EVIDENCE FOR LATE-GLACIAL AND HOLOCENE ENVIRONMENTAL-CHANGES FROM SUBFOSSIL ASSEMBLAGES IN SEDIMENTS OF LAKE NEUCHATEL, SWITZERLAND

Ostracod and diatom assemblages, mollusc remains, plus pollen and sedi ment stratigraphy from three cores taken at sites 40, 80 and 135 m dee p in Lake Neuch (a) over cap tel, Switzerland, show distinct changes s ince deglaciation, Shifts are interpreted in terms both of climatic ch ange and major changes in the catchment input caused by the Aar River entering or bypassing Lake Neuch (a) over cap tel. Three palaeoenviron mental phases are recognized: (1) the Late Glacial, with the inflow of the river before 10,000 yr B.P.; (2) the early to mid-Holocene, with temporary bypassing of the river between 10,000 and 4850 yr B.P.; and (3) the mid- to late Holocene, without river input after approx. 4850 yr B.P. During periods with river input, rhythmites were deposited, Os tracods indicate a well oxygenated hypolimnion, suggesting deep ventil ation enhanced by the river inflow. The diatom assemblage indicates eu trophic to mesotrophic conditions and is dominated by generally well p reserved periphytic species, The high number of unidentifiable pollen suggests oxidation during transport in the river. During periods witho ut river input, a massive calcareous silt with abundant authigenic cal cite was deposited, diatoms indicate a change to mesotrophic and oligo trophic conditions, and decreased sedimentation rates and higher alkal inity enhanced the dissolution of diatoms. Terrestrial and littoral mo lluscs as well as periphytic diatoms indicate erosion of the shore whe n the lake level rose or fell with onset or offset of the river inflow , Before 12,600 yr B.P., the patchy vegetation cover led to intensive soil erosion and high sedimentation rates. The lake was still frozen d uring the winter months. After approx, 12,600 yr B.P. the sediment cha nged from elastic rhythmites to thinly bedded, non-glacial rhythmites, the vegetation cover became denser, the abundance of ostracods increa sed, and molluscs started to colonize the profundal zone of the lake. The Younger Dryas (approx. 10,700-10,000 yr B.P.) displays maximum abu ndance of ostracods, indicating deep ventilation. The transition to th e Holocene is characterized by a change in the ostracod assemblage and the river bypassed the lake between approximately 10,000 and 8900 yr B.P. When the river reentered the lake after approximately 8900 yr B.P ., a new ostracod species appeared, After two short periods of river b ypassing between approximately 7200 and 6950 yr B.P., diatoms indicate a higher trophic level about 6000-5000 yr B.P. associated with higher erosion rates and increased nutrient input due to Neolithic agricultu ral activities. After approximately 4850 yr B.P, the river bypassed th e lake again and the trophic level decreased. In contrast to the early to mid-Holocene periods of decreased oxygen supply, the mid- to late Holocene is characterized by deep ventilation. This is attributed to t he northward retreat of the polarfront and enhanced westerlies consequ ent to decreased insolation after 6000 yr B.P. The change of the river flow pattern correlates with glacier oscillations and may suggest a c limatic control. The final cessation of the inflow is attributed to di version of the river consequent to isostatic uplift and late Neolithic clearance activity. (C) 1998 Elsevier Science B.V. All rights reserve d.