Saturday, 15 July 2006

Quantitative Mineralogical Indices to Diagnose Cryogenic and Pedogenic Weathering in Soils of the Northern Part of the East European Plain.

Victor V. Rogov and Dmitri L. Golovanov. Geographical Faculty, Moscow Sate University, Vorob'evy gory, Moscow, Russia

Cryogenic weathering is a complex physicochemical process; it cannot be reduced to the mechanical disintegration of bedrock due to the wedging action of ice appearing in the freezing rock mass. The further lowering of temperatures induces the cryohydration mechanism of weathering related to the dynamics of film water. In the case of mechanical disintegration, its products are of the sandy-gravelly texture; in the case of the cryohydration, they are of the predominantly coarse silt size typical of loess. The most active development of cryogenic loess-like material is typical of loose sandy loamy substrates rather than of hard rocks, because of the larger range of film moisture and the better manifestation of cryogenic structures. The results of cryogenic weathering by the cryohydration mechanism are seen in the accumulation of the coarse silt (0.01-0.05 mm) fraction and in the enrichment of this fraction in comminuted quartz grains. This is explained by the existence of the lower size limits for the comminution of different minerals by the cryohydration mechanism: for quartz, this is the coarse silt size; for feldspars, fine sand; and for mica, fine and medium sand. The resistance of minerals to cryohydration also depends on the initial grain size; therefore, the series of minerals' resistance to weathering depend on the particular size fraction. They are also different for different conditions of weathering, including, in particular, cryogenic areas and the areas with temperate humid climate.

V.N. Konishchev (1981) studied the distribution of different minerals by particle-size fractions for sediments subjected to weathering under different climatic conditions. On this basis, he suggested an index to estimate the conditions of weathering and called it the coefficient of cryogenic contrast (CCC).

CCC = (Quartz / Feldspars (0.01 0.05 mm)) : (Quartz / Feldspars (0.1 0.05 mm))

The CCC values above 1.0 attest to the formation of sediments under strongly cryogenic (permafrost) conditions; the values below 1.0 suggest that these sediments were formed under warm humid conditions (without cryogenesis). For permafrost environments, a linear negative correlation exists between the CCC value and the mean annual temperature at the soil surface.

The specificity of mineral distribution by particle-size fractions in the permafrost-affected soils can be used as a diagnostic feature of the Gelic material. It is suggested that the mineralogical composition and specificity of soil microfabric should be taken into account in the definition of Gelic material. The CCC values in this material should exceed 1.0.

The notion of a polygenetic character of Holocene soils in the Russian Plain should also be taken into account in analyses of the mineralogical composition of coarse fractions. Different stages of the development of these soils and their parent materials the Pre-Holocene cryogenic stage and the Holocene noncryogenic humid stage are characterized by different directions of weathering processes. The Holocene soils on mantle loams can be considered the soils developing from the previous cryogenic weathering mantle.

The cryogenic stage is marked by the enrichment of the coarse silt fraction in quartz; the recent weathering processes delete the results of the cryogenic weathering, as quartz grains of this size are partly dissolved. The dissolution of quartz in the coarse silt fraction is comparable in intensity with the involvement of silica into the biological turnover and its hydrochemical discharge with river flows.

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