Monday, 10 July 2006

Micromorphological and Microbiological Diagnostics of Elementary Pedogenic Processes in Extremely Arid Desert Soils of Mongolia and the Problem of Their Classification.

Dmitri L. Golovanov, Geographical Faculty, Moscow Sate Univ, Vorob'evy gory, Moscow, Russia and Marina P. Lebedeva (Verba), V.V. Dokuchaev Soil Science Institute, Pyzhevskii per. 7, Moscow, Russia.

The main outcome of pedogenesis in the extremely arid desert soils of Mongolia is the development of soluble salts in the upper soil horizons, the accumulative distribution pattern of the wind-blown and deposited clay fraction, and the development of the surface desert crust and subcrust horizons underlain by the rubified illuvial-metamorphic horizon; the total thickness of these horizons is comparable with the maximum depth of soil wetness in the long-term climatic cycle. The pebbly desert pavement with desert varnish on the surface protects the profile of extremely arid desert soils of Mongolia from wind erosion. The mechanism of its formation is related to the blowing out of the fine particles during strong winds and the specific "floating" of pebble inclusions in the soil mass during rare rainstorm episodes. Micromorphology, revealed, the presence of pores filled earlier by salt crystals, which most probably have dissolved (pores-phantoms) in the crust and subcrust horizons, the features of dissolution and migration of salts and gypsum, and the presence of partially dehydrated gypsum with increased birefringence. The temperature of gypsum dehydration is about 60-80C. These features are combined with the features of dissolution of primary grains of carbonates and the mobility of iron compounds as bleaching of the soil mass around pores, ocherous mottles in the soil plasma, and iron coatings of quartz grains in the illuvial-metamorphic rubified horizon. The presence of algae in the crust horizon, and the presence of iron-reducing microorganisms in the subcrust and rubified horizons suggest that the redistribution of iron compounds in the soil mass is a recent process taking place upon the short-term development of stagnic conditions in the former during the long-term cycle of pedogenesis. In the presence of high concentration of sulfates, reducing conditions and the development of gleyization are accompanied by the reduction of sulfates followed by the oxidation of iron sulfides to hydrolytical acid sulfates. The hydrolytically acid sulfates favor the local dissolution of primary calcite grains. A similar process was described by Minashina and Feofarova in the gray cinnamonic soils developing from the sulfate-containing rocks. The results of our study suggest that pedogenesis in the extremely arid deserts of Mongolia is strong rather than weak. Although the developing soil horizons are not thick, the horizonation itself is very distinct with the soil profile consisting the following horizons (as specified in the new Russian Soil Classification system): K (crust)-Lcs,z-BFMcs-C(cs). The rubified BFM (illuvial-metamorphic) middle horizon is suggested as a criterion to separate extremely arid desert soils of Mongolia from brown arid desert soils. This distinction should be performed at the high taxonomic level (the level of soil orders), as it is suggested in the distinction of the iron-differentiated alkaline soils. Besides the extremely arid desert soils of Mongolia, a separate type of crusty solonchakous solonetz can also be distinguished in this order. This study has been supported by the Russian Foundation for Basic Research, project no. 05-04-49098.

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