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Lower to middle Weichselian pedogenesis and palaeoclimate in Central Europe using combined micromorphology and geochemistry: the loess-palaeosol sequence of Alsheim (Mainz Basin, Germany)

Peter Kühn1, Astrid Techmer2, Michael Weidenfeller3
1 Forschungsbereich Geographie, Lehrstuhl Physische Geographie und Bodenkunde, Uni Tübingen
2 Leibniz-Institut für Angewandte Geophysik, Hannover
3 Landesamt für Geologie und Bergbau Rheinland-Pfalz, Mainz

V 2.7 in Löss und terrestrische Archive (Fortsetzung)

18.09.2012, 14:30-14:50, H8

 

The focus of this study lies on the lower to middle Weichselian sections of a loess-palaeosol sequence situated in the Mainz Basin near Alsheim (Central Europe). We characterize particularly the lower Weichselian loess and loess derivatives and the weathering intensities of buried Bw, Btw and Ah (humus zones, Chernozems) soil horizons with grain size, micromorphological and geochemical data. The geochemistry of distinct buried soil horizons and sediments is additionally used as a proxy for palaeotemperature and palaeoprecipitation.

High rates of sedimentation with alternating phases of relocation seem to be the main cause for a much less differentiation into Middle and Upper Weichselian palaeosol-loess units of the Alsheim loess-palaeosol-sequence compared to other loess-palaeosol sequences (e.g. Nussloch near Heidelberg), whereas the Lower Weichselian shows distinct buried Ah, Bw and Btw horizons (Figure 1).

The degree of fineness, i.e. the accumulated sum in percent of single grain size fractions divided by the number of fractions, is an easily applicable and suitable tool for distinguishing between loess and loess derivatives within one region. If loess deposits have a distinct influence by local components the degree of fineness seems to be inappropriate for interregional comparisons.

Micromorphology shows compacted granular structure and moderately to strongly developed pedality as characteristic properties for aquatic loess, whereas channel microstructure with no pedality is typical for loess deposits. Spongy microstructure suggests a classification of the Lower Weichselian Mosbach Humus Zones as Chernozems. The Eemian palaeosol (Btw horizon in Als III) shows only weak clay illuviation, characteristic for drier regions in Europe.

The chemical index of alteration (CIA) index is low (<50 = no weathering) for loess deposits in the Alsheim loess-palaeosol sequence, which is in contrast to the world wide loess samples ranging from >53 to < 70 (Gallet et al. 1998). The highest weathering was detectable for Btw horizons with CIA values >70.

Calculations based on geochemical indices (cf. Sheldon and Tabor, 2009) yielded palaeoprecipitation data for the Mainz Basin, which – compared to the current mean annual precipitation (MAP) of 789 mm - suggest around 150 mm higher MAP for the Last Interglacial (Btw horizon) and distinctly lower precipitation of around 300 – 400 mm MAP for periods of loess and sandy loess deposition, and a MAP of < 500 mm for Weichselian Interstadials (humus zones and Bw horizons) .

The calculated mean annual palaeotemperature (MAT) for Interstadials with 8.9°C for Bw horizons and with 9.6°C for humus zones (6.9°C or 7.6°C, considering the relation of the present MAT of the Mainz Basin and Germany) seems to be a good approximation of the MAT for Lower and Middle Weichselian Interstadials. A MAT of 8.7°C (6.7°C, taking into account the relation of the present MAT of the Mainz Basin and Germany) for Stadials (loess and sandy loess samples) seems to be higher than other temperature estimations for Stadials.

The first direct quantitative estimation of mean annual palaeotemperature and palaeoprecipitation from the geochemistry of soil horizons and sediments (loess and sandy loess) in Central Europe shows mostly comparable palaeotemperatures reconstructed from other proxies. Additionally palaeoprecipitation data can be provided. This suggests that palaeopedological-based temperature and precipitation reconstruction could be successfully implemented in Europe and that palaeosols may provide a useful alternative to other proxies for correlating European records during the last Glacial-Interglacial cycle.

The future investigation of reference loess-palaeosol-sequences using the same methods - in Central Europe: e.g. Mainz-Weisenau for lower to middle Weichselian, Nussloch for middle to upper Weichselian and Dolní Věstonice for the Weichselian period - would give the possibility of a regional and supra-regional validation of our data and could be the starting point to establish a palaeosol-proxy-data base for palaeotemperature and palaeoprecipitation.

 

 

References

Gallet, S., Jahn, B., Van Vliet Lanoë, B., Dia A., Rossello, E., 1998. Loess geochemistry and its implications for particle origin and composition of the upper continental crust. Earth and Planetary Science Letters 156, 157-172.

Sheldon, N.D., Tabor, N.J., 2009. Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols. Earth-Science Reviews 95(1-2), 1–52. doi:10.1016/j.earscirev.2009.03.004.

 

Techmer, A., Rolf, C., Weidenfeller, M., 2006. Lumineszenz-Chronologie und paläomagnetische Untersuchungen des Lößprofils Alsheim (Mainzer Becken). Mainzer geowissenschaftliche Mitteilungen 34, 113-134.

Sections Als I, IIb and III with sedimentological information and key soil horizons. IRSL ages are from Techmer et al. (2006).
Sections Als I, IIb and III with sedimentological information and key soil horizons. IRSL ages are from Techmer et al. (2006).



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Letzte Änderung 26.07.2012