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Buried Soil Horizons in the Profile of Surface Soils of the Russian Plain
Alexander O. MAKEEV, Tatyana E. Yakusheva, Dokuchaev Soil Institute, 109017, Pyzhevsky lane, 7, Moscow, Russia.
Soils with second humus horizons are common for high flat interflues in late Pleistocene periglacial areas of the Russian Plain. They can be met in steppe, forest-steppe and southern part of forest areas with Chernozem, Gray Forest Soils and Sod-Podzolic soil cover regardless modern bioclimatic zonality. But in all the cases they can be met only on loess and loess-like sediments and abruptly disappears in the margin zones between loess mantles and other adjacent sediments.
Second humus horizons occurs as a black lenses, 20-40 cm thick, and may be separated from surface humus horizons of a modern soil by a less humidified material, or situated immediately after it. They may be met only in the soils within rounded depressions, 30-100 cm deep and 20-100 m wide, that are common for such flat surfaces and are the remains of the former paleocryogenic depressions. Towards the margins of depressions black lenses gradually ceases. The formation of second humus horizons was simultaneous to the final stages of sedimentation. The environment in the depressions had been changing by that time between that of a temperate thermokarst lake and a wet frost meadow. The deposition of an eolian dust was accompanied by intense annual freezing from the bottom and sides, as well as slope processes. This had resulted in the formation of dirty-brown stripes and bands in loess strata, similar to those of a thermokarst sediments of a modern cryolithozone (Central Yakutia). Frost cracks and pseudomorphs, that complicate the lower border of second humus horizons, also indicate severe impact of cryogenesis.
Second humus horizons, now situated at a depth of 30-50 cm, are former surface horizons of a frost-meadow soils, that had been developed in a break in sedimentation. They were then buried by a final loess layer. It is evidenced, for instance, by a maximum in phytoliths distribution in soil profile. The buried nature of a second humus horizons are especially obvious an a contact zones between loess and ground moraine. Evaporational concentration of free carbonates above former permafrost layers in the soils on an interdepression surfaces supports the idea of an arid climatic conditions in most of the period of loess deposition.
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