Seasonal variability and threshold effects of the organic detrital sedimentation in lakes: imbalances between organic records and climatic fluctuations (Chaillexon basin, Doubs, France)

Optical analysis (palynofacies) of the organic matter (OM), occurring in va rious compartments (soils, bedrock formations, lacustrine chalks) of the la custrine basin of Chaillexon (Doubs, France) has led to the definition of o rganic markers. Their Variable contributions were observed in suspended mat ters taken over a period of a year (annual scale) and in lacustrine sedimen ts covering the last 12,000 years B.P. (millennial scale). Results obtained lead to three main conclusions. Firstly, there exists a seasonal variation in the detrital organic fluxes, directly linked to the autumnal litter production. This allows us to distin ct the detrital OM loaded during autumn-winter organic floods and spring-su mmer ones. Secondly, the organic record in Boreal to modern sediments of th e lake of Chaillexon, fed by autumn-winter fluxes, appears mainly controlle d by the autumnal productivity of vegetal biomass. Thirdly, the organic record does not point out a progressive evolution of o rganic fluxes linked with successive climatic phases. It shows abrupt ruptu res of the balance (threshold effects) corresponding with the passage of th e tree line in the watershed. These ruptures are not necessarily synchronou s with climatic fluctuations.