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

Abstract

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.

The significance of climate fluctuations for lake level changes and shifts in subsistence economy during the late Neolithic (4300–2400 b.c.) in central Europe

“…Lake level data also supply information for modelling experiments and can be used to refine palaeoclimatic reconstructions based on pollen data (Guiot et al 1993;Magny et al 2001Magny et al , 2003Magny et al , 2005). In particular, lake level records give specific information on precipitation changes, in contrast to most climatic markers documenting often only temperature changes.…”

Strengths and Weaknesses of Quantitative Climate Reconstructions Based on Late-Quaternary Biological Proxies

“…However, if the climate response of the proxy is well understood, the use of a constraint may provide a more consistent set of results than those from an unconstrained MAT (e.g. Magny et al 2001Magny et al , 2003Muller et al 2003;Bordon et al 2009). The need of using such constraints raises questions about the suitability and robustness of MAT as a climate-reconstruction procedure when applied to pollen data, especially when the constraints applied are themselves derived from the pollen data (e.g.…”

“…Muller et al (2003) use MAT on the same modern pollen data-set in eastern Canada to reconstruct annual precipitation, mean annual temperature, evaporation:potential evaporation, mean January and July temperatures, run-off, and growing degree-days above 0°C and above 5°C. See also Magny et al (2001Magny et al ( , 2003 for reconstructions of seven climate variables from one data-set. de Vernal et al (2001; use the same dinoflagellate cyst assemblages to reconstruct summer and winter seasurface temperatures, the number of months of ice cover, and summer sea-surface salinity (see Telford 2006 for a critique).…”

“…Other proxies used as constraints have included biome reconstructions derived from assemblages of plant macrofossils or fossil pollen (e.g. Magny et al 2001Magny et al , 2003Ortu et al, 2006Ortu et al, , 2010, beetle-based climate reconstructions, and sedimentary parameters (see Birks 1995 for a review). The use of certain proxies as a constraint has the disadvantage that these proxies can no longer be used to verify independently the reconstruction results .…”

Rövid távú koraholocén (8200 év) klímafluktuációk vegetációra gyakorolt hatása a Déli-Kárpátok Retyezát-hegységében

“…A nagyobb mennyiségben beáramló édesvíz az áramlat átfordulási pontjának délebbre tolódását eredmé-nyezheti, mely a mélységi víz keletkezési intenzitásának csökkenésében nyilvánul meg. Következménye lehet a magasabb földrajzi szélességeken elhelyezkedő területek lehűlése, vagy éppen nagyobb csapadékmennyiség elő-fordulása Közép-Európa területein (2. ábra) a megerősö-dő ciklonikus tevékenység miatt (Magny, Bégeot, Guiot, Guiot, Marguet, & Billaud, 2003;Magny, 2007).…”