Hypogenic speleogenesis, dissolution by aggressive and deep-seated fluids, is the dominant cave formation process for the most important karst systems in Italy. Endogenic H2S and CO2 are the main corrosive agents

of underground karst corrosion. Throughout the region, evidence of all end-members of karst processes can be found, from solution limestone caves to deposition of carbonate travertine. The redox reactions in the groundwater and in the atmosphere involve chemoautotrophic microbial activity. The presence of active hypogene branches in many caves permits a direct study of the karst-forming processes, as well as a comparison with resultant morphologies

Galdenzi, S.

Menichetti, M.

Hypogenic speleogenesis, dissolution by aggressive and deep-seated fluids, is the dominant cave formation process for the most important karst systems in Italy. Endogenic H2S and CO2 are the main corrosive agents
of underground karst corrosion. Throughout the region, evidence of all end-members of karst processes can be found, from solution limestone caves to deposition of carbonate travertine. The redox reactions in the groundwater and in the atmosphere involve chemoautotrophic microbial activity. The presence of active hypogene branches in many caves permits a direct study of the karst-forming processes, as well as a comparison with resultant morphologies

MENICHETTI, MARCO

Archivio istituzionale della ricerca - Università di Urbino

177DeepKarst 2016ACTIVE HYPOGENIC KARST IN ITALYSandro Galdenzi Viale Verdi, 10, Jesi, 60035, Italy, galdenzi.sandro@tiscali.itMarco MenichettiUniversity of UrbinoPure and Applied Sciences Department, Campus Scientifico Urbino, 61029, Italy, marco.menichetti@uniurb.itAbstractHypogenic speleogenesis, dissolution by aggressive and deep-seated fluids, is the dominant cave formation process for the most important karst systems in Italy. Endogenic H2S and CO2 are the main corrosive agents of underground karst corrosion. Throughout the region, evidence of all end-members of karst processes can be found, from solution limestone caves to deposition of carbonate travertine. The redox reactions in the groundwater and in the atmosphere involve chemoautotrophic microbial activity. The presence of active hypogene branches in many caves permits a direct study of the karst-forming processes, as well as a comparison with resultant morphologies.IntroductionLimestone karst speleogenesis is a morphological process involving the removal of mass from a host rock via an aggressive chemical agent transported by a fluid vector. Exogenous, soil-derived CO2 is the chemical agent responsible for the formation of epigenic limestone caves, while endogenic agents are responsible for the corrosion seen during hypogenic cave speleogenesis (chemical, physical, or both; e.g., CO2, H2S, cooling, mixing, etc.). The fluid vector flowing through the rock mass could be water or a gas. The main difference between epigenetic and hypogenic speleogenesis, however, is related to the origin of the corrosive agents. Epigenic speleogenesis is a process that normally propagates downward from the surface through soil zone to depth through the subsurface drainage network. On the other hand, hypogene speleogenesis is related to the upward flow of deep-seated solutions generated at depths.The Italian Peninsula is probably the world’s best location to observe both the active and fossil hypogenic speleogenesis processes in different geological contexts (Galdenzi and Menichetti, 1989; Galdenzi, 2009; Menichetti, 2009) (Figure 1). All the end-members of the karst processes can be found here, from epigenetic limestone caves to hypogenic mazes to deposition of carbonate travertine. For more than a century of speleological research, many hypogenic limestone caves have been explored, mapped, and studied (Principi, 1931). In some of these cave systems, speleogenetic corrosional processes linked to the endogenic CO2 and H2S vents are active and directly observable.Active and fossil hypogenic caves are known in the regions of Tuscany, Umbria, Marche, Latium, Campania, and Calabria. These caves are characterized by a variety of patterns and morphological sizes, including three-dimensional maze systems and deep shafts. In many cases, the caves are developed at several levels, which is related to the evolution of an external hydrographic network. The local geological and structural features of the different areas, and the hydrology and groundwater flow modalities have led to the development of different cave morphologies.Hydrogeological FrameworkAnalyses of the different morphological aspects of these karst systems in Italy have allowed the identification of primary formation processes within a geological and hydrogeochemical framework. Hypogene speleogenesis driven by deep-seated fluids is the cave formation processes for the most important karst systems in the Italian Apennines. Here, H2S and endogenic CO2 are the main agents of underground karst corrosion.\In many caves, hypogenic processes can be linked to the oxidation of H2S that produces sulfuric acid as oxygen-rich groundwaters mix. In caves, the resulting limestone corrosion produces gypsum deposits that could be assumed to be indicators of hypogene speleogenesis. The cave morphologies show that the H2S oxidation zone is not restricted to the shallow groundwater levels but can be extended to the deeper sections of the aquifer, which are fed by freshwater via a complex regional hydrogeological flow paths. Along the Peninsula, general low temperature CO2 gas emissions are known, 178 DeepKarst 2016with flow rates estimated at 1011 mol y-1 in proximity of the main outcrops of travertine deposits. The pCO2 values of the groundwater range from 0.03 to 0.1 atm. This increases the solubility of CaCO3 by an order of magnitude with respect to the soil pCO2 typicall of epikarstic waters. The action of endogenic CO2 in the cave formation, especially at lower temperatures, is not easy to detect, and the resulting cave morphology is not indicative of any specific cave formation process (Menichetti, 2011). The travertine and tufa deposits represent the other end-member of karst processes as the product of the limestone dissolution by deep-seated fluids. These deposits form as a result of degassing of surfacing CO2-rich groundwater containing >2 mmol L–1 calcium.Geothermal anomalies are particularly common along the Tyrrhenian side of the Peninsula (Figure 1). The cooling of thermal water during its ascent along conduits increases its CO2 aggressiveness, with corrosion acting almost uniformly along fracture surfaces and producing a dramatic increase in hydrologic flow and karst void development.The Hypogenic CavesAlong the Peninsula, karst is not homogeneously distributed, despite the presence of large limestone outcrops (Figure 1). In the Umbria-Marche Apennines area, the presence of major hypogenic caves is well-documented by several decades of research and exploration. Caves in this region are known for their vertical karst systems, such as Monte Cucco and Faggeto Tondo, and the maze systems of the still-active Frasassi Gorge and Acquasanta Terme caves. The main cave systems consist of a few tens of kilometers of solutional passages with galleries and shafts, which are characterized by large rooms, cupola and blind pits, anastomotic passages, bubble trails, roof pendants, knife edges, and phreatic passages (Galdenzi & Menichetti, 1995). The phreatic morphologies are widely diffuse in both active and fossil karst systems, indicating the important actions operated by aggressive gas during its ascent into the upper levels of the groundwater.In the Lazio and Campania regions, where many Jurassic limestones experienced a Plio-Quaternary K- alcaline and calcalkaline volcanism, the hypogenic karst is marked by the presence of the Pozzo del Merro shaft and active large travertine deposits a few kilometers northeast of Rome. In these regions, high values of CO2 and H2S have been recorded in several cave systems.In the southernmost regions, other hypogenic active karst systems have been identified at Capo Palinuro along the Tyrrhenian Sea coast. These systems are located in correspondence with several submarine H2S vents in a setting where speleogenetic evolution is linked to sea level oscillations. In the northern Calabria region, hypogenic caves related to thermal sulfidic springs are known in three different areas, and active cave-forming processes occur in the Sibari area.The Active CavesThe presence in many hypogenic caves of an active section permits a direct study of karst-forming processes as well as a comparison with the derived morphologies. In the whole Italian Peninsula, the main active cave-forming processes can be related to the oxidation of H2S to sulfuric acid in the groundwater as well as in the atmosphere where the redox reactions involve chemoautotrophic microbial activity (Sarbu et al., 2000). Stratification and mixing of different waters with significant variations in chemistry, temperature, TDS and pH are well documented in almost all of the active caves., A significant amount of organic matter can grow in the sulfidic streams and close to the water table, as white filaments can be seen as meter-long seaweed-like structures in deep cave pools in Frasassi.   The sulphur-oxidizing bacterial communities, which use H2S as an energy source, support invertebrate life, producing a wholly chemoautotrophic aphotic ecosystem in the Frasassi sulfidic cave sections (Sarbu et al., 2000). The role played by these microbial communities in the limestone corrosion is not well understood. Data from the Frasassi caves show that oxidizing bacteria living in the cave streams do not play an active role in the production of acidity (Jones et al., 2015).Close to the sulphidic streams, air moisture is rich in H2S and CO2, which is released from the groundwater and acts as a main agent of limestone-wall corrosion. The corrosion first appears on limestone as small white spots of about 1 cm in diameter. In these spots the calcium carbonate is replaced by gypsum, which can expand into a continuous rim of microcrystalline gypsum. At the limestone/gypsum interface, a sulphur-oxidizing bacteria biofilm plays an important role in the limestone corrosion. This biofilm, consisting of organic mucous matter, arranges in small formations like spider webs and thin “stalactites” with acid drops having pH <1 (Sarbu et al., 2000). The gypsum is often squashy and can easily fall in the floor, producing large deposits, as directly observed in the Acquasanta Caves.This depositional mechanism can explain the large gypsum deposits in non-active hypogenic caves, also  comprising the “gypsum glaciers” produced by post-depositional flow and accumulation. A similar origin 179DeepKarst 2016is consistent with stable isotope data. All gypsum deposits in M.Cucco, Frasassi Gorge, Pozzi della Piana, Acquasanta, and the Lepini Mountains have negative values of δ34S ratio, which are compatible only with a derivation from H2S released by groundwater. In the sulfidic groundwater, δ34S is significantly different in sulphide (which is always negative) and dissolved sulphate (which is always highly positive).The rate of limestone corrosion due to endogenic H2S action on calibrated limestone slabs has been tested in a five-year experiment. The slabs were positioned in the sulphidic branch of the Frasassi cave system, both in air and water. The slab surface was replaced with gypsum at a rate of limestone corrosion of about 0.05 mm y-1 (Galdenzi et al., 1997). A subsequent experiment in groundwater showed that corrosion rate is influenced by flow dynamics and can increase to 1 mm y-1 with a weakly turbulent flow. A comparison of the measured corrosion rate with cave wall retreat has shown that active corrosion processes have acted at the same intensity throughout the caves’ history (Galdenzi, 2012).ModelingEven though general speleogenetic reactions are known, the precise geological, hydrogeological, and geochemical conditions of their occurrence need to be documented. In particular, the reactions at the gas/water interface (H2S, CO2) and especially the role played by organic matter require further and more detailed research. Building a fluid flow model through a limestone mass could be one way to understand hypogenic karst processes and explore the relationship between lithology, tectonics, hydraulics, geochemistry, and geothermal setting. A simple regional ground water flow model is critical to understanding heat and mass transport in hydrogeological systems related to active hypogenic caves. A multiphysics model that takes into account the hydraulic, thermal, and physical-chemical conditions of the Acquasanta Terme karst system indicates that a simple thermal convection at basin scale is not sufficient to explain the water temperature and geochemistry recorded in the sulphidic spring; an external heat source is necessary. ConclusionsA more dynamic view of cave pattern development and evolution in space and time that will take into account the general altitude variations of the regional water table level together with episodic gas emissions needs to be considered.The best conditions for hypogene cave development typically occur in small limestone outcrops covered by low-permeability units, in which fluids rising from depth can reach an oxidizing environment or mix with descending freshwater. However, some hypogenic caves also occur in open hydrogeologic massifs, where epigenic caves prevail, due to the high intensity of the corrosion.Hypogenic caves generally have a multistage evolution related to the progressive incising of surface streams and the depression of the related base level. The great variety of examples in different geological settings offered by the Apennines make it a good place to evaluate the influence of geologic and geomorphic factors on hypogenic cave formation.ReferencesGaldenzi S. 2009. Hypogene caves in the Apennines (Italy). In: Klimchouk AB, Ford DC, editors. Hypogene speleogenesis and karst hydrogeology of artesian basins. Simperfol Ukrainian Institute of Speleology and Karstology, Special Paper 1: 101-115Galdenzi S. 2012. Corrosion of limestone tablets in sulfidic ground-water: measurements and speleogenetic implications. International Journal of Speleology 4 (2): 25-35. http://dx.doi.org/10.5038/1827-806x.41.2.3Galdenzi S, Menichetti M. 1989. Evolution of underground karst systems in the umbria-marche Apennines in Central Italy. Proceedings of the 10th International Congress of Speleology, Budapest, (3) p. 745-747.Galdenzi S, Menichetti M. 1995: Occurrence of hypogenic caves in a karst region: examples from central Italy. Environmental Geology 26: 39-47. http://dx.doi.org/10.1007/BF00776030Galdenzi S, Menichetti M, Forti P. 1997. La corrosione di placchette calcaree ad opera di acque sulfuree: dati sperimentali in ambiente ipogeo. Proceedings of the 12th International Congress of Speleology, (1) p. 187-190.Jones DS, Polerecky L, Galdenzi S, Dempsey BA, Macalady JL. 2015. Fate of sulfide in the Frasassi cave system and implications for sulfuric acid speleogenesis. Chemical Geology 410: 21-27. http://dx.doi.org/10.1016/j.chemgeo.2015.06.002Menichetti M. 2009. Speleogenesis of the hypogenic caves in Central Italy. Proceedings of the 15th International Congress of Speleology, p. 909-915.Menichetti M. 2011. Hypogenic cave in Western Umbria (Central Italy). Acta Carsologica 40 (1): 129-145. http://dx.doi.org/10.3986/ac.v40i1.33180 DeepKarst 2016Principi P. 1931. Fenomeni di idrologia sotterranea nei dintorni di Triponzo (Umbria). Le Grotte d’Italia, 5: 1- 4.Sarbu SM, Galdenzi S, Menichetti M, Gentile G. 2000. Geology and biology of the Frasassi Caves in Central Italy, an ecological multi-disciplinary study of a hypogenic underground ecosystem. In: Wilkens H, et al., editors. Ecosystems of the world. New York (NY): Elsevier. p. 359-378.Figure 1.Legend: (a) stratigraphic succession from Quaternary to Miocene; (b) Quaternary volcanic rocks; (c) the main calcareous stratigraphic succession from Jurassic to Miocene; (d) the main karstic spring; (e) the main mineralized spring; (f) important caves; (g) hypogenic caves; (h) close valley with karstic drainage; (i) CO2-rich gas vent; (k) spring rich in CO2 gas; (j) Apennines water divide; (l) geothermal heat flux, the thick dotted line indicates 100mW/m. In Larderello geothermal field, the heat flux reaches 400 mW/m.

ACTIVE HYPOGENIC KARST IN ITALY

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