Vegetational development during the Late-Würm at Lobsigensee (Swiss Plateau)

Abstract

Lobsigensee is a small lake situated northwest of Bern at 514 m asl and was covered by Rhone ice during the Wurm glaciation. Palynological and plant macrofossil studies of a Late-Wurm deposit in the littoral are presented. The stratigraphy of the sediments is from bottom to top : sand, sandy clay, clay, lake marl, peat. The Oldest Dryas consists of three local pollen assemblage zones recording the gradual establishment of a treeless vegetation rich in heliophilous and pioneer species and also containing dwarf shrubs in its third phase. At the transition from clay to lake marl a sharp Juniperus peak initiates the Bolling which is mainly dominated by tree-birches. This shift from dwarf birch to tree-birches is confirmed by the macrofossils analyzed. An equivalent of the Older Dryas is not found. The beginning of the Allerod is characterized by the expansion of Pinus and its end by the volcanic ash from Laach. There are slight but consistent indications of a more open vegetation during the Younger Dryas. The transition from lake marl to peat coincides with the boundary between Late-Wurm and Holocene.