Long-term monitoring of drip water and groundwater stable isotopic variability in the Yucatan Peninsula: Implications for recharge and speleothem rainfall reconstruction

Hydroclimate interpretation of stalagmite delta O-18 records from tropical regions requires an understanding of the temporal integration of rainfall amount and its isotopic composition by drip waters that form stalagmite deposits. This study presents oxygen (delta O-18) and hydrogen (delta D) isotopic results from over 1200 groundwater, rainfall and drip water samples, collected at similar to weekly time intervals, over three hydrological years at Rio Secreto Cave, in the Yucatan Peninsula, Mexico. Cave environmental conditions and the isotopic composition of drip water were monitored in three chambers with different degrees of air ventilation, along with temperature and relative humidity conditions at the surface. We examined 16 drips and observed that annual delta D and delta O-18 variability reflects the isotopic variability of rainfall to varying degrees. The observed annual amplitude of drip water isotopic variability represents between 5% and 95% of that of rainfall, reflecting epikarst water reservoir size and the complexity of flow paths. Drips that closely reflect the isotopic variability of rainfall and best preserve the isotopic signal of individual rainfall events are observed, but they are uncommon. Only two drips out of 16 were found to have potential to record rainfall isotopic shifts associated with tropical cyclones if sampled at weekly resolution. The relationship between delta D and delta O-18 in drip water suggests that recharge is biased toward the rainy season (June to November), which represents up to 80% of total annual precipitation. We find that over the course of a year most drips reflect the annual delta O-18 composition of rainfall, in support of quantitative precipitation estimates from stalagmite delta O-18 records. We find evidence that the effective recharge in this cave system is controlled by precipitation amount and that recharge is not limited to the months when precipitation exceeds evaporation. (C) 2018 Elsevier Ltd. All rights reserved.