Caves and snoticles

If ever there was “received wisdom” in the geosciences the most pervasive is the notion that the weak acid formed when carbon dioxide dissolves in rainwater is the cause of carbonate solution.  Anyone hearing it in the spiel from a cave guide, while admiring caverns as big as cathedrals, is not surprisingly awe-struck by such an innocuous sculpting agent.  Many speleologists have long wondered if there might be other mechanisms, and the discovery of bacterial films that generate strong sulphuric acid provides a good candidate.  They can take the form of floppy, stalactite-like masses, that have become fondly known as “snoticles”.  However, their role in cave formation had not been substantiated until April 2004.  Microbial geochemists at the University of Texas carefully studied the geochemical balances in a cave system in Wyoming where such bacteria are abundant (Summers Engel, A. et al. 2004.  Microbial contributions to Cave formation: New insights into sulfuric acid speleogenesis.  Geology, v. 32, p. 369-372).  The bacteria are members of two groups that live in aerated conditions and use the oxidation of sulphide ions (from hydrogen sulphide) as a source of metabolic energy.  Oxidation results in sulphuric acid, which rapidly dissociates in water to generate abundant hydrogen ions (the source of acidity and low pH) and sulphate ions.  So, to thrive the bacteria need a continuous source of hydrogen sulphide, of which more later.  The study by Annette Summers Engels and two colleagues shows that hydrogen sulphide is efficiently consumed by the bacteria, so that little if any enters the cave’s atmosphere.  Interestingly, water flowing through the cave isn’t particularly acid either, yet the bacteria generate a great deal of sulphuric acid.  It is rapidly neutralised by reaction with calcium carbonate near the colonial mats, to increase the flux of calcium and sulphate ions into solution.  The effect extends to limestone pebbles on the beds of the cave streams, so the bacteria encourage solution beneath water as well as near snoticles hanging from the roof.  That suggests that they can live below the water table, where many caves are thought to have formed in the past, being left as open caverns as the water table fell as bulk permeability increased with solution.  The studied cave does experience a constant flux of hydrogen sulphide, but where does that come from?  There are other groups of bacteria that generate sulphide from dissolved sulphate ions, but under highly reducing conditions.  They are the source of the “sour gas” that is a constant danger in oil production in some petroleum fields, consumed gleefully in dissolved form at a great many spas and generated in our own guts.  These sulphate-sulphide reducing bacteria get their energy from dead organic matter, that many sediments deposited under reducing conditions contain in substantial volumes.  Interestingly, connectivity between oxygen-rich and oxygen-starved groundwater might create a recycling of sulphur that involves both bacterial groups.  Many limestones contain strata that are rich in organic remains and metal sulphides, in which conditions become reducing.  Equally, interbedded, black shales might play a role.

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