BIOLOGICAL AND ABIOTIC CONTROLS ON SULFIDE OXIDATION, ACID PRODUCTION, AND CARBONATE DISSOLUTION IN THE FRASASSI CAVE SYSTEM, ITALY

Sulfidic caves form in carbonate bedrock where H₂S-rich groundwaters interact with oxygenated surface waters and cave air. At the cave watertable, the fate of dissolved sulfide is controlled by three processes that independently influence the rate of cave formation: degassing of H₂S(g) to the cave atmosphere, biological oxidation in sulfur-oxidizing biofilms, and abiotic sulfide oxidation. In order to evaluate how each of these processes contribute to H₂S disappearance and acid production, we used a combination of field measurements and transport modeling to quantify degassing and oxidation in sulfidic streams throughout a large sulfidic cave complex containing more than 26 km of passages (Frasassi cave system, Italy).

Unexpectedly, we found that H₂S(g) degassing accounts for the majority of sulfide disappearance from Frasassi streams, highlighting the potential importance of subaerial sulfide oxidation and acid production in the evolution of sulfidic caves. Consistent with this result, measured limestone dissolution rates are equally high above and below the cave watertable. In cave streams, biological oxidation accounts for the majority of subaqueous sulfide oxidation, and measured microbial rates are several orders of magnitude faster than predicted for abiotic oxidation. However, we found little evidence for sulfuric acid production within the microoxic, S-oxidizing stream biofilms. Microsensor profiles of H₂S, O₂, and pH in the biofilms suggest that elemental sulfur is the primary end-product of biological sulfide oxidation. pH decreases rapidly with depth in anoxic sediments below the biofilms, likely due to microbial fermentation. This phenomenon represents a secondary biological influence contributing to limestone dissolution in sulfidic caves.