THE SECRET TO SULFURIC ACID SPELEOGENESIS

Speleogenetic models are almost entirely based on abiotic chemical and hydrologic controls, while biological controls are typically considered too mysterious, too complicated, or are just ignored. Recently, the sulfuric acid speleogenesis (SAS) model has amended the classic limestone dissolution dogma, revealing the impact of acids other than carbonic acid to cave formation processes. The original SAS model, formulated from observations of Lower Kane Cave (Wyoming), was based entirely on chemical oxidation of H₂S to sulfuric acid, and the reaction of the acid with limestone to form gypsum that quickly dissolves. However, reduced sulfur compounds (e.g. H₂S) also serve as rich energy sources for microorganisms. A more recent investigation of Lower Kane Cave reveals a subsurface system colonized by microbial mats of surprisingly complex consortia of sulfur-oxidizing bacteria, as well as fermenting, methanogenic, and sulfate-reducing microbes. These microbes tightly cycle C and S, and affect SAS in three ways as a direct result of their specific metabolisms: 1) sulfuric acid production, 2) local generation of H₂S gas, and 3) colonization of cave-wall surfaces. Observations of cave limestone and in situ microcosm experiments demonstrate that the bulk of subaqueous limestone dissolution is done by sulfur-oxidizing bacteria that directly colonize surfaces, despite cave water being slightly oversaturated with respect to calcite. Laboratory enrichment cultures and cave gas measurements reveal the presence of a vigorous anaerobic community in the stream mats that produces autochthonous H₂S. The H₂S that volatilizes into the cave air is oxidized at the cave walls where aggressive dissolution and gypsum replacement of carbonate surfaces proceed. Interactions between cave-wall biological (microbial colonization) and physicochemical (interfacial phenomena) factors influence subaerial speleogenesis. The net results of microbial processes are the acceleration of limestone dissolution by H₂S oxidation to sulfuric acid, enhanced H₂S volatilization into the cave atmosphere, and ultimately cave formation. Recognition of these microbial contributions reveals the non-secretive role of microbes to speleogenesis and fundamentally refines the SAS model.