Dating cave drip water by tritium

Speleothems are increasingly used as an archive of past climate, but some of the proxy signals encoded in these deposits reflect hydrological characteristics of the karst aquifer (and not necessarily climate variability). A central aspect in karst hydrology is the time required for the rainwater to reach the point of discharge in a cave, e.g. the tip of the stalactite. One promising approach in determining this residence time is drip-water dating by tritium ( 3H). In contrast to traditional tritium dating, we do not refer directly to tritium concentrations in precipitation as input function, but to an infiltration-weighted annual mean of the rainwater values. Using concentration differences between this infiltration-weighted mean and the drip water, an age is calculated from the radioactive decay law, assuming piston flow. The approach was tested in three adjacent caves in northwestern Germany which were monitored for about two years. All of the studied drip sites yielded drip water ages between 2 and 4 years with uncertainties on the order of 1 year. These results were confirmed at several drip sites by oxygen isotope data which show rather constant values with insignificant intra-annual variability. Attempts to apply the 3H– 3He method resulted in comparable ages, despite several complicating factors.