The transformation of mineral composition of permafrost soils formed on sedimentary deposits of the northern taiga in Central Siberia

In recent years, soil formation processes have been actively studied in areas with harsh climatic conditions and dominating post-lithogenic soils. It is assumed that there is no active accumulation of fresh material in these soils and there is no differentiation of chemical composition along the horizons. The prediction of intensity and direction of soil processes is crucial due to climate warming and the expansion of mineral resource extraction. One of the least studied soil parameters is the mineralogical composition of coarse fractions and its changes during soil formation. The unique geological structure and rich mineralogical composition of deposits, combined with harsh climatic conditions, result in highly specific mineral transformation processes during cryogenic weathering. The paper reflects the possibility of using a geochemical approach to analyse the transformation of the soil column composition in relation to the genetic horizons and the parent rock. For the research on the territory of the Daldyn-Alakit mining district (66°24'N, 112°17'E) located in the northeastern part of the Central Siberian Plateau, soil sections of the most predominant soil types (Cryosols, Gleysols, and Leptosols divisions) have been established. The soils are formed on eluvial-deluvial-solifluction deposits with close occurrence of bedrock, represented by dense dolomites, yellowish limestones and oolitic dolomite limestones. The climate of the region is strongly continental with long cold winters and short summers, the average monthly temperature in January is -33...-38°C, in July +16...+18°C, the amount of precipitation is low (about 247 mm). The most typical pedons (See Fig. 1), physicochemical properties, and bulk composition of the soils (See Tables 1, 2) are described along with X-ray diffraction (XRD), thermogravimetric (TGA), and scanning electron microscopy (SEM) analyses (See Figs. 2-4). The main physicochemical indicators include a slightly acidic reaction in the upper organogenic horizon and a near-neutral reaction lower in the profile, relatively high carbon content (up to 10% in the upper and 1-3% in the lower horizons), and a heavy granulometric composition (See Table 1). The bulk chemical composition of the studied soil types in the Daldyn-Alakit mining district is quite similar (See Table 2). By weight, the major oxides follow this decreasing order: SiO2 > Al2O3 > CaO > MgO > Fe2O3 > K2O > TiO2 > Na2O > MnO. The behavior of dominant oxides in the soils depends not on the subtype or main soil-forming process but correlates well with organic matter content. The studied soil types are similar in mineralogical composition, consisting of quartz, feldspars, carbonates, and layered silicates. Scanning electron microscopy revealed common micromorphological features of pedogenesis. Soil aggregates in Cryosols and Gleysols have a more rounded shape and smaller size (about 10 pm) in the upper horizon compared to the lower one. The thermograms of the soils differ in exothermic and endothermic effects, reflecting key genetic characteristics within each soil type. The predominance of humic acid combustion in Cryosols, compared to other soils, indicates more intensive decomposition of plant residues due to microbial activity (See Figs. 2-4). For the geochemical characteristics of the soils, calculations of oxide ratios and a series of coefficients (chemical weathering in soils of G.J. Retallack, CIA and ICV) were carried out on the basis of gross chemical analysis data. The weak eluvialilluvial differentiation is also evident in the molar ratios of SiO2 ÷ R2O3 and R2O ÷ Al2O3, with differences between horizons observed in the RO ÷ Al2O3 ratio. Overall, biological activity and productivity in permafrost-affected soils of the northern taiga are low, but the calculated coefficient values increase in the upper organogenic horizons compared to mineral horizons (See Table 3). Analysis of geochemical coefficients revealed soil heterogeneity relative to the parent material at the level of morphogenetic characteristics (See Figs. 5a, 5b). Soil-forming processes are least pronounced in Leptosols, while Gleysols occupy an intermediate position. Cryosols are the most sensitive to pedogenesis. The main morphogenetic differences occur between soils with higher organic carbon content. The use of geochemical coefficients helps identify changes in the mineral composition of permafrost-affected soils and more distinctly reflects the differentiation of post-lithogenic soil profiles. All studied soils exhibit relatively low weathering indices, indicating minor alterations in mineral composition. However, an increase in the clay fraction relative to the parent material is observed. Biochemical weathering is likely to play a greater role in the transformation of the mineral soil component. In terms of the degree of soil material transformation, the investigated permafrost-affected soils of the northern taiga landscapes can be ranked as follows: Leptosols → Gleysols → Cryosols. The article contains 5 Figures, 3 Tables, 44 References. The Authors declare no conflict of interest.

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