Corrosion Rates Assessment in the Mixed Zone of Coastal Karst Caves by Means of Mass-Loss Rock Tablets (Sa Gleda Cave, Mallorca, Western Mediterranean)

Limestone corrosion and coastal karst cave (flank margin cave) enlargement are closely related by the mixing zone between meteoric and seawater, yet quantitative data on corrosion rates in these environments remain scarce. Recent speleodiving exploration in flanking margin caves in Mallorca revealed numerous submerged cavities with different haloclines between 0 and 25 m below m.s.l. To investigate rock-decay mechanisms along these haloclines, exposure trials were conducted in Cova de sa Gleda. Three sets of water-loss rock tablets (WLRT), composed of bioclastic calcarenite limestone and crystalline aragonite (aragonite crystal aggregates), were deployed along a water-column depth profile ranging from 5 to 16 m. After 749 exposure days, tablets were explored by SEM and XRD. Differences in mass show that calcarenite tablets lost an average of 1.89% of their initial mass, while aragonite tablets have lost 8.05%. Corrosion rates varied along haloclines: at 5 m depth (10 to 16 PSU), rates were 3.10% for calcarenites and 11.08% for aragonite; at 10 m (19 to 29 PSU), corrosion increased respectively to 10.8% and 17.93%; at 16 m (>35 PSU, seawater), corrosion decreased to 1.97% and 3.48%, respectively. These haloclines coincide with the height position of notches and other observable corrosion features within the cave. Consequently, these corrosion features present along the cave can be interpreted as proxies of the former position of the groundwater mixing zone.