Evolution of Chernozems in the Northern Caucasus, Russia during the second half of the Holocene: carbonate status of paleosols as a tool for paleoenvironmental reconstruction

A chronosequence of Chernozems buried under kurgans (funeral mounds) dated archaeologically to >5000, 3800–4000 and 1600–1700 years BP as well as the modern surface Chernozems were studied to reconstruct environmental change in the Central Northern Caucasus in the second half of the Holocene. Special emphasis was made on the detailed morphological and analytical characteristics of pedogenic carbonates and microscopic investigation of phytoliths. In the paleosols buried >5000 years BP, carbonates represented by compact micritic pedofeatures are in the uppermost part of the solum; neoformed gypsum is found below. This differs from the features of the modern soils and the paleosols buried 1600–1700 years BP in which the upper part of the profile is leached, pedogenic carbonates occur mostly in the form of diffuse pseudomycelium, impregnations and fine films, whereas gypsum is absent. The comparison of morphological features of the paleosols and the modern soils confirms the general tendency of paleoenvironment evolution from warm and dry in the Atlantic period to cooler and more humid in the Late Subboreal and Subatlantic periods. The compaction of soil mass and tonguing of humus horizon together with some dissolution of carbonate pedofeatures and signs of high biological activity in the paleosols buried 3800–4000 years BP are thought to indicate contrasting seasonal and annual variations of precipitation in early Subboreal period. Presence of calcified root cells of perfect habit in the paleosols of this chronointerval is also interpreted as evidence of contrasting seasonal climate. Hard carbonate nodules were found in all Chernozems of the chronosequence; the size of nodules, their internal porosity and recrystallization phenomena increase with age. We assume nodules to be relict pedofeatures in the Subatlantic paleosols and modern soils, which were transformed but not completely destroyed during the late Holocene humidification and enhancement of leaching. Thus, a multiphase model of carbonate pedofeatures formation is dominant in the soils studied. Phytolith assemblages indicate meadow–steppe vegetation in the Atlantic period; later in the Subboreal and Subatlantic periods, forest vegetation appears. Meadow (with some steppe elements) associations also determined in the late Holocene profiles probably reflect anthropogenic deforestation. These results are in good agreement with the interpretation of paleosolʹs properties. We conclude that complex characteristic of pedogenic carbonates, which we define with the term “carbonate status” is one of the most valuable sources of the paleogeographical information in the chronosequences of steppe and forest–steppe soils.