Drip water measurements from Carlsbad Cavern: implications towards paleoclimate records yielded from evaporative-zone stalagmites

Stalagmites can host numerous potential climate proxies (stable and radiogenic isotopes, trace elements, annual and non-annual banding, grayscale, growth hiatuses, mineral assemblage). Reproducibility and/or integration of proxy results between one or more stalagmites will become increasingly important, and ideally, climate records generated by multiple stalagmites from the same cave or cave room are expected to be near-identical. The reality is that stalagmites from the same cave room can yield differing results to some degree, especially in cave environment zones that are evaporative. Our drip water study in an evaporative shallow-depth cave environment in Carlsbad Cavern shows that adjacent drip sites produce differing drip rate behavior, but share some similarities. Drip water collected from four sites in this evaporative cave environment shows Ca, Mg, Sr, Ba, U, and Th elemental concentrations and 234U/238U (expressed as δ234U) to vary seasonally, and all but U have higher values during the winter months when the instrumented cave site exhibited slower drip rates and was drier. Results from our Carlsbad Cavern drip sites indicate that increased relative humidity in the cave and decreased surface and cave atmospheric pressure combined with increased precipitation (rain and snow) are responsible for faster drip rates. Changes in atmospheric pressure play an essential role, although less directly during summer months. We therefore conclude that stalagmites within the same cave room may not record and produce identical proxy records in these more evaporative cave environments, but that differing records are simply recording the same climate signals expressed uniquely by the individual proxies, and that each stalagmite simply has differing sensitivities to the climate signals. Integrated, these proxy differences serve as important past climate indicators. Our drip sites respond to seasonal variations in climate more so than individual rain/snow events, and we favor the interpretation that seasonal changes reflect regional as well as local climate changes