Diagnostics of modern and relict soil processes in solonetz complex of the Caspian lowland.
Authors: Churilin Nikita , Lebedeva Marina , Varlamov Evgheni 
Published: March 2022

&lt;p&gt;We made a comprehensive mineralogical and micromorphological study of soils in the area with unexpressed microrelief (nano-depressions with a height difference of 5-10 cm). The soils are located on the territory of the Urdinsko-Torgunskaya elevated flat plain (+32 m A.S.L.) in the northern part of the Caspian lowland.&lt;/p&gt;&lt;p&gt;In solonetz at a depth of 46 cm appear Fe-Mn patches, carbonate formations and gleying indicators. Salinization begins at a depth of 25 cm. In kastanozem humus cutans and streaks along large pores appear from 30 cm. Carbonate formations appear at 50 cm and below they are represented by white carbonate soft spots. The soil is washed from salts up to 200 cm.&lt;/p&gt;&lt;p&gt;Clay fraction (&lt;1um) of the profiles contains illite, mixed-layer phase, kaolinite and chlorite. Studies of the mineralogy of soils showed that in the kastanozem the prevalence of illite over the mixed-layer phase is observed not in the whole profile but only in the upper 50 cm (which differs from the literature data). In the solonetz domination of illite is observed in the whole profile, with the exception of the parent rock from a depth of more than 120 cm.&lt;/p&gt;&lt;p&gt;Reconstruction. During the recession of the Early Khvalyn Caspian Sea, the coastline had a heterogenic relief inherited from the unevenness of the seabed with saline sediments. Then the subaerial development of the surface at the site of kastanozem there was a slight depression, which was gradually washed out of salts due to atmospheric precipitation more intensively than the surrounding area. First halophytic vegetation began to grow here. During the rewinding of the material this vegetation trapped aeolian material and then micro-elevation formed. The duration of subaerial development determines the development of the soil humus horizon on this elevation. On this eolian elevation at the microlevel we found signs of soil formation at a depth of 55-60 cm. On the surrounding flat surface, there was a hydromorphic gypsum-bearing solonchak where remained a large number of gypsum formations from a depth of 120 cm in the profile of the modern solonetz. Then the surface was overlain by aeolian sediment with a different ratio of mineral phases. Relative to the modern surface the thickness of the sediment is 50 cm, and on the surrounding surface with modern solonetz is 120 cm. In this surface illite began to dominate over the mixed-layer phase in clay, which indicates a different source of material drift. In this upper eolian sediment, modern solonetz began to form with the local development of kastanozem in nano-depressions due to subsidence of the surface during compaction of the most biogenically aggregated material.&lt;/p&gt;&lt;p&gt;In combination with micromorphological studies, the analysis of the properties of the solonetz complexes allows us to conclude that the modern soils in this area are shallowly buried paleosols. The mineralogical and micromorphological features diagnose modern soils as a pedocomplex.&lt;/p&gt;&lt;p&gt;This work was supported by the Russian Science Foundation, grant &amp;#8470;21-74-20121.&lt;/p&gt;