Oxygen and hydrogen isotopes of rainfall and dripwater at DeSoto Caverns (Alabama, USA): Key to understanding past variability of moisture transport from the Gulf of Mexico

The Southeast and the US Gulf Coast in particular are notably lacking isotope data in the water cycle despite the fact that moisture transport from the Gulf of Mexico (GOM) has a considerable influence on both regional and continental rainfall patterns. This study reports time-series of oxygen and hydrogen isotopes acquired over a 3-year period (2005-2008) from GOM-derived rainfall, cave dripwater and shallow groundwaters, and offers valuable insights on the links between factors controlling regional rainfall and the ubiquitous karst hydrology.Amount-weighted mean monthly rainwater δ¹⁸O and δD values in Tuscaloosa, Alabama range from −1.5 to −8.3‰ and −1.2 to −49.5‰, respectively, and show mean seasonal amplitudes of ∼4‰ (δ¹⁸O) and ∼25‰ (δD). In comparison d-excess values display large seasonal amplitudes of 10-20‰ resulting from differences in the degree of evaporation from falling raindrops between summer and winter months, and correlate well with the coeval air temperature (r² = 0.59; p < 0.05). Deviations of the Gulf Coast Meteoric Water Line (GCMWL) slope and d-excess from the global meteoric water line (GMWL) are attributed to different rates of evaporation after condensation, and to humidity contrast between the cloud boundary layer and the surrounding atmosphere in the vapor source region, respectively. Rainfall amounts declined during the study interval from an excessive “wet” year, ascribed to six tropical storms incursions during an unusually active hurricane season in 2005, to an onset of a regional drought during 2007-2008 with monthly rainfall amounts substantially below normal values (30-year monthly means). An interannual trend of ¹⁸O and ²H enrichments is discerned from 2005 to 2008 (1.4‰ and 11.6‰, respectively) coeval with the decline in rainfall amounts.Dripwater samples from nearby DeSoto Caverns show weak δ¹⁸O and δD seasonal variations and record only 20% and 51% of the ¹⁸O and ²H enrichments, respectively, discerned in the rainwater 3-year time-trends. The seasonal and interannual amplitude attenuations in the dripwaters are attributed to a relatively thick overlying bedrock (∼30-40 m) and a relatively large, well-mixed, epikarst-storage reservoir. Residence time of water in the cave’s epikarst is estimated to be 1-3 months based on high-resolution flow-rate data.Our investigation suggests that global atmospheric circulation patterns (ENSO and Bermuda High) likely govern the interannual δ¹⁸O and δD isotope trends discerned in the water cycle compartments but much longer time-series are required to confirm our conjectures. The results of this study form a solid basis for future acquisition and interpretation of climate proxy records from regional speleothems.