Picture of Diana Northup     You need a headlamp, not a yellow brick road to get there. And while there's no Toto or Cowardly lion, Associate Professor Diana Northup has uncovered a world more remarkable than anything the Wizard could have dreamed up. Along the way, she and Penny Boston of New Mexico Tech in Socorro, and Mike Spilde of the University of New Mexico Institute of Meteoritics have helped develop new ideas about how large subterranean caves are created.

     It's always been a bit of a mystery how large caverns like those at Carlsbad were created. Clearly, acids eating away at limestone deposits were involved, but which acids and where did they come from? Caves are full of noxious gases that might be converted into acids, but how is the conversion done? For example, geologists have long understood that some caves are rich in hydrogen sulfide (H2) which can be oxidized to sulfuric acid (H2SO4). But who or what was doing the oxidizing?

Northup and her colleagues now have the answer. They've been working in a cave in southern Mexico called “Cueva de Villa Luz,” which is still in the process of expanding. Northup and Boston found H2S levels in the cave as high as 210 parts per million (ppm). When H2S levels rise to 10 ppm, humans must don protective respirators, but for some “sulfur loving” bacteria, the sky-high hydrogen sulfide levels are a bonanza. Such bacteria oxidize the energy-rich H2S, releasing H2SO4as a waste. Could such bacteria be involved in cave formation?

Picture of a snottite     Within the cave, Northup and Boston found stalactite-like structures oozing with slimy goo that literally dripped sulfuric acid. Geologists call these bizarre formations “snottites.” The two New Mexico researchers performed a molecular analysis of snottite samples and found that they were brimming with bacteria closely related to Aciditheobacillus theooxidans, a known sulfur-loving organism.

     Other odd globs of microbes, appropriately called “phlegm balls,” were found in underground streams in the cave, precisely where H2S was seeping out of the rock strata. Still other strange assemblages of bacteria and archaea called “biovermiculations” adorn the cave walls. Thus, these subterranean microbial communities provide the missing piece in the cave formation puzzle. The microorganisms oxidize compounds like H2S, producing acid that dissolves the cave's limestone walls, producing gypsum. The gypsum falls into the underground streams, where it is carried away. As the lining of the cavern is slowly sloughed off, the cavern expands. Just wait a few million years, and a first-rate cavern is ready for visitors!

     You've gotta see snottites, phlegm balls and other underground oddities, right? Go to:   or .


Photo Credits:  Top: "Diana Northup and Mike Spilde taking oxidation-reduction potentials at the Teapot Pool, 12/19/01," photo copyright 2001 by Kenneth Ingham, used by permission;  Bottom: "A snottite with midges on it," photo copyright 2003 by Kenneth Ingham, used by permission.

Oct. 13, 2003

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