May 10, 2004 | David F. Coppedge

Caves Are Made by Bacteria

Caves seem like archetypes of slow, gradual, ancient processes.  Tourists have long been told that caves form slowly over many tens or hundreds of thousands of years by the slow dissolution of limestone by weak carbonic acid in water carried down from surface rainfall.  That explanation took a dramatic turn in the 1970s when scientists realized that a stronger agent, sulfuric acid, might enlarge subaerial chambers much more quickly. It was a revolutionary discovery to realize that sulfuric acid could be formed by oxidation of subterranean hydrogen sulfide.  Now, the same “type locality” where the initial studies were done, Lower Kane Cave in Wyoming, has produced another revolutionary discovery; bacteria make caves.
    A trio of geologists from the University of Texas at Austin revisited Lower Kane Cave for a three-year research program.  They measured very little hydrogen sulfide emitting into the air able to dissolve limestone on subaerial surfaces.  Instead, they found two species of bacteria that feed on hydrogen sulfide produce more sulfuric acid.  Concentrated in dense microbial mats, these bacteria essentially focus the acid on the phreatic (groundwater) limestone surfaces.  “Our observations show that sulfur-oxidizing bacteria colonize subaqueous carbonate surfaces, localize dissolution by generating acidity, and therefore are integral to sulfuric acid speleogenesis,” they state in their revised model of sulfuric-acid speleogenesis (SAS; speleogenesis meaning cave formation).  Their paper is published in the May issue of Geology.1
    Although some of the hydrogen sulfide is autocatalyzed, escaping into the cave atmosphere to contribute to dissolution of subaerial surfaces as previously suggested, it appears that most of it is biogenic – catalyzed by microbes that extract energy from hydrogen sulfide: “cave enlargement via dissolution of the cave floor is microbially mediated,” they theorize.  Because these bacteria concentrate sulfuric acid formation on the cave floor and are operative where subaerial dissolution would be kinetically limited, ”microbial catalysis extends the phreatic depths to which porosity and conduit enlargement could occur in carbonate systems, including oil-field reservoirs and aquifers.  The metabolic consequences of an active microbial ecosystem change the model for sulfuric acid speleogenesis.”
    See a summary of this paper on Nature Science Update, which states, “Although the exact age of Lower Kane Cave is controversial, Engel says that it probably formed about 10,000 years ago.  That is relatively recent; caves formed by the more common action of carbonic acid grow considerably more slowly.”  They claim Carlsbad Caverns is 10-14 million years old; “They show just what the bacteria can achieve, given time.”

1Engel, Stern and Bennett, “Microbial contributions to cave formation: New insights into sulfuric acid speleogenesis,” Geology, Vol. 32, No. 5, May 2004, pp. 369�372, doi: 10.1130/G20288.1.

The authors do not give any indication of how rapidly this process would occur relative to other cave-formation processes or to the old SAS model, nor do they indicate that microbial sulfide oxidation would be the only or dominant process.  Nevertheless, if the dense microbial mats described by the researchers focus sulfuric acid on the rock surfaces, it seems plausible that cave formation, at least in some cases, could be much more rapid than usually assumed.  Despite NSU’s guesses, nobody was present even 10,000 years ago to tell us if conditions were the same back then; how much less for millions of years.  If enough bacteria and the right conditions were available, why could not even large caves form rapidly?  NSU claims that most caves form by standard carbonic acid solution, but look at how revolutionary the SAS model was in the 1970s.  Who knows what other mechanism might be discovered?
    According to these researchers, an open-air environment is no longer required for sulfuric acid speleogenesis; cave enlargement by concentrated sulfuric acid could be occurring within groundwater-saturated regions.  It would be interesting to find out if this extends the temperature range in which cave formation could take place.  Enzymes are known to facilitate reactions that normally take much longer or require narrow temperature ranges.
    Share this finding with the guide on your next cave tour right after the “millions of years” story.  It’s fun watching the summer hires get befuddled when something off the script happens.  Have the kid record it on the camcorder.

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