Saturday, 15 July 2006

Identification of Parent Material of Soils along a Lithotoposequence in a Sedimentary Area using Particle-size Distribution and Mixing Equation.

CÚline Collin Bellier1, Dominique Arrouays1, Denis Baize1, Vincent Champdavoine1, Dominique King1, and Jean-Pierre Rossignol2. (1) I.N.R.A., av de la Pomme de Pin, B.P. 20619, Ardon, ORLEANS, France, (2) I.N.H., 2 rue LeNotre, Angers, France

Under temperate climate, one of the main pedogenic factor is the parent material. In many cases, the parent material is difficult to identify because it is too deep to be seen easily or it may be totally turned into soil. Moreover, soil may be issued from several parent materials by mixing processes due to slope redistribution, wind transfer or human activities. The objective of this study is, using particle-size distribution, 1) to identify the signature of parent materials origins and 2) to calculate the percentage of each parent material in the case of mixing. The study took place in France, in a sedimentary area, thin layers with different facies, in the Paris Basin. The data were obtained from field study, along a lithotoposequence, that is, a serial arrangement in which geologic layers and soils are ordered logically along the main slope. Nine pits were dug enough to reach the bedrock. Each horizon and bedrock, which is a potentially parent material, was described and sampled. We used eight fractions of particle-size distribution measurements to characterize these samples along this lithotoposequence. Two quantitative indicators, based on the skeleton fractions (2 silts and 5 sand fractions) are used to compare 1) each horizon to the different parent materials and 2) each horizon to the neighboring bellowed horizon in all soil profiles. The results determine, for each profile, the range of 1) allochthony or autochthony, and 2) soil heterogeneity or homogeneity. In some cases, horizons were not in correlation with the bedrocks characterized but were a mix of those materials. To quantify this mix, we solved mixing equations. In conclusion, thanks to particle-size distribution measurements, we were able to characterize and quantify the origin of soil parent materials and to understand pedogenesis. In our case, the deposits create mixing rather than superposition. From these results, we propose a soil spatial organization model which summarizes 1) the regional soils distribution, and 2) the possible pedogenic scenario through time.

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