis and the MISECA development index, and to evaluate the applicability of these methods for estimating paleoprecipitation in this region.
Methodology: The study area, Sa'adabad 3, is located on the northern slopes of the Alborz Mountains in Golestan Province, near Gorgan city, at coordinates 54°22'30" E longitude and 36°49'23" N latitude. The region has a mean annual precipitation of 620 mm and is classified as temperate and semi-humid, with xeric and thermic soil moisture and temperature regimes. The sequence, approximately 12 meters thick, comprises 28 distinct horizons. Following field reconnaissance, the Sa'adabad 3 sequence was selected as the most complete loess-paleosol sequence in the region. Scaffolding was installed for precise sample collection from accurate heights. Soil horizons were classified according to the USDA Soil Taxonomy (Soil Survey Staff, 2014) and WRB (2006) systems. Undisturbed samples were collected from each horizon for thin section preparation. For micromorphological sample preparation, undisturbed soil samples were air-dried and impregnated with polyester resin. A mixture of 60% polyester resin with 40% acetone as thinner was used, with 20 drops of stearic acid as catalyst and 10 drops of cobalt hardener added. Impregnation was performed in multiple stages using a vacuum pump in a desiccator to remove trapped air. After hardening, samples were cut into 1 cm-thick slabs, with one surface ground and polished flat. Polished surfaces were mounted on frosted glass slides using a mixture of resin, hardener, and catalyst. Mounted samples were cut to approximately 2 mm thickness using a cutting machine, then ground to 70-100 μm thickness, and finally polished to approximately 30 μm thickness following Murphy's methodology. Micromorphological descriptions were conducted using a polarizing microscope under plane-polarized light (PPL) and cross-polarized light (XPL), following the methodologies of Bullock et al. (1985), Stoops (2003), and Stoops et al. (2018). The MISECA index was calculated by assigning weights to micromorphological parameters including b-fabric, frequency of clay coatings, decarbonated zones, microstructure, iron and manganese oxides, and mineral weathering degree. The cumulative score determined the soil development degree, with higher scores indicating more developed soils.
Results: Micromorphological analysis of the Sa'adabad 3 sequence revealed distinct pedological features across the loess and paleosol horizons. Microstructure analysis showed massive microstructure in CBk, BCk, CBkk, and BCkg horizons, while weak to strong angular and subangular blocky microstructures dominated the paleosol horizons (Btkgss2, BAk1, BAtkg1, Bk1, Btk, Btkgss1). The transition from massive to well-developed blocky microstructure reflects increasing pedogenic development under more humid conditions. Void analysis identified channels, chambers, vughs, and planar voids (planes) in the sequence. Paleosols exhibited abundant channels, chambers, and planes indicating intense biological activity (roots and fauna), while loess horizons were dominated by channels and vughs. The b-fabric was predominantly crystallitic (calcitic) throughout most horizons, with speckled b-fabric occurring in some argillic horizons, and a combination of both types observed in certain horizons. Coarse fragments were dominated by subangular to angular quartz grains. Clay coatings (clay cutans) were observed as orange-colored features around voids and within the soil matrix, predominantly in argillic horizons (Btkgss, Btkg, Btk, BAtkg), indicating active clay translocation (lessivage) under humid conditions. Carbonate pedofeatures included nodules, concretions, coatings and hypocoatings, needle-fiber calcite, micrite, and sparitic calcite. Needle-fiber calcite indicated suitable moisture conditions and decomposable organic matter in paleosols. Calcitic crystallitic b-fabric resulted from carbonate leaching and reprecipitation in lower horizons. Decarbonated zones were observed, indicating carbonate dissolution and translocation. Micritic and sparitic calcites represented different precipitation rates and time scales. Iron and manganese pedofeatures included coatings, hypocoatings, quasi-coatings, and nodules, formed under alternating oxidation-reduction conditions associated with periodic wetting and drying. Biological remains included mollusk shell fragments (more abundant in loess horizons indicating cold, dry conditions) and faunal excrements (indicating suitable environmental conditions for biological activity). The MISECA index classified soil horizons into well-developed, moderately developed, and weakly developed categories. Horizons Btkgss (2350-2250 cm), Btkg (2195-2250 cm), Btk3 (1390-1440 cm), BAtkg (1882-1970 cm), Btkg (1600-1670 cm), and Btk (1390-1520 cm) were classified as well-developed soils. Decarbonated zones, iron and manganese oxides, and clay coatings had the greatest influence on the index and soil development. Based on the MISECA index and correlation with precipitation graphs established by Khormali et al. (2012), Taheri et al. (2016), and Shahriari et al. (2018), annual precipitation estimates for the Sa'adabad 3 region were derived. The most developed horizons (Btkgss1 and Btk1) indicated estimated annual precipitation of approximately 741 mm, while the least developed horizon (BCkg) indicated 470 mm. In horizons Bkkm and CBk, where simultaneous soil distribution and loess particle deposition occurred, precipitation decreased to approximately 370 mm.
Conclusion: Integration of micromorphological features and the MISECA development index for the Sa'adabad 3 sequence provides robust evidence for multiple-stage climatic fluctuations during the Pleistocene-Holocene period in northern Iran. The observed variations in horizon development degree, pedofeature types, and patterns of mineral accumulation or depletion indicate significant climatic oscillations during paleosol formation. The MISECA index demonstrates a strong positive correlation with mean annual precipitation, enabling quantitative estimation of past rainfall conditions. Horizons with low index values, characterized by weak structure, primary carbonate accumulations, and limited evidence of clay translocation, represent weakly developed soils formed under semi-arid climatic conditions with low weathering intensity and short pedogenesis duration. Conversely, horizons with higher index values, exhibiting evidence of active deep leaching, chemical weathering, and advanced soil structure development, indicate more advanced pedogenesis under semi-humid to humid conditions. The presence of argillic horizons with clay coatings (clay cutans), angular to subangular blocky structure, and weak reaction to hydrochloric acid indicates active clay translocation (lessivage) and gradual carbonate removal from upper horizons through water infiltration. These features typically develop under conditions of sufficient moisture, denser vegetation cover, and longer pedogenesis duration, indicating relatively stable humid climatic conditions during interglacial periods. The occurrence of needle-fiber calcite in paleosols provides further evidence for suitable moisture conditions and decomposable organic matter, consistent with warmer and more humid climates. Iron and manganese pedofeatures, including coatings and nodules, indicate alternating oxidation-reduction conditions associated with periodic water saturation and drainage, reflecting climate variability. Mollusk shell fragments in loess horizons indicate cold, dry glacial conditions, while faunal excrements in paleosols reflect favorable environmental conditions for biological activity. The estimated precipitation values (370-741 mm) derived from the MISECA index demonstrate that the Sa'adabad 3 region experienced progressive climatic shifts from semi-arid to semi-humid and ultimately to humid conditions during paleosol formation. These climatic changes likely occurred in response to periodic fluctuations in precipitation, temperature, and vegetation expansion or retreat during Quaternary interglacial and post-glacial periods, providing the necessary conditions for advanced genetic and structural soil evolution. This research confirms that integrated micromorphological analysis and the MISECA index are powerful and reliable tools for paleoclimate reconstruction in loess-paleosol sequences, with significant implications for understanding regional climate dynamics and their impact on landscape evolution in northern Iran. The findings contribute to the broader understanding of Quaternary climate variability in the Eurasian loess belt and highlight the importance of northern Iranian loess-paleosol sequences as exceptional archives for paleoenvironmental research.