New isotopic results on bulk carbonate and mollusc (gastropods and
bivalves) samples from Lake Geneva (Switzerland), spanning the period
from the Oldest Dryas to the present day, are compared with pre-existing
stable isotope data. According to preliminary calibration of modern
samples, Lake Geneva endogenic calcite precipitates at or near oxygen
isotopic equilibrium with ambient water, confirming the potential of
this large lake to record paleoenvironmental and paleoclimatic changes.
The onset of endogenic calcite precipitation at the beginning of the
Allerod 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
bioproductivity and/or to a `'catchment effect'', the contribution of
biogenic CO2 from the catchment area to the dissolved inorganic carbon
reservoir of the lake water becoming significant only during the
Preboreal.
Gastropods are confirmed as valuable for studies of changes in
paleotemperature and in paleowater isotopic composition, despite the
presence of a vital effect.
Mineralogical evidence indicates an increased detrital influence upon
sedimentation since the Subboreal time period. On the other hand, stable
isotope measurements of Subatlantic carbonate sediments show values
comparable to those of pure endogenic calcite and of gastropods (taking
into account the vital effect). This apparent disagreement still remains
difficult to explain.