Influence of the composition and properties of Khvalynian deposits on the evolution of soils of the Volga-Ural interfluve (based on the results of mineralogical and micromorphological studies)

In the northwestern part of the Caspian Plain, a comparison of macro-, micromorphological, granulometric, physico-chemical parameters and mineralogical quartz-feldspar cryogenic contrast coefficient (CCC) was carried out for two different types of soils underlain by layered sediments. The values of the СCC for the underlying sediments are ≥1, which indicates cryogenic transformations of the sediment. The abrupt distribution of the ССС depending on the texture class of layers (ССС <1 for loamy material, ССС≥1 for clay) of layered chocolate clays near Lake Elton in the Fox Dry Valley outcrop (+12 m altitude) indicates the seasonal introduction and deposition of material from the upper part of the Lower Khvalynian marine sediments into reservoirs with weak flow at the boundary of the Holocene-Pleistocene. For the territory with of +26 m altitude (Janybek station), loess-like layers with CCC≥1 were also identified, in which inclusions of fragments of chocolate clays are similar to clays near Lake Elton. This is evidence that the deposits of layered chocolate clays from Fox Dry Valley are one of the sources of the lower loess-like layers of sediments on the territory of Janybek station. In the compared soils at the same depth (about 100 cm), signs of synlithogenic cryoarid pedogenesis were noted: granular structure and gypsum accumulations with signs of dissolution and recrystallization. Different types of soils were formed in the surface “warm” layers of sediments (CCC<1), which is associated with different factors of soil formation in modern and paleoclimatic conditions of the Holocene. We assume that after the stage of increased atmospheric humidification in the chronointerval 3500– 3000 years ago, the development of surface soils occurred in different ways. On the undrained territory of the station, with a shallow occurrence of saline water-table, a characteristic three-component solonetz complex with a microrelief was formed, including the studied soil pit of the Gypsic Solonetz on a micro-elevation. On a drained flat surface near Lake Elton, the washing of the soil from easily soluble salts to a depth of 70 cm marks the stage of increased atmospheric moisture. The carbonate content of loess-like “warm” sediments, deep water-table and modern arid pedogenesis did not allow the solonetzic pedogenesis to manifest in this area. As a result, Haplic Calcisol was formed on this surface.  

Since the 2000s, there has been an increase in climate aridity on the Janybek plain, accompanied by a slight increase in winter precipitation, which causes deeper spring soil wetting. As a result, in the upper 50(70) cm of the studied soils, increased biogenic activity and increased humus accumulation, removal of soluble salts and redistribution of calcium carbonate were noted. The microrelief determines the relative severity of these processes. 

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