Petit-Lac (western Lake Geneva) environment and climate history from deglaciation to the present: a synthesis



During the past decade, the presentation of seismic and sedimentological data has allowed reconstruction of the environment and climate history of the Petit-Lac (western Lake Geneva). Methods such as high-resolution seismics, sediment core analysis (macroscopic description, grain-size analysis, mineralogy) and palynology have been used to infer the changes in the lake's environment from deglaciation to the present. However, no final synthesis has been attempted to link this information in the development of a comprehensive evolution model of the Petit-Lac and its surrounding region. The Petit-Lac deglaciation occurred in three phases during the Rhône glacier retreat: the Geneva stage and the Coppet and Nyon re-advances. In the Versoix area, rivers developed just after the retreat of the Rhone glacier from the Nyon stage. The Nyon fan delta started at the end of the Bølling, and its lobe fluctuated in size and orientation in six phases from the Lateglacial to the present. The action of bottom currents (i.e. erosion, non-deposition surfaces) arising at the beginning of the Holocene indicates that the frequency and direction of strong wind regimes varied greatly. Lacustrine mass failures occurred at different time intervals: two between deglaciation and the end of the Oldest Dryas, two between the Bølling and the Younger Dryas, and four during the Holocene. From the Oldest Dryas to the Contemporary Epoch, the vegetation changed from a steppe to a climate-influenced forest, and finally to a mostly human-controlled forest.