STABLE ISOTOPES IN LAKE GENEVA CARBONATE SEDIMENTS AND MOLLUSKS - REVIEW AND NEW DATA

New isotopic results on bulk carbonate and mollusc (gastropods and biv alves) samples from Lake Geneva (Switzerland), spanning the period fro m the Oldest Dryas to the present day, are compared with pre-existing stable isotope data. According to preliminary calibration of modern sa mples, Lake Geneva endogenic calcite precipitates at or near oxygen is otopic equilibrium with ambient water, confirming the potential of thi s large lake to record paleoenvironmental and paleoclimatic changes. T he onset of endogenic calcite precipitation at the beginning of the Al lerod biozone is clearly indicated by the oxygen isotopic signature of bulk carbonate. A large change in delta(13)C values occurs during the Preboreal. This carbon shift is likely to be due to a change in biopr oductivity and/or to a ''catchment effect'', the contribution of bioge nic CO2 from the catchment area to the dissolved inorganic carbon rese rvoir of the lake water becoming significant only during the Preboreal . Gastropods are confirmed as valuable for studies of changes in paleo temperature and in paleowater isotopic composition, despite the presen ce of a vital effect. Mineralogical evidence indicates an increased de trital influence upon sedimentation since the Subboreal time period. O n the other hand, stable isotope measurements of Subatlantic carbonate sediments show values comparable to those of pure endogenic calcite a nd of gastropods (taking into account the vital effect). This apparent disagreement still remains difficult to explain.