δ18O values of cave drip water: a promising proxy for the reconstruction of the North Atlantic Oscillation?

Abstract

We present cave monitoring data and two drip water δ18O models for the Herbstlabyrinth-Adventhohle (HL) cave system, Germany. Winter climate of the cave region is influenced by the North Atlantic Oscillation (NAO), as documented by the positive correlations between winter temperature, winter rainfall δ18O values and the winter NAO index. Cave monitoring data show that recharge of the aquifer at the HL cave system mainly occurs during winter months, confirming our drip water model, which calculates recharge as the difference between precipitation and potential evapo-transpiration. Cave drip water from the HL cave system should, thus, be a sensitive proxy for reconstruction of the NAO. The infiltration models accounting for mixing effects occurring in the karst aquifer show a very good agreement with the monitoring data. Comparison of the modelled and the monitored drip water δ18O values suggests a mean residence time of the water in the aquifer of 12 months and a transmission time of 10 months. Using these parameters, a long-term dataset from the nearby meteorological station Frankfurt am Main and the rainfall δ18O data from our monitoring program, we model the δ18O values of the cave drip water for the last 144 years. Despite of the favourable conditions of the HL cave system for NAO reconstruction, the modelled drip water δ18O time series only shows a moderate correlation with the winter NAO index (r = 0.33). The major reason for the relatively low correlation is the substantial contribution of precipitation from the remaining seasons to the drip water, for instance, due to heavy precipitation events occurring during summer. These rainfall events bias the seasonality of infiltration and weaken the winter NAO signal recorded in the drip water δ18O values. Reconstruction of the NAO from speleothems is, thus, challenging, in particular considering further complications arising from chronological uncertainties and calcite precipitation under conditions deviating from isotopic equilibrium. These problems may, at least in part, be avoided by multi-proxy studies of annually laminated stalagmites.