Global analysis reveals climatic controls on the oxygen isotope composition of cave drip water

The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ(18)O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ(18)O is most similar to the amount-weighted precipitation δ(18)O where mean annual temperature (MAT) is < 10 °C. By contrast, for seasonal climates with MAT > 10 °C and < 16 °C, drip water δ(18)O records the recharge-weighted δ(18)O. This implies that the δ(18)O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ(18)O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.