Sulfuric acid caves of the world: A review

In soluble rocks such as limestone and dolostone there are two main types of cave genesis (speleogenesis); (i) epigenic speleogenesis, in which the aggressiveness that forms caves is derived from the surface, and (ii) hypogenic speleogenesis where upwelling gases and fluids acquire their aggressiveness at depth dissolving carbonate rocks to form caves. Whereas epigenic (surface) waters can be undersaturated respect to carbonates obtaining their aggressiveness mainly from soil CO2, hypogenic (deep rising) fluids can obtain their carbonatedissolving capacity at depth from various processes and mechanisms. Hypogenic speleogenesis that involves dissolution of carbonate rocks by sulfuric acid, mainly derived by oxidation of upwelling hydrogen sulfide, is called sulfuric acid speleogenesis (SAS). Although this type of process has been described during the early 19th century, the first elaborated theories are rather recent, and a detailed model was proposed only in the mid-1970s. Only a few cave areas were known to host sulfuric acid caves, but studies were boosted with the discovery and understanding of the inactive Lechuguilla Cave in New Mexico (USA), and the still active Movile Cave in Romania, both discovered in 1986. Today 84 areas globally are known to contain sulfuric acid caves, offering a wide variety of environments. This review gives a historical overview of studies concerning these caves, explains the chemistry behind the formation of the dissolving fluids and their interaction with the carbonate host rock, and describes the chemical, mineralogical, geomorphological, and microbiological signatures typical of sulfuric acid dissolution. The significance of SAS caves in landscape evolution and their possible role in deep karstification, and thus hydrocarbon reservoirs and deep hydrogeology, is also briefly analyzed.