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Snail biogeography, palaeoenvironment and human settlements in western Europe during the Upper Weichselian

Olivier Moine1
1 Laboratoire de Géographyie Physique, UMR CNRS 8591

V 5.1 in Klima und Umwelt der letzten 130.000 Jahre

18.09.2012, 08:30-08:50, H6

 

Archaeological databases of Upper Weichselian human settlements show changes in the northern limit of Aurignacian, Gravettian, and Lower Magdalenian and Solutrean in north-western Europe. Large fluctuations of the southern limit of the Scandinavian ice cap during the Upper Weichselian Pleniglacial (38-17 ka BP) undoubtedly influenced the zonation of vegetation biomes in the European Great Plain, and indirectly territories occupied by Upper Palaeolithic peoples. However, the quasi-absence of continuous and high resolution pollen records in this area limit the reconstruction of vegetation dynamics in space and time, which also suffer a lack of modern analogues as indicated by pollen analyses and vegetation biome modelling.

During the last glaciation, loess/palaeosol sequences developed all over the European Great Plain, and especially during the Upper Weichselian. In addition, loess deposits, which do not preserve pollen, sustain important populations of terrestrial molluscs and ensure the preservation of their shells, which are good proxy for palaeoenvironmental reconstructions.

The creation of a malacological database and the study of high resolution molluscan records exclusively for attested Upper Weichselian loess deposits of north-western Europe highlights several important characteristics of both biogeography and dynamics of the malacofauna, and thus of palaeovegetation.

The composition of the molluscan faunas highlights a cold, humid and poorly vegetated “western domain” including only 7 taxa and surrounding the coasts of the Channel. Eastward, 40 taxa have been listed for the “central domain” and reflect also cold but drier conditions and a more developed vegetation. Vegetation types are also more diversified: steppe in the Middle Rhine Valley and in Saxony, tundra and herb-tundra in the Upper Rhine Valley, damp tundra and poorly vegetated places here and there.

The comparison with several other proxies highlights a remarkable correspondence of the limit between both “western” and “central” domains with loess sedimentary facies, humid on the coast and drier to the interior, the area of highest ice-wedge density corresponding to the “western domain”, the fauna of ungulates including more Reindeer on the Atlantic facade and more Horse and Mammoth in Germany and the northern expansion of Aurignacian and Gravettian lithic cultures (see figure).

In the European loess belt, loess sequences are composed of loess units alternating with tundra gley horizons. Ice-wedges open only in these latter. With regards to millennial-timescale climate changes, loess units have been correlated with stadials, the formation and the degradation/cryoturbation of tundra gley horizons with stadial-interstadial transitions and interstadials respectively. Consequently, the development of tundra gley horizons is associated with phases of reduced eolian activity and dust transportation. A northward shift of the polar front, possibly favoured by a retreat of the sea ice cover, may have thus induced higher precipitations on the continent, which benefited the growth of Alpine and Scandinavian ice caps.

In Nussloch and Wiesbaden (Germany), and in Curgies and Villers-Carbonnel (France), decreases in richness, due to losses of vegetation-requiring taxa, occur throughout tundra gley horizons and lead to minimums at their top. Indeed, the presence of ice in the soil in winter and of water-logged environment in summer can be unfavourable for both vegetation and malacofauna. Besides, the few comparable data available show higher molluscan richnesses within tundra gley horizons than within loess units from northern France (poor vegetation) and from the Middle Rhine Valley downstream of Frankfurt (steppe), whereas it is the contrary in the Upper Rhine Valley (tundra and herb-tundra).

Humidity/precipitation increases associated with the formation of tundra gley horizons can thus be favourable in dry loessic contexts from the “western domain” and from the north-western part of the “central domain”. In any case, the melting of the permafrost, which causes tundra gley degradation during interstadials, induces water excesses in soil and rather unfavourable conditions in the moderately humid context of the Upper Rhine Valley.

As only minor changes differentiate northern expansions of Aurignacian and Gravettian lithic cultures, this oscillating system does not seem to have strongly affect the geographical distribution of human settlements in north-western Europe. Nevertheless, an eventual effect on the seasonal presence of human populations at the northern fringe of their territory could be explored by a thorough linkage of human settlements within the malacological-environmental frame in construction.

 

On the contrary, Solutrean and Lower Magdalenian cultures are associated with a significant retreat to the south of human populations. The evolution of richness throughout the few available high resolution molluscan records show around 27 ka BP an extreme impoverishment in Villers-Carbonnel and a strong decrease in Remagen, the malacofauna becoming almost as poor as in the “western domain”, and possibly around 27 ka BP, a strong decrease also in both Wiesbaden records, unlike in Nussloch where it seems to occur only around 22 ka BP, and led to the quasi-disappearance of the malacofauna in the uppermost four meters of loess in profile P4. Such a strong vegetation impoverishment and dryness increase are contemporaneous to the Last Glacial Maximum. Despite the southern location of Nussloch compared to the other sites, other molluscan records are required to confirm the delayed occurrence of this environmental shift in the Upper Rhine Valley.

Comparison between the malacological “western domain” (solid line), the area of high density in ice-wedges (dashed line) and the northern limit of expansion of Aurignacian (green), Gravettian (yellow), Lower Magdalenian and Solutrean lithic (blue) industries. Arrows indicate population movements.
Comparison between the malacological “western domain” (solid line), the area of high density in ice-wedges (dashed line) and the northern limit of expansion of Aurignacian (green), Gravettian (yellow), Lower Magdalenian and Solutrean lithic (blue) industries. Arrows indicate population movements.



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Letzte Änderung 26.07.2012