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ACCROTELM Informe resumido

Project ID: EVK2-CT-2002-00166
Financiado con arreglo a: FP5-EESD
País: France

Synthesis of lake-level change data

A first synthesis of available lake-level data was established for west-central Europe. A data set of 180 radiocarbon, tree-ring and archaeological dates obtained from sediment sequences from 26 lakes in the Jura Mountains, the northern French Pre-Alps and the Swiss Plateau was used to construct a Holocene mid-European lake-level record. The dates do not indicate a random distribution over the Holocene, but form clusters suggesting an alternation of lower and higher, climatically driven lake-level phases. For the last 4500 years, they provide evidence of an unstable Holocene climate punctuated by phases of higher lake level at 4150-3950, 3500-3100, 2750-2350, 1800-1700, 1300-1100, 750-650 cal BP and after AD 1394. A comparison of this mid-European lake-level record with the GISP2-Polar Circulation Index record, the North Atlantic ice-rafting debris events, and with the 14C record suggests teleconnections in a complex cryosphere-ocean-atmosphere system. This also suggests that changes in solar activity played a major role in Holocene climate oscillations over the North Atlantic area.

New investigations have been made in west-central Europe to document better the regional pattern of palaeohydrological variations over the last 4500 years. Recent archaeological investigations around Neolithic and proto-historic lake-shore settlements have provided the opportunity to obtain additional lake-level data. New sediment sequences have been observed and analysed from Lakes Pluvis and Bourget (sites of Tresserve and Chindrieux) in the southern Jura, and from Lakes Morat (site of Montilier) and Neuchâtel (site of Marin-les-Piécettes) on the Swiss Plateau.

Additional lake-level data were also obtained from Lakes Morat and Neuchâtel on the Swiss Plateau. Sediment sequences marked by an alternation of peaty-organic and carbonate lake-marl layers give evidence of successive lake-level fluctuations during the last 7 millennia. The lake-level changes recognised at Lakes Pluvis and Bourget in the southern Jura, and at Lakes Morat and Neuchâtel on the Swiss Plateau fully agree with the regional pattern defined from previous data.

On the basis of glacier records established in the Swiss Alps and lake-level records established in west-central Europe, a comparison has been attempted between high-resolution palaeohydrological data in west central Europe over the past 3300 years. A data set of tree-ring, radiocarbon and archaeological dates in addition to historical sources was used to reconstruct fluctuations of the Great Aletsch, the Gorner and the Lower Grindelwald Glaciers in the Swiss Alps. The three ice-streams experienced nearly synchronous advances at c. 1000-600 BC, 500-600, 800-900, 1100-1200 and 1300-1860 AD. These glacier fluctuations show strong correspondences with lake-level variations reconstructed in eastern France (Jura Mountains and Pre-Alps) and on the Swiss Plateau.

Pollen and lake-level data provided by the sediment sequence from Tresserve (Lake Bourget, France) that documents the period 3800-2500 cal BP allow us to reconstruct climatic parameters. Results suggest that phases of higher lake level coincided with an increase in annual precipitation by ca 100mm, a decrease in summer temperature by 1°C and a shortening of the growing season. Conversely, periods of low lake level corresponded to a decrease in annual precipitation, an increase in summer temperature and a longer growing season. This general pattern appears to be consistent with correlations observed between high-level phases of regional lake levels and glacier advances in the Swiss Alps. It may have resulted from alternate southward-northward displacements of the Atlantic Westerly Jet Stream.

Lake Joux in the Swiss Jura was investigated to establish a high-resolution record of environmental changes (lake-level and vegetation cover) over the last millennium.
The Lake Joux sediment sequence offers a detailed lake-level record for the Little Ice Age (LIA) and more particularly for the transition between the Medieval Warm Period (MWP) and the LIA. Generally speaking, the late MWP (around AD 1250-1400) and the last century appear to be characterised by lower lake-level conditions than the LIA that experienced marked high-level phases.

The comparison between the Lake Joux lake-level record and the radiocarbon record used as an indicator of past variations in solar activity gives evidence of a general coincidence between phases of decrease in solar irradiance (higher 14C values) and phases of higher lake level. In particular, the Wolf, Spörer, Maunder and Dalton minimums of solar activity corresponded to phases of higher lake level.

With the support of the program ECLIPSE and the MSH-Besançon of the French CNRS, additional investigations have been undertaken in northern Italy to prolong towards southern Europe the palaeohydrological transect documented by Danish and German mire sites in northern Europe.

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Reported by

Université de Franche-Comté
Université de Franche-Comté, UFR des Sciences et Techniques, 16 route de Gray
25 030 Besançon
France
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