SULFUR DYNAMICS DURING SULFURIC ACID SPELEOGENESIS WITHIN LOWER KANE CAVE, WYOMING

Lower Kane Cave is forming from sulfuric acid speleogenesis, and the exchange of S between redox states in the air, water, sediment, and microbial biomass alters the transport and distribution of S in the cave. Dissolved sulfide enters the cave passage via four springs. The carbonate cave walls are being replaced by gypsum and the cave floor is covered with fallen gypsum and chert cobbles. Sulfide is an energy source for microbes, and thick mats dominated by chemoautotrophic S-oxidizing bacteria have developed downstream from each spring. S exists in multiple redox states, including S gases, aqueous sulfide, aqueous sulfate, elemental S in biomass, and the mixed-redox species of sulfoxy-anions that exist briefly as aqueous intermediates of biotic and abiotic S cycling. H₂S gas was measured by GC directly above the stream surface, and the concentration averaged 30 ppmv. No H₂S was measured over the spring orifice, although high aqueous sulfide concentrations were detected at the spring orifice (~0.85 ppm). The highest gas concentration was measured directly over the microbial mats 20 m downstream where aqueous sulfide decreases to ~0.03 ppm. The aqueous sulfate concentration is constant at 110 ppm and the cave waters remain highly undersaturated with respect to gypsum. Low levels of aqueous thiosulfate (0.09 ppm) and sulfite (1 ppm) were detected. The stream-bed sediment contains S in various forms, determined by a modified Johnson-Nishita method, with acid-volatile sulfur (AVS) being dominant; AVS content is greatest in the stream deposits, at 0.8% (wt/dry wt) of total sediment. The AVS fraction contributes to the cycle of iron and other metals in the cave. Microbial mats contain 2% to 50% sulfur by weight; mat samples with the highest sulfur content contain abundant elemental sulfur. Although the stream water is at equilibrium or supersaturated with respect to calcite, microcosm experiments indicate that dissolution of calcite occurs along the stream bed at microsites of lowered pH associated with S-oxidizing bacterial filaments. Microbial S-oxidation coupled to primary productivity contributes to the available ecosystem energy. The S-cycle is tightly linked to both biotic and abiotic C-cycling through the metabolisms of S-oxidizing and S-reducing microbes and by driving dissolution of the carbonate host rock.