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

摘要

Lower to middle Weichselian loess, loess derivatives and buried soils of the loess-paleosol sequence Alsheim (Central Europe) were characterised by particle size distribution, geochemical and micromorphological data. High rates of sedimentation with alternating phases of relocation are the main cause for a much less differentiation into Middle and Upper Weichselian loess-paleosol units of the Alsheim loess-paleosol sequence compared to other loess-paleosol sequences (e.g. Nussloch near Heidelberg), whereas the Lower Weichselian has distinct phases of pedogenesis resulting in Ah, Bw and Btw horizons.To distinguish between different loess deposits locally and intraregional, the degree of fineness is an easily applicable and suitable tool, though inappropriate for interregional comparisons. The chemical index of alteration (CIA) is low (<50=no weathering) for loess deposits in the Alsheim loess-paleosol sequence, which is in contrast to the worldwide compiled loess samples with CIA values ranging from >53 to <70 (Gallet etal., 1998). The highest weathering was detectable for Btw horizons with CIA values >70.A direct quantitative estimation of mean annual palaeotemperature and mean annual palaeoprecipitation can be provided by calculations based on geochemistry of soil horizons and sediments. The present mean annual precipitation (MAP) in the Mainz Basin is 789mm. In contrast, palaeoprecipitation data suggest a 150mm higher amount for the Last Interglacial (Btw horizon), a much lower amount of around 300-400mm MAPP (periods of loess and sandy loess deposition) and a MAPP of <500mm for Weichselian Interstadials (humus zones and Bw horizons). The calculated mean annual palaeotemperature (MAPT) for Interstadials with 8.9°C for Bw horizons or with 9.6°C for humus zones (or to 2K lower, considering the relation of the present MAT of the Mainz Basin with the MAT of Germany) seems to be a good approximation of the MAPT for Lower and Middle Weichselian Interstadials. A MAPT of 8.7°C (or 6.7°C) for Stadials (loess and sandy loess samples) is higher than other temperature estimations for Weichselian Stadials in Europe.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 paleosol (Btw horizon in Als III) has only weak clay illuviation, characteristic for drier regions in Europe.Palaeoclimate and soil formation of the Last Glacial-Interglacial cycle calculated from geochemistry and micromorphological data are in good accordance with other proxy data in Central Europe. This suggests that paleoclimate reconstruction based on palaeopedological analyses could be successfully implemented in Europe. Such data may provide a useful alternative to other proxies for correlating European records.