Vegetation and lake-level data from the archaeological site of Tresserve, on the eastern shore of Lake Le Bourget (Savoie, France), are used to provide quantitative estimates of climatic variables over the period 4000-2300 cal BP in the northern French Pre-Alps, and to examine the possible impact of climatic changes on societies of the Bronze and early Iron Ages. The results obtained indicate that phases of higher lake level at 3500-3100 and 2750-2350 cal BP coincided with major climate reversals in the North Atlantic area. In west-central Europe, they were marked by cooler and wetter conditions. These two successive events may have affected ancient agricultural communities in west-central Europe by provoking harvest failures, more particularly due to increasing precipitation during the growing season. However, archaeological data in the region of Franche-Comté (Jura Mountains, eastern France) show a general expansion of population density from the middle Bronze Age to the early Iron Age. This suggests a relative emancipation of proto-historic societies from climatic conditions, probably in relation to the spread of new modes of social and economic organisation.

Pollen and sediment analyses were used to reconstruct vegetation and lake-level changes over the mid-Holocene period at Saint-Jorioz, Lake Annecy (northern French Pre-Alps). Episodes of forest clearings point to Neolithic cultural activities in the northern French Pre-Alps from c. 6500 cal. BP in agreement with other pollen and macrofossils records from eastern Switzerland and the Jura mountains. The lake-level record shows rises atc. 8300–8200, 6400, 5900 and after 5730 cal. BP. The rise at c. 8300–8200 cal. BP shows a tripartition. Lower water levels developed before 8300–8200 and 6665 cal. BP, at c. 6050 and 5730 cal. BP. Rises at c. 8300–8200, 6400, 5900 and after 5730 cal. BP can be related to the 8200-yr event and North Atlantic ice-rafted debris (IRD) events occurring at c. 6200, 5800 and 5500 cal. BP, respectively. Using a model based on pollen and lake-level data, the palaeoenvironmental changes reconstructed at Saint-Jorioz were translated into quantitative climate parameters. The results suggest that rises in lake level coincided with increasing annual precipitation, runoff and available moisture, and decreasing mean summer temperature and shortening of the growing season probably as a result of alternate southward/northward displacements of the Atlantic Westerly Jet. At Saint-Jorioz, the period around the 8200-yr event corresponded to a c. 2.5°C summer temperature cooling and a c. 130 mm P-E (precipitation-evaporation) increase in agreement with other European and marine palaeoclimate records; a c. 4°C summer-temperature cooling and a rise in annual precipitation by c. 175 mm are reconstructed at c. 5900 cal. BP and after 5730 cal. BP, i.e., close to the time of the North Atlantic IRD events dated at 5800 and 5500 cal. BP, and are to be tested by further investigations. A general trend toward temperature cooling and precipitation increase appears over the period documented by the sediment sequence of Saint-Jorioz possibly related to an orbitally induced reduction of summer insolation.

On the basis of sedimentological analysis of two cores taken at Chatillon, Lake Le Bourget (northern French Pre‐Alps), and well dated by radiocarbon dates in addition to tree ring dates obtained from an archaeological layer, this paper presents a high‐resolution lake‐level record for the period 4500–3500 cal. a BP. The collected data provide evidence of a complex palaeohydrological (climatic) oscillation spanning the ca. 4300–3850 cal. BP time interval, with major lake‐level maxima at ca. 4200 and 4050–3850 cal. a BP separated by a lowering episode around 4100 cal. a BP. The lake‐level highstands observed at Chatillon between 4300 and 3850 cal. BP appear to be synchronous with (i) a major flooding period recorded in deep cores from the large lakes Le Bourget and Bodensee, and (ii) glacier advance and tree line decline in the Alps. Such wetter and cooler climatic conditions in west‐central Europe around 4000 cal. a BP may have been a nonlinear response to decrease and seasonal changes in insolation. They may also provide a possible explanation for the general abandonment of prehistoric lake dwellings north of the Alps between 4360 and 3750 cal. a BP. Copyright © 2012 John Wiley & Sons, Ltd.

Depuis plus de 30 ans, dans les massifs montagneux des Alpes françaises du Nord, des programmes de recherche ont renouvelé nos connaissances sur des occupations humaines de la Préhistoire récente. Des gisements ont livré des séquences chrono-culturelles et naturelles de référence. Bien calées chronologiquement, elles autorisent une bonne approche des évolutions culturelles, du Mésolithique moyen à la fin de l 'âge du Bronze, ainsi que bioclimatiques, du Préboréal au début du Subatlantique. Pour répondre à des problématiques d'occupation des territoires, des prospections thématiques ont été mises en œuvre par des chercheurs d'horizons différents ; quelques-uns contribuent ici à cet échange de données. Retracer les épisodes du peuplement préhistorique dans un espace ouvert aux influences aussi vaste et sur un temps aussi long n 'étant pas possible dans cette contribution, il est proposé de porter un regard documentaire sur trois secteurs géographiquement individualisés et bien caractérisés géomorphologiquement : -dans les domaines lacustres savoyards, dans l 'Avant-Pays au contact du Jura méridional, les principaux épisodes de l 'occupation littorale sont présentés (abattages dendrochronologiques entre les années 3842 et 805 av. J.-C.) ; -dans les massifs subalpins calcaires de moyenne altitude, Chartreuse et Vercors, proches des grands axes de circulation (sillon alpin, couloir rhodanien), les modalités de passage du Mésolithique moyen alpin (de 8000 à 7000 av. J.-C.) au début du Néolithique moyen (de 4800/4700 à 4500 av. J.-C.) sont exposées ; -dans la zone montagneuse orientale cristalline d'altitude plus élevée, les travaux menés dans les hautes vallées de Maurienne et de Tarentaise montrent la précocité et l 'intensité des conquêtes néolithiques.

No abstract