Climatic influences on the offset between d18O of cave drip waters and precipitation inferred from global monitoring data

We present a meta-analysis of data from 22 caves and 96 drip sites from 4 continents where both the cave drip water d18O and the weighted mean d18O of precipitation have been measured. Drip water d18O is similar to the weighted mean d18O of precipitation (within ± 0.3 ‰) for sites where mean annual temperature (MAT) is less than 15 °C (85% of drips where MAT < 15 °C) and an aridity index (annual precipitation (P) / annual potential evapotranspiration (PET)) is greater than 0.65 (74% of drips at sites where P/PET > 0.65). In contrast, at warmer locations with increased water deficit, drip water d18O deviates from the weighed mean precipitation d18O by +3 ‰ and -1.5 ‰. We argue that this is due to evaporation in the soil and shallow vadose zone (thereby increasing drip water d18O) and lower water storage in the vadose zone, leading to relatively less mixing (thereby increasing the range in drip water d18O to more closely reflect recharge water d18O). Speleothems that have formed close to isotopic equilibrium are likely to have an oxygen isotope composition that contains a mixed signal of cave air temperature and precipitation d18O only in cool and temperate regions (T < 15 °C), or very wet climates where P/PET >> 0.65. In contrast, in warmer and drier environments, speleothems which have formed close to equilibrium will have d18O that reflects cave air temperature and a seasonal bias toward the d18O composition of rain in periods of high recharge, as well as the extent of evaporative fractionation of stored karst water.