1. The survival of dissolved H 2 S is very pH, temperature, and salinity dependent: H 2 S + H 2 O → H 3 O + HS diprotic acidpKa 2 = 19 ± 2 Fig. 1

2. Chemistry of Spring Water of Active Sulfuric Acid Caves CAVE/COUNTRY CAVE SPRING WATER PARAMETERS Temperature °CpHSO 4 mg/LCl mg/LH 2 S mg/LDO mg/L Cueva de Villa Luz, Mexico 28-306.61-7.3920-940782-814100-500ND-2.2 Frasassi Caves, Italy 13.1-13.76.9-7.4≤240658.7≤22.2-- Acquasanta Terme Caves, Italy 33.1-43.46.58-6.75669-9571697-310013.0-28.1-- Montecchio Cave, Italy 31.3-366.86131318.8-- Parrano Cave, Italy 276.3317320810-- Serpents Cave, France 35.5-46.66.560-23015-305-- Aghia Paraskevi Caves, Greece 35-39.26.35-7.0--19,467.5101.6-- Lower Kane Cave, USA 22.6-246.8-7.25182-31912.3-14.55-6ND Fairy Cave, USA 49.66.39110297061.650.48 Fig. 2

3. H 2 S + 2O 2 H 2 SO 4 2H 2 S (aq) + O 2(aq) → 2S + 2H 2 O Further oxidation yields SO 3, S 2 O 3, and finally SO 4. Biotic oxidation enhances the rate by 3 or more orders of magnitude, but the outcome is the same. Sulfuric acid is the world’s number one industrial chemical, and over 230 million metric tons were produced in 2012. It is produced in a 4-step process: S (s) + O 2 → SO 2, 2SO 2 + O 2 → 2SO 3, SO 3 + H 2 O → H 2 SO 4 2H 2 S (g) + 3O 2(g) → 2SO 2 + 2H 2 O >200 ⁰C Fig. 3 Or as an alternative sulfur source:

4. gypsum rinds biofilms pipe A modern analog outside of speleogenesis: Biogenic Sulfide Corrosion Internal corrosion of concrete wastewater pipes and tanks (man-made caves) due to anaerobic sulfate-reducing bacteria generating H 2 S, which is oxidized to sulfur in the condensate, which aerobic sulfur-oxidizing bacteria metabolize into H 2 SO 4. Only occurs if the pipe or tank is partially full (air space above water is required) The acid converts CaCO 3 into gypsum A process studied since the 1940s Fig. 4