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 delta O-18 and delta C-13 values in the context of palaeoclimate research, it is important to quantify the isotope fractionation processes influencing the delta O-18 and delta C-13 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 delta O-18(calcite) and delta C-13(calcite) values in dependence on relative humidity and wind velocity. Our results reveal that evaporation may have a significant effect on delta O-18(calcite) and delta C-13(calcite) 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 delta O-18(calcite) and delta C-13(calcite) can be as high as changes caused by variations of temperature, drip interval, pCO(2).(drip) and pCO(2.cave). Furthermore, the change of pCO(2.cave) due to ventilation effects, may also affect speleothem delta O-18(calcite) and delta C-13(calcite) values because of the effect of pCO(2).(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, pCO(2.drip), pCO(2.cave)) can have a comparable effect on delta O-18(calcite) and delta C-13(calcite) values depending on the magnitude of variability. We validated the ISOLUTION-model by modelling the delta O-18(calcite) values from a cave-analogue experiment and successfully reproduced the values for high temperatures. (C) 2012 Elsevier Ltd. All rights reserved.