Saturday, 15 July 2006

Transformation of Parent Material (Mantle Loam and Moraine) Composition, Related to Soil Formation in the Upper Volga Region, Russia.

Olga A. Samonova and Elena N. Aseyeva. Faculty of Geography, Moscow State Univ, Leninskiye Gory, 119892, Moscow, Russia

The compositional transformation of unconsolidated sedimentary rocks during pedogenesis has been the subject of much attention in pedology and landscape geochemistry, providing insights into the nature of spatial variability of soil characteristics. This study was undertaken to analyze the transformation of two different types of parent material (mantle loam and moraine) within the central part of the Russian plain related to soil formation. The research focused on a quantitative comparison of compositional characteristics of A1 horizons and corresponding properties of underlying geological strata. In total 27 soil samples and 34 samples of parent material were analyzed for major elements (SiO2, Al2O3, TiO2, Fe2O3, CaO, MgO, Na2O, K2O, P2O5, MnO, including ŃÎ2) and selected trace elements (Zr, Cu, Ni, Co, Cr, Pb, Zn). Granulometric analyses were performed for all samples to determine the relative abundances of 6 grain–size fractions (1.0–0.25 mm, 0.25–0.05 mm, 0.05–0.01 mm, 0.01–0.005 mm, 0.005–0.001mm,<0.001 mm). Two selected granulometric fractions of the A1 and C horizons (0.25 - 0.01 mm and <0.001 mm) were analyzed for major and trace element contents. Data analysis shows that 1) highly significant difference between the granulometric compositions of the moraine and the mantle loam C horizons is poorly preserved in the studied A1 samples. The moraine granulometric distribution has been modified to a greater extent than that of the mantle loam. On average in A1 horizons we observe an increase of the 0.25 - 0.05 mm fraction and a decrease of the <0.001 mm fraction. 2) all studied A1 horizons are rich in biogenic components such as K2O, P2O5, MnO, CO2 and low in mobile elements (Fe2O3 and Al2O3), compared to parent strata. However, the chemical difference between A1 horizons and a parent material is greater for soils on the mantle loam. 3) there is no significant difference between the 0.25 - 0.01 mm fractions of the A1 horizons on different parent rock material considering major element abundances, whereas such difference is observed in the same fraction of the C horizons. 4) the A1 horizons show higher concentrations of Zr, Pb, Zn, Ti compared to the parent strata. The maximum concentrations of these microelements are found in the A1 horizons on the moraine. The moraine appears to have been modified to a greater extent than the mantle loam when considering the transformation of microelement contents. 5) Considering studied trace elements and such metals as Fe, Mn, Ti in the 0.25 - 0.01 mm fraction, the compositional difference between two types of parent material is significant. After soil formation the difference is preserved, which suggests little effect of local soil forming processes on the chemical composition of the studied fraction. On average higher abundances of Ti, Zn, Cu are observed in the 0.25 - 0.01 mm fraction of A1 horizons as compared to the same fraction of corresponding parent materials. Zr, Rb are more abundant in A1 horizons on the moraine. While Fe, Mn and Co are enhanced in concentration in the studied fraction of the parent materials. 6) in the fraction < 0.001 mm the initial compositional difference between two types of parent materials becomes less evident after soil formation. The studied fraction of the A1 horizons has higher average concentrations of Mn, Cu, Co, Cr, Pb, Zn as compared to the same fraction of parent rocks. Fe, Ti and Cr are more abundant in parent materials. 7) soil formation modifies relationships between element concentrations and abundances of grain-size fractions. In A1 horizons Zr and Zn are associated with finer fractions whereas in C horizons they correlate nicely with abundances of coarser fractions. On the contrary, Ti relates best to coarser fractions in A1 horizons whereas in the C horizons it is associated with finer fractions. Summarizing, soil formation increases compositional similarity of A1 horizons derived from mantle loam and moraine parent materials in the Upper Volga region. The extent of transformation depends on the nature of parameters studied.

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