Tritium, Chlorine-36, and water isotopologue profiles from Lechuguilla Cave and Carlsbad Cavern, New Mexico, USA

Abstract

Carlsbad Cavern and Lechuguilla Cave are large limestone caves located within Carlsbad Caverns National Park, southeastern New Mexico, USA. Both caves feature numerous pools that enable sampling of locally perched water bodies at different depths within the thick vadose zone. Isotopic analysis of multiple water samples from both caves revealed clear evidence of bomb-pulse tritium (up to 6.1% bomb-pulse maximum, BPM) and 36Cl (up to 5.2% BPM) in many pools. In all cases, isotope concentrations were well below peak bomb-pulse levels, indicating complex mixing processes between the ground surface and the cave pools. The deepest bomb-pulse detections in the caves indicate vertical transport velocities of up to 6–9 m/yr for the fastest flowpaths. Peak tritium levels in Lechuguilla are lower than those in Carlsbad, potentially due to mixing and dilution via vapor-phase exchange through the high relative humidity cave atmosphere, processes that may be less effective in Carlsbad's drier atmosphere. Peak 36Cl/Cl ratios in Carlsbad are lower than those in Lechuguilla, possibly due to dilution by introduced stable Cl− associated with tourist developments in the cave. Stable isotope results demonstrate the different evaporative regimes of the two caves, caused by Carlsbad Cavern's large natural entrance, in contrast to Lechuguilla's complete lack of a present-day natural entrance. Our results bolster the use of speleothem records for paleoclimate reconstruction, demonstrating the effective transmission of the bomb-pulse signal to depths of hundreds of meters within a few decades. At the same time, the pronounced and variable impact of evaporation on some samples shows the potential importance of local conditions on specific sample results.