WATER, GAS, AND PHYLOGENETIC ANALYSES FROM SULFUR SPRINGS IN CUEVA DE VILLA LUZ, TABASCO, MEXICO

Cueva de Villa Luz in Tabasco, Mexico is an active hypogenic cave system that produces prodigious quantities of hydrogen sulfide. More than twenty springs originate in the cave. Groundwater inlets into the cave fall into two categories: those with high H₂S content (300-500 mg/L) and no measurable O₂, and those with modest O₂ and very low H₂S content. Measured outbursts of sulfide and other gases occur sporadically and rapidly, with concentrations of H₂S in the cave air rising from <30 ppm to >200 ppm within four minutes.

The source of the H₂S in the spring waters has not been clearly identified. Either the Villahermosa petroleum basin (~65 km to the north) or the El Chichon volcano (~50 km to the west) may serve as the source area for the water and H₂S. Water analyses from four different springs in the cave yield similar results, typical of cave water: supersaturated with respect to calcite and aragonite, undersaturated with gypsum, near equilibrium with silica, and supersaturated with CO₂ relative to atmosphere. Tritium values suggest that the water is modern; stable isotope values plot near the Global Meteoric Water Line. A gas sample collected from one of the sulfur springs contained CO₂, H₂S, N₂, He, and CH₄, minor Ar, H₂ and O₂ but no measurable CO. The N₂/He (4500) and He/Ar (0.9) of the sample indicate that the gas is volcanic in origin and contains a mantle contribution. Helium isotope ratios (R/R_A=1.8 where R_A=³He/⁴He of air) are significantly higher than crustal values (Å0.02 Ð 0.05 R_A) and confirm an upper mantle contribution to the gases discharging into the cave. This suggests that the water and the gas may have separate sources, the water representing near surface ground water while the gas has a volcanic/upper mantle origin.

Throats of the sulfide-rich springs host thick, gelatinous colonies of sulfur-utilizing bacteria. Preliminary results of rDNA phylogenitic analysis have identified  clones with closest relatives in other subsurface sulfur systems: an Epsilonproteobacteria found in Sulphur River in Parker Cave, KY and in Kane Cave, WY and a symbiont of the polychaete Alvinella pompejana that resides in deep ocean hydrothermal systems. This commonality hints of an indigenous sulfur-utilizing subsurface.