Transference of isotopic signal from rainfall to dripwaters and farmed calcite in Mediterranean semi-arid karst

This study was conducted in Ejulve cave (Spain, southwestern Europe) over a period of four years (2013-2016) to identify the factors affecting the isotopic composition of rainfall, dripwaters, and farmed calcite. The results indicate that δ18O of rainfall in the study area was mainly controlled by the temperature effect (r2 = 0.82, p-value < 0.001) with source effect and amount effect being secondary controls. Additionally, it exhibited a seasonal pattern with lower values indicated during winter and higher in summer. The use of effective infiltration to evaluate the recharge of the epikarst is extremely important in semi-arid karst climate conditions. This analysis revealed that while the amount effect exerted a moderate control on the annual δ18O in rainfall, it had a lower influence on the isotopic variability of the rainfall potentially infiltrating the karst aquifer. The sampling of 12 drip locations in Ejulve cave indicated that seepage and seasonal drips featured similar δ18O values in dripwaters. The homogenisation process during the infiltration in the epikarst was not complete, therefore, the δ18O seasonality seen in rainfall was also observed in dripwater δ18O, albeit attenuated and lagged. With the seasonal increase in rainfall in autumn, drip rates of seasonal and seepage groups increased within a few weeks. However, the lower δ18O of autumn-winter rain was observed in dripwaters with a lag as long as 21 weeks. The δ18O in farmed calcite showed a small variation of 0.11‰ while the δ13C varied seasonally with a variation of about 0.22‰ and indicated higher values during summer and autumn and lower values in winter and spring. Seasonal and seepage sites exhibited similar δ18O variability. However, δ13C was more positive in seepage sites and was enhanced by degassing kinetics to produce a variability of up to 0.42‰ in contrast to the 0.29‰ observed in the seasonal sites. The δ18O of calcite showed dependence on the δ18O of dripwater with a weak influence of the drip rate. However, δ13C exhibited a higher correlation with the drip rate and high δ13C was explained by enhanced degassing and low water availability while low δ13C values were in concordance with higher microbial activity, vegetation productivity, and greater water availability. The ventilation of the cave during late autumn through spring favoured the higher calcite deposition rates found during that time.