PHREATIC OVERGROWTHS ON SPELEOTHEMS IN THE LITTORAL CAVES OF MALLORCA, WESTERN MEDITERRANEAN: HYDRO- AND GEOCHEMICAL MONITORING

Mallorca’s phreatic overgrowths on speleothems (POS) are an ideal paleo-sea level proxy because they precipitate from brackish water in littoral caves, where the water table is coincident with sea level. POS are documented above, below, and at the current Mediterranean sea level and can be accurately dated using U-series techniques, thus precisely constraining the elevation of each sea-level stand.

POS mineralogy alternates between calcite and aragonite; sometimes both minerals occur within the same cave, but at different sea level stands. To determine the controls of aragonite/calcite precipitation, this study monitored an aragonite- and a calcite-precipitating site in Vallgornera and Drac caves, respectively. In the continuous aquatic (temperature, pH, and specific conductivity) and cave atmospheric (temperature and CO₂) records, higher variability was recorded in Vallgornera, whereas Drac was more stable. CO₂ levels were higher in Vallgornera (2,493 ppm maximum) than Drac (1,110 ppm maximum). While cave ventilation is more efficient in the latter cave, water pCO₂ calculations (using total alkalinity and pH) indicate a greater CO₂ concentration gradient from the cave water to the cave atmosphere, documenting the role of CO₂ degassing.

Salinity ranged from 6.2 - 8.6 PSU in Drac, higher than the 5.5 - 6.5 PSU range observed in Vallgornera; this indicates a greater marine influence over the calcite-precipitating POS in Drac. Monthly cave water samples and vertical profiles indicate higher δ¹⁸O values and higher salinity with increasing depth, with the surface influenced by meteoric water. δ¹⁸O and salinity both trend toward the Mediterranean Sea endmember with depth in the caves.

Water samples collected from 0.2 m and 1.0 m depths in Drac show more positive δ¹³C values at the surface. Vertical δ¹³C profiles collected every 0.5 m show more positive values in the surface layer in Drac. δ¹³C profiles collected in Vallgornera did not have a positive offset at the surface. Monthly vertical pH profiles record similar results, with positive offsets of 0.5 – 1 pH unit at the water surface in Drac and only intermittently in Vallgornera. Combined with considerations of CO₂ gradient between the cave water and atmosphere, it is evident that degassing is episodic in Vallgornera Cave, and more consistent in Drac Cave.