Isotope disequilibrium effects: The influence of evaporation and ventilation effects on the carbon and oxygen isotope composition of speleothems - A model approach

In order to improve the understanding of variations of speleothem δ18O and δ13C values in the context of palaeoclimate research, it is important to quantify the isotope fractionation processes influencing the δ18O and δ13C values of stalagmites. Here we present an extended version of speleothem stable isotope model that accounts for evaporation and condensation effects during precipitation of calcite. The ISOLUTION-model allows to calculate the effect of evaporation on δ18Ocalcite and δ13Ccalcite values in dependence on relative humidity and wind velocity. Our results reveal that evaporation may have a significant effect on δ18Ocalcite and δ13Ccalcite due to the loss of H2O from the solution layer and the related increase of the Ca2+ concentration. This leads to higher precipitation rates and, consequently, larger isotope fractionation effects. The effect on speleothem δ18Ocalcite and δ13Ccalcite can be as high as changes caused by variations of temperature, drip interval, pCO2.drip and pCO2.cave. Furthermore, the change of pCO2.cave due to ventilation effects, may also affect speleothem δ18Ocalcite and δ13Ccalcite values because of the effect of pCO2.cave on the equilibrium concentrations of the CO2-H2O-CaCO3-system and, thus, the calcite precipitation rate. The variability of other cave parameters (temperature, drip interval, pCO2.drip, pCO2.cave) can have a comparable effect on δ18Ocalcite and δ13Ccalcite values depending on the magnitude of variability. We validated the ISOLUTION-model by modelling the δ18Ocalcite values from a cave-analogue experiment and successfully reproduced the values for high temperatures.