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The mystery of loess dolls: structure and origin - a case study

Gabriella Barta1
1 Department of Physical Geography, Eötvös Loránd University

P 2.1 in Löss und terrestrische Archive

The genesis and morphology of carbonate concretions from loess-paleosoil sequences was always an exciting subject, which still has got yet opened questions. These carbonate concretions are also known as loess dolls, because their different appearances make possible various associations concerning the shape.

Carbonate concretions appear frequently under paleosoils which draw attention to a connection with leaching processes. Since soil development is characterized with more moist conditions and relatively stable surface compared to dust accumulation environments, it makes possible a more effective carbonate migration. Carbonate leaches out not just after the development of a soil, but during its evolution when the changes of the porosity of loess lead to a loss of carbonate content. The dissolved carbonate of coated and detrital grains is transported by the percolating solutions or fluctuating groundwater levels. The growth of concretions is related also to later on infiltration. Loess dolls are may formed in cavity systems well exposed to air, where the partial CO2 pressure is lower. These properties also provide adequate conditions for carbonate precipitation. It cannot be excluded that the morphological types of loess dolls are in connection with climate and vegetational changes, though it is sure that different shapes are partly the consequence of hydromorphological processes.

Investigations were carried out on loess doll samples from the loess-paleosoil sequence of Süttő (Hungary), which site provides high resolution information on the paleoenvironmental changes of the Late Pleistocene. Altogether 29 samples were analysed from the sandy loess layers belonging to the profiles MIS 6 and MIS 2. Besides the morphological description two separate methods were used to recover the inner structure of the concretions: cutting into two or more parts and treatment with 10% hydrochloric acid.

Three main types of loess dolls resulted from the experiments: a.) concretions with perpendicular and/or longitudinal cracks; b.) concretions containing one or more condensation nucleus; c.) combination of these two properties. Concerning these structures some conclusions can be drawn on the origin of loess dolls. Carbonate may have been precipitated in or around cavity systems of biogenic origin (root-related channels, biogalleries) and of abiogenic origin (structural properties of the sediment). Precipitation occured around condensation nuclei (as certain microscale secondary carbonate types). In most cases a multiphase development history seems to be characteristic for the loess dolls.  

Letzte Änderung 26.07.2012