(Submitted Abstract to the 2003 National Speleological Society Meeting, Porterville, California)

Consequences of low pH, cave-wall condensation and biofilm development to sulfuric acid speleogenesis

Annette Summers Engel*, Libby A. Stern, Philip C. Bennett

Jackson School for Geosciences, University of Texas at Austin, Austin, Texas 78712; aengel@mail.utexas.edu

Volatilization and oxidation of H₂S to sulfuric acid on cave-wall surfaces causes aggressive carbonate rock dissolution and replacement by gypsum during sulfuric acid speleogenesis. Reddish-brown crusts cover the gypsum in Lower Kane Cave (Wyoming, USA). The crusts are composed principally of C, O, and Si, with abundant microorganisms and euhedral quartz microcrystals. Condensation droplets hang from cave-wall surfaces and average droplet pH was 1.7, ranging from pH 1.25 on crust to 2.92 on gypsum. Droplets on crust had contact angles >90^o (avg. 121.6^o), indicating hydrophobicity. Droplets with pH < 2 were undersaturated with respect to gypsum, while droplets with pH > 2 were in equilibrium. Condensate solutions on gypsum will approach pH 2, but typically not exceed it, due to buffering by the bisulfate-sulfate weak acid/base pair (pK = 1.92) combined with the gypsum-sulfate. Droplets on crusts had pH values below the critical HSO₄^-:SO₄^2- pK as a result of crust hydrophobicity and acid-producing bacteria. Therefore, armoring of the cave walls by gypsum and biofilms fundamentally impacts how a cave enlarges during sulfuric acid speleogenesis. Microbial colonization of the low pH, moist gypsum habitat forms an organic film that eventually becomes impermeable. Condensation becomes separated and out of equilibrium with respect to the underlying gypsum, precluding diffusion of sulfuric acid through the gypsum to the underlying limestone, limiting or shutting-off sulfuric acid dissolution completely. Only when fresh limestone is exposed will subaerial speleogenesis be reinitiated, and the replacement-colonization cycle start again.

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