Human impact and soil erosion during the last 5000 yrs as recorded in lacustrine sedimentary organic matter at Lac d'Annecy, the French Alps

Sedimentary organic matter has been systematically studied in an eight-metr e long core from the centre of the Petit Lac (Annecy, French Alps). The pal ynofacies composition identifies different terrestrial organic sources incl uding forest floors, soil-horizons and geological substratum. The amount of recycled organic matter derived from the geological substratum is estimate d and subtracted from the other contributions from the catchment area. The palynological record indicates that the relative variations in organic sour ces are directly dependent on human landuse. From ca. 5000 to 1700 BP, the human impact on soil cohesion is very low and organic matter is mainly expo rted from the surficial forest floor. The Roman invasion (ca. 1700 BP) mark s the most important ecological and hydrological change. From 1700 to 900 B P, the clearing of forests released deeper-soil components. This trend is i ncreased after 900 BP with agriculture intensification, which resulted in a higher sedimentation rate. In all the periods, extreme events such as floo d or intensive run-off are characterised by notable increases of organic ma tter from surface (5000 to 1700 BP) and deep (1700 BP to now) soils.