Geochemistry

Bacterial Spelunkers

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Science  07 May 2004:
Vol. 304, Issue 5672, pp. 799
DOI: 10.1126/science.304.5672.799a

Caves are typically formed in limestone deposits, made up primarily of calcium carbonate (CaCO3), and cave formation is driven by carbonic acid dissolution of the carbonate rocks. In the Lower Kane Cave in Wyoming, the presence of a hydrogen sulfide-rich (H2S) thermal spring and gypsum deposits (CaSO4) suggested that volatilization of H2S from the spring water, oxidation of the H2S gas to sulfuric acid, and reaction of the sulfuric acid with calcium carbonate had produced gypsum and that dissolution of the gypsum by water would contribute to cave formation.

Engel et al. have found that sulfur-oxidizing bacteria, not aqueous geochemistry, actually control cave growth. The bacteria oxidize H2S either completely to sulfate for energy or partially to elemental sulfur that is stored intracellularly for later use. The locally produced sulfuric acid attacks the rock matrix on which the microbial mats live, converting the carbonate to gypsum. This sequence of reactions not only buffers the environmental pH of the microbial ecosystem but can occur over a wide range of conditions, potentially affecting porosity and conduit growth in deeper caves, oil field reservoirs, and aquifers. — LR

Geology 32, 369 (2004)

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