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Geochemical characteristic of the Late Pleistocene loess-soil sequences in south-western Poland

Jerzy Raczyk1
1 Department of Physical Geography, University of Wrocław

P 2.16 in Löss und terrestrische Archive

Polish loess areas are situated in the central part of the European loess belt, along its northern margin. The loess cover in SW Poland forms a thin (3-5 m, maximum 15 m) discontinuous mantle consisting mainly of Late Pleistocene deposits. The thickness, continuity and stratigraphic differentiation of loess covers increases towards the east where several loess cycles form thick sequences locally exceeding 30 m in depth. Loess patches in SW Poland have a small thickness and often contain significant admixture of clay and sand fractions. Probably it was one of the main reasons, they have been interpreted by most of researches as hard to define loess-like deposits. Although the aeolian origin of the silt deposits inŚlęża Massif and Trzebnica Hills was postulated already by Orth in 1872. Thick loess covers occur mainly on the forefield of the Sudetes Mountains (e.g. Głubczyce Upland and Niemcza-Strzelin Hills). They can also be found far from the Sudetes Mountains (e.g. Trzebnica Hills) and within the mountainous depressions (e.g. Kłodzko Basin; Jary et al., 2002).

Three Late Pleistocene loess-soil sequences in SW Poland were chosen to detail geochemical examination: Dankowice (Niemcza-Strzelin Hills – 35 km toward south from Wrocław), Skarszyn and Zaprężyn (Trzebnica Hills – ca 20 km toward north-east from Wrocław).The thickness of the last interglacial-glacial sequences ranges between 6 and 9 m. All these profiles were sampled in vertical continuous sections at close intervals (5–10 cm) and documented with respect to their sedimentology, palaeopedology and stratigraphy. The main litho- and pedostratigraphic units were established during the field investigations for soils and sediments of the natural moisture content. To avoid confusion we use labelling system elaborated by Kukla and An (1989) and slightly modified by Marković and co-workers (2008). Late Pleistocene interfluve loess sequence in SW Poland consists of four units: two loess units (upper younger loess unit - L1L1 and lower younger loess unit - L1L2) and two polygenetic palaeosol complexes (intra-loess palaeosol L1S1 and basal Late Pleistocene pedocomplex S1). In the top of younger loess unit recent soils S0 have developed (Fig. 1).

The following analyses were conducted: grain-size distribution (laser diffraction method), low-field magnetic susceptibility, carbonate and organic carbon content, clay fraction mineralogy as well as TL, OSL and AMS 14C dating. Geochemical analyses were performed in Soil Laboratory of Physical Geography Department of Wrocław University. Atomic absorption spectrometry (AAS) was used for analysis of major elements. The concentrations of major elements (Na, K, Ca, Mg, Fe, Mn, Ti, Al and Si) were measured for bulk loess samples and later converted to oxides.The results were presented as weight percentage of components. Then chemical composition was recalculated on volatile-free basis. Weathering index WI= (Al2O3+Fe2O3)/(Na2O+K2O) acc. Pye and Johnson (1988) was calculated and several molar weathering ratios:CIA - Chemical Index of Alteration= (Al2O3/Al2O3+CaO+Na2O+K2O) x 100, SiO2/TiO2 and Fe2O3/TiO2 were considered (Fig.1).

The results of loess geochemical analyses show a certain regional differentiation of chemical composition between Trzebnica Hills and Niemcza-Strzelin Hills (e.g. the amount of MgO and CaO). These differences derive mostly from varied sources of loess material and confirm the importance of local conditions. Major element concentrations usually confirm previous designation of main lithostratigraphic units. However, the variation of chemical composition found in Dankowice loess section is much more pronounced if compared with Trzebnica Hills loess sections.The highest indexes of chemical weathering are usually attributed to soil units, particularly for both S1 and S0 units. The substrate of these soils shows considerable transformation by soil processes. Weathering indices of loess units are generally much lower. However some portions of loess sections directly above soil units show similar (high) indices. This is probably connected with local recycling of weathered silty sediments in the beginning of the loess formation phase. Slight variations of major element concentrations detected within loess units may arise from different soil-weathering conditions during the main loess deposition phases.Geochemical investigations of loess-soil units enable us to recognise Late Pleistocene weathering conditions and some weathering ratios might be used as a potential proxy of environmental changes.

 

References

Jary, Z., Kida, & J., Śnihur, M. (2002): Lessy i osady lessopochodne w południowo-zachodniej Polsce (Loess and loess-derived sediments in SW Poland) - Czasopismo Geograficzne, 73 (1-2): 63-100.

Kukla, G.J. & An, Z.S. (1989): Loess stratigraphy in central China. Palaeogeography, Palaeoclimatology – Palaeoecology, 72: 203-225.

Markovic, S.B., Bokhorst, M.P., Vandenberghe, J., McCoy, W.D., Oches, E.A. &Hambach, U. (2008): Late Pleistocene loess-paleosol sequences in the Vojvodina region, north Serbia. Journal of Quaternary - Science, 23: 73-84.

Nesbitt, H.W. & Young, G.M. (1982): Early Proterozoic climates and plate motions inferred from major element chemistry of lutites - Nature, 299: 715–717.

Orth, A. (1872): GeognostischeDurchforschung des SchlesischenSchwemmlandeszwischendemZobtener und TrebnitzerGebirge - Wiegandt u. Hempel. Berlin. LVIII, 361: pp.

Fig. 1. Weathering indices within litho- and pedostratigraphic units of Late Pleistocene loess-soil sequences at Dankowice, Skarszyn and Zaprężyn: WI acc. Pye& Johnson (1988); CIA acc. Nesbitt & Young (1982).
Fig. 1. Weathering indices within litho- and pedostratigraphic units of Late Pleistocene loess-soil sequences at Dankowice, Skarszyn and Zaprężyn: WI acc. Pye& Johnson (1988); CIA acc. Nesbitt & Young (1982).

Letzte Änderung 27.07.2012