LATE HOLOCENE LEVEL FLUCTUATIONS OF THE LAKE ILAY IN JURA, FRANCE - SEDIMENT AND MOLLUSK EVIDENCE AND CLIMATIC IMPLICATIONS

The level fluctuations of Lake Ilay, Jura (France) during the last thr ee millennia are reconstructed from sedimentological and malacological analyses of a core that is well-dated by tree-ring, radiocarbon and p ollen datings. Changes in sediment facies, in carbonate concretion ass emblages and in mollusc assemblages highlight a major lowering phase a t c. 1550 BP and minor lowering phases at c. 2800 BP and shortly befor e AD 1394. Rises in lake level developed during the early Subatlantic and between c. 1500 and 1000 BP. These data are in good agreement with other proxy data from higher European and American latitudes. These c orrelations support the climatic origin of the level fluctuations of t he Lake Ilay during the late Holocene. They suggest that the mediaeval climatic optimum is centred rather in the early than the late Middle Age.