Mineralogy and Magnetic Properties of the Loess-Soil Formation as the Reflection of the Landscape and Climatic Conditions in the Terek–Kuma Lowland in the Pleistocene

A detailed mineralogical and geochemical study of the Otkaznoe reference section, representing a loess–soil complex confined to the Terek–Kuma Plain, has been performed. The section provides a detailed record of the history of this region in the Pleistocene. The thickness of the sediments reaches 140 m and the age can exceed 800 thousand years. One of the main tasks of the work is to demonstrate the capabilities of a set of environmental magnetism methods in combination with Mössbauer spectroscopy and mineralogical and geochemical data for paleogeographic reconstructions of natural environment and sedimentation conditions in the Pleistocene. The observed changes in the main magnetic properties in the Otkaznoe section suggest a high sensitivity of iron minerals to the changing climatic conditions during soil formation and sedimentation. Characteristic of soil horizons is an increased content of ferrimagnets. The ferrimagnetic contribution is considerably increased in paleosol (PS) horizons, reaching 80% of the total magnetic susceptibility as compared with loess horizons (50–60%). Up to six levels of paleosol development with different degrees of manifestation of the changes in mineral composition are distinguishable based on the vertical distribution of clay minerals in the studied section. An important point is the presence of chlorites in the PS horizons most likely explainable with erosion of the most weathered PS top. A noticeable increase in the content of smectite phase in PS horizons suggests a sufficient duration of soil formation. Changes in magnetic susceptibility distribution and clay mineralogy clearly identify the PS horizons and distinctly specify the Mikulino interglacial (MIS5) and Dnieper interstadial (MIS6). Weakly pronounced Bryansk paleosol belongs to the latter. The observed changes in the composition of the loess–soil complex confirm the stratigraphic constructions. The quantitative characteristics (the set of magnetic, mineralogical, and geochemical parameters) of the loess–soil complexes in the Terek–Kuma Plain are used to reconstruct the dynamics of climatic conditions in the Pleistocene and to confirm the trend of a gradual aridization during the Pleistocene. Characteristic of the epochs of interglacials, when the soil complexes were formed, is increased climate humidity as compared with glaciation stages, with the maximum aridity index IDM of 35 (annual precipitation to 700 mm) observable in the Inzhavino pedocomplex (PC4).