Climatic significance of glacier retreat and rockglaciers re-assessed in the light of cosmogenic dating and weathering rind thickness in Clarée valley (Briançonnais,French Alps)

Recent advances in geochronology provide opportunities to identify the glacial and periglacial sequences in mountain area. In this paper, we focus on the Upper Durance catchment (Southern French Alps), in the Clarée valley, where no absolute chronological benchmark was defined. Glacial remnants and periglacial features were investigated, integrated within a chronological scenario thanks to relative (weathering rind thickness) and absolute (CRE technique) dates. We find evidence of the existence of a Late-Glacial valley glacier, that has never been reported before. Then the upper valleys became ice-free during a period corresponding to the Early-Holocene Climate Optimum, whereas two sequences of cirque glaciation are identified during the second half of the Holocene. We also show evidence of three main generations of rockglaciers, which also mainly developed during the second half of the Holocene, probably at the end of the Subboreal period. The location and the altitudes of rockglaciers are strongly dependent on the geomorphic setting (especially the location of the sediment sources), hence complicating their palaeo-climatic interpretations.     

The use of micromorphology in soil pollen analysis: The interpretation of the pollen content of slope deposits in Galicia, Spain

Humic horizons of soil profiles often contain pollen and spores. Comparison of the sequence of pollen associations of soil profiles with pollen zones of reference diagrams offers palyno-ecological information, relevant for reconstruction of landscape ecological development. In palynological studies of semiterrestrial and aquatic deposits, pollen is considered to be part of the sediment and anaerobic conditions in water-saturated deposits promote its conservation. This is not true for pollen in soils. Aerobic conditions in drained soils result in bio-oxidation of unprotected pollen grains. Interpretation of soil pollen associations requires knowledge of the processes of infiltration and conservation of pollen grains in the soil material. Micromorphology is an important technique to investigate these processes. This is demonstrated in a case study of polycyclic slope deposits in Galicia (Spain). The buried cambisols, in pleistocene solifluction deposits, contain post-sedimentary infiltrated pollen. The pollen associations reflect successive phases in soil ecological development. The holocene colluvial deposits contain syn-sedimentary incorporated pollen. The pollen associations reflect environmental conditions during the deposition of colluvial layers.