The development of deep soil processes in ancient kurgans of the North Caucasus

• In humid climate, Luvisols on kurgans are developed in less than 4000 yrs.
• Deep soil processes affect the 10-m-deep layer in 4000 yrs.
• Deep soil features are represented by calcite, gypsum, and iron concentrations.
• Deep soil processes differ from soil processes in longer characteristic times.
• Deep soil horizons represent the first stage of the development of the weathering mantle.

In the past four thousand years, Luvisols with a clearly differentiated profile have developed on kurgans of the Bronze Age under beech and oak stands in humid climatic conditions of the foothills of the North Caucasus. The great height of the kurgans (up to 10 m) and their homogeneous composition (the kurgans were constructed of Chernozemic soil and are underlain by Chernozem) make it possible to study the direction and rate of pedogenetic and diagenetic processes acting at a considerable depth. In these kurgans, the profiles of Luvisols are underlain by the carbonate-illuvial horizon that has been shaped by illuviation processes acting for about 4000 years. The major part of this horizon lies at the depths of 110–350 cm, though carbonates penetrate as deep as 9 m along the deep earthworm burrows. At this depth, they occur within the noncalcareous mass of the lower part of the kurgans' bodies and in the upper part of the underlying Chernozem. At the depth of 5–9 m, in the kurgans and in the buried Chernozem, gypsum and iron pedofeatures (nodules and red-brown films on ped faces) have appeared. Humus-clayey infillings are present in some of the earthworm burrows at this depth. The formation of such a deep profile with the carbonate-illuvial, gypsiferous, and ferruginated horizons is related to the local bioclimatic conditions characterized by high precipitation and the growth of forest vegetation. Deep washing of the soil with atmospheric precipitation is combined with the manifestation of illuviation processes and alteration of the soil mass at a considerable depth. The mineralization of humic substances and some alkalization of the soil mass also take place in the deep horizons. However, these processes may occur at different depths independently from the illuviation processes and can be attributed to the group of diagenetic processes.