Abstract: In cave environments, water vapor condensation occurs naturally when warmer/wet air masses flow close to colder cave surfaces. Artificial microclimate perturbations in show caves can enhance this process, leading to potential deterioration of rock art and degradation of speleothems. Here we investigate the triple oxygen and hydrogen isotopic compositions of condensation water in Altamira Cave (Cantabria, northern Spain) to evaluate the potential of stable isotopes in the study of condensation mechanisms in caves. We assess the role of cave ventilation in the spatiotemporal isotopic variability of condensation water in Altamira Cave. To this end, water drops that condense naturally on artificial supports in different parts of the cave were collected for 7 years and their isotopic compositions (?17O, ?18O, ?D and derived parameters 17O-excess and d-excess) were compared to those of droplets with no apparent dripping taken from the cave ceiling (i.e. presumably condensation water) and fast dripping points (i.e. infiltration water) during the same period. Condensation waters in the outmost cave sectors, closer to the entrance, show higher ?17O, ?18O and ?D values during the cave ventilation period (June to October) compared to the rest of the year. This seasonal pattern can be explained by changes in the contributions of two moisture sources for condensation: advection of allochthonous water vapor from outside during the cave ventilation period and recycling of autochthonous vapor generated from cave dripwater during the stagnation period. In contrast, the isotopic values of condensation waters in the inner cave sectors are similar to those of infiltration water, with insignificant seasonal variability. This suggests that water condensation in the inner cave sectors is sourced by autochthonous vapor, with no significant contributions of external moisture, even during the cave ventilation period. We conclude that allochthonous water vapor condenses preferentially in the Entrance Hall and does not affect significantly the rest of the cave. These results are relevant for the management of Altamira Cave and for future investigations on condensation mechanisms in cavities elsewhere.

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