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Reconstruction and palaeoclimatic interpretation of mid-Holocene vegetation and lake-level changes at Saint-Jorioz, Lake Annecy, French Pre-Alps

Laboratoire de Chrono-Ecologie, CNRS-UMR 6565, Faculté des Sciences et Techniques, 16 route de Gray, 25030 Besançon, France; michel.magny{at}univ-fcomte.fr

Carole Bégeot

Laboratoire de Chrono-Ecologie, CNRS-UMR 6565, Faculté des Sciences et Techniques, 16 route de Gray, 25030 Besançon, France

Joël Guiot

CEREGE BP 80, Europòle Méditerranéen de l'Arbois, 13545 Aix-en-Provence cedex 4, France

André Marguet

Direction des Recherches Archéologiques Sous-Marines et Sub-aquatiques, 58bis avenue des Marquisats, 74000 Annecy, France

Yves Billaud

Direction des Recherches Archéologiques Sous-Marines et Sub-aquatiques, 58bis avenue des Marquisats, 74000 Annecy, France

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.

Key Words: Lake-level changes • vegetation history • human impact • quantitative palaeoclimate reconstruction • Central Europe • Atlantic IRD events • 8200-yr event • mid-Holocene

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