Palaeoecological study of the lacustrine sediment of Ribains maar (Haute-Loire, Massif Central, France) during the transition between the last interglacial and the last glacial periods

Abstract

AS a part of a multidisciplinary research programme in palaeoecology on the crater of Ribains (French Massif Central), pollen and diatom analyses were undertaken. Results show major chronological variability in both terrestrial and algal communities during the transition between the last interglacial (Eemian) and the last glacial (early Wurm) periods. The end of the Eemian was marked by an expansion of Pinus on the slopes of the lake and by a diatom community consisting of planktonic and eutrophic species indicating falling lake levels. Early Wurm began with severe climatic degradation, characterized on the catchment by the sharp decline of the forest to the benefit of steppic species, and by the shallowing of the lake as indicated by the development of a larger littoral diatom community and the dominance in the planktonic community by Aulacoseira subarctica. Subsequently warming-up occurred, as indicated by the development of a Pinus woodland and a return to higher lake-water levels as indicated by abundant densities of planktonic species. Another oscillation of the lake level towards shallower depths was recorded by the development of aquatic macrophytes associated with a maximum development of littoral diatoms. The last episode of the sequence presented here witnessed the expansion of thermophilous trees with the simultaneous development in the lake of a planktonic and eutrophic diatom assemblage characterizing higher water levels and nutrient enrichment. On the whole, diatom analysis showed a good correlation with pollen analysis and both indicated changes in the precipitation/evapotranspiration ratio as the main cause for lake-level changes. This study, which combined results obtained from different palaeoecological approaches, provides a synoptic view of fluctuations in the aquatic and terrestrial palaeoenvironments.