Disequilibrium carbon and oxygen isotope fractionation in recent cave calcite: Comparison of cave precipitates and model data

Speleothem proxy data provide important information in continental palaeo-climate research due to their precise chronology and wide geographic distribution. Despite a continuously growing number of field and numerical studies designed to study stable isotope fractionation effects, many aspects remain a matter of debate. Here, carbon and oxygen isotope ratios from cave drip water and calcite precipitates sampled on watch glasses in the Bunker Cave (Western Germany) are compared with the values expected for isotopic equilibrium. Furthermore, the field data are compared with the results of a numerical model simulating stalagmite growth and stable isotope ratios.Two drip sites with different drip rates were investigated. Drip site TS 1 is characterised by a high drip rate, and drip water of this site is characterised by a high saturation index with respect to calcite. TS 1 shows no evidence of prior calcite precipitation. Conversely, drip site TS 8 shows a low drip rate, and drip water from this site is characterised by a low saturation index. TS 8 shows evidence of prior calcite precipitation. Whereas the mean δ13CDIC values of the drip water are significantly different between the two drip sites, the mean δ18Odrip water values are similar. Calcite precipitation rates are higher at watch glass site U I corresponding to drip site TS 1. This is probably due to the higher SIcalcite and drip rate. The δ18O and δ13C values of the calcite precipitated on watch glasses U IV corresponding to drip site TS 8 are significantly higher than those of the calcite collected on watch glasses U I. Calcite precipitation occurs close to isotopic equilibrium at watch glass site U I, whereas the calcite precipitated at watch glass site U IV shows clear evidence of isotopic disequilibrium.The results suggest that cave calcites precipitated at lower drip rates have higher δ13C and δ18O values than predicted by equilibrium precipitation. Furthermore, our data show that stalagmites from the same cave, which are fed by drip water with similar δ18O values, may have different δ18Ocalcite values.The δ13C and δ18O values predicted by the numerical model are generally lower by 1.9‰ and 0.8‰, respectively, than those of the natural cave calcite. Nevertheless, the first-order trends observed in the cave data are well reproduced by the model. The offset between cave and model data is probably due to the fractionation factors used in the model.