Oxygen and carbon isotopes in modern fresh-water ostracod valves: assessing vital offsets and autecological effects of interest for palaeoclimate studies

The isotopic composition of modern benthic ostracod and mollusc carbonate, the oxygen isotope composition of lake water (δ18OW), the carbon isotope composition of dissolved inorganic carbon (δ13CDIC), and water temperatures were monitored during a year, at different water depths in two lakes (Ammersee and Starnberger See, southern Germany). The aim was to test whether the valves are built in isotopic equilibrium and how seasonal water temperatures are recorded in the shells. We find systematic offsets between δ18O of valve carbonate compared to δ18O expected for a theoretical calcite formed in isotopic equilibrium. Offsets are higher than +2‰ for different species of the Candona group, +1.5‰ for Cytherissa lacustris, and around 0.8‰ for Limnocythere inopinata and Darwinula stevensoni. They seem to be constant for all instars of a species and temperature independent. Mean water temperatures of the calcification period are readily reflected in the valve-δ18O if corrected for the offsets. δ13C of valve carbonate is mainly controlled by δ13C of the dissolved carbonate in ambient lake water, but seems to be biased by micro-habitat effects and vital offsets. The study provides information on the seasonality of valve formation, thus potentially allowing the quantification of temporal and spatial water temperature gradients from specific δ18O records.