Speleogenesis – oral 2013 ICS Proceedings

KARST PROCESSES AND CARBON FLUX IN THE FRASASSI ,

Marco Menichetti Earth, Life and Environmental Department, Urbino University, Campus Scientifico, 61029 Urbino, Italy National Center of , 06021, Costacciaro, Italy, [email protected]

Hypogean speleogenesis is the main formation process in the Frasassi area. The carbon flux represents an important proxy for the evalution of the different speleogenetic processes. The main sources of CO2 in the underground system are related to endogenic fluid emissions due to crustal regional degassing. Another important CO2 source is hydrogen sulfide oxidation. A small amount of CO2 is also contributed by visitors to the parts of the cave open to the public.

1. Introduction and are developed over at least four main altimetric levels, related to the evolution of an external hydrographic The Frasassi area is located in the eastern side of the network. The lowest parts of the caves reach the phreatic Apennines chain of Central Italy and consists of a 500 m- zone where H2S-sodium-chloride mineralized ground- deep gorge formed by the West-East running Sentino River waters combine with CO2-rich meteoric circulation (Fig. 1). (Fig. 1). More than 100 caves with karst passages that The carbonate waters originate from the infiltration and occupy a volume of over 2 million cubic meters stretch over seepage from the surface and have total dissolved solids a distance of tens of kilometres at different altitudes in both (TDS) contents of 500 mg/L. Mineralized waters with a banks of the gorge. temperature of about 14 °C and more than 1,500 mg/L TDS The main karst system in the area is the Grotta del Fiume- rise from depth into a complex regional underground Grotta Grande del Vento that develops over more than drainage system. Here, the H2S can reach concentrations of

Figure 1. Schematic geological cross-section through the Frasassi Gorge showing the groundwater pathways.

20 km, about 1 km of which is open to the public and is up to 0.5 mmol/L and the endogenic CO2 in the water can visited by about 200,000 tourists each year. The caves are have values close to 100 mg/L. hosted in Jurassic carbonate banks with a well developed The Frasassi karst system is mainly horizontal and has a syngenetic porosity. The groundwater drainage is controlled ramiform pattern; several big rooms with wide ceilings end by N-S trending fault systems and a network of conjugate abruptly in narrow passages or fissures; phreatic passages joint sets distribuited in the NE-SW and NW-SE directions. (1–10 m size), often anastomotic, also spread over large The caves consist of a maze system of solutional passages, parts of the cave where they form some network zones. The 376 Speleogenesis – oral 2013 ICS Proceedings prevailing underground morphology is 3D-mazes with through carbonate rocks found in correlation with several cupolas, bubble trails, roof pendants, and knife edges also travertine outcrops are well known in Central Italy rather common. (Menichetti, 2009). In the study region, groundwater P CO2 values range from 0.03 to 0.1 atm. Various types of gypsum deposits fill the cave passages. At present, gypsum formation only occurs on the cave walls The rate of sulfuric acid speleogenesis was tested over a above the watertable, owing to H2S oxidation (Menichetti five year period using calibrated limestone tablets located et al., 2008). in the deep sulfidic branch of the Grotta del Fiume containing a fast flowing sulfidic stream. The tablet surfaces where replaced with gypsum at a limestone corrosion rate of about 0.05mm/y. The weight loss was strongly influenced by the location of each tablet within the cave chamber and by the concentration of hydrogen sulfide in the cave atmosphere (Menichetti et al. 2008). Calcium carbonate deposition occurs in many parts of the caves, and the experimental values obtained are especially important for the cave branch open to the public, where in the vicinity of the underground tourist pathway coins and dust are incorporated into the thin carbonate flowstones.

3. Carbon flux Figure 2. A record of the CO2 air values near the sulfuric lake from March 1st through May 10th, 2005. The CO2 values are Measurements of CO2 in the air were carried out over many corrected for altitude. The thick line is the moving average. In the years using various techniques, with both spot and inset graph shows the CO2 bimodal frequency distribution. automated sample aquisition. The measurement density is higher close to the branch of the cave open to the public where electric power is available; measurements were 2. Karst processes also taken close to the sulfuric lakes and streams. Spot The hypogenic origin of the Frasassi cave, as well as other measurements were acquired using vials through wich a karst systems in the Central Italy, is well known and studied calibrated air sample was drawn with a pump. Continuos (Menichetti 2009). Hypogean speleogenesis is related to the measurements were recorded with infrared sensors (accuracy circulation of hydrogen sulfide rich groundwater through of ± 5ppm) connected to a datalogger. The carbon dioxide lower Jurassic limestone (Galdenzi and Menichetti 1989, concentration in the deep branch of the cave varies from 600 1995; Menichetti et al. 2008). The same speleogenic to 4000 ppm and is related to the discharge of rising sulfidic mechanisms were also suggested for caves with similar water and to the seasonal biological cycles (Fig. 2). morphologies in other parts of the world (Egemeier 1981; Ford and Williams 2007; Palmer 2007). The main karst processes in Frasassi occur close the water table where most of the H2S-rich groundwater is circulated. The oxidation of hydrogen sulfide to sulfate [1] happens in the upper phreatic zone, near the watertable, in the presence of oxygen from dripping waters and from the cave atmosphere. Limestone dissolution [2] takes place both in phreatic and vadose conditions.

+ 2- [1] H2S + 2O2 ↔ 2H + SO4

+ 2- [2] 2H + SO4 + CaCO3 + H2O ↔ CaSO4. 2 H2O + CO2 Carbonate corrosion produces sulfate ions in the phreatic Figure 3. A record of the CO2 air values in the branch of the Grotta zone and replacement of limestone in the walls of the vadose Grande del Vento open to the public. A total of 250,000 visitors sectors of the caves with gypsum (Menichetti et al. 2008). entered the cave from February 1st through October 15th, 1993. The CO values are corrected for altitude. The cave’s sulfuric streams contain much floating organic 2 matter. Sulfur-oxidizing bacterial communities that use H2S In the part of the cave open to the public, the natural as an energy source and are part of a chemoautotropic background CO2 concentration in the air is augmented by aphotic ecosystem supporting invertebrate life are known an anthropogenic component. Tourists’ breath, as well the in the Frasassi caves (Sarbu et al. 2001). The role played by opening of the air-tight doors, contributes significantly to this organic matter in limestone corrosion has not been well the variations in carbon dioxide concentration in the cave defined. The chemical process described in [2] releases atmosphere. The carbon dioxide levels recorded close to the fresh CO2 into the upper portion of the groundwater, as well tourist pathway in 1993 show seasonal variations with a as into the cave atmosphere, augmenting the aggressivity summer maximum that exceeds 1,000 ppm and a winter of the environment toward carbonates. Endogenic CO2 minimum of about 400 ppm. Daily and hourly fluctuations sources resulting from the significant groundwater flow of up to 1,100 ppm can be correlated to the number of 377 Speleogenesis – oral 2013 ICS Proceedings

visitors (Fig. 3). In the summer, the drop of the CO2 the cave open to the public but is minor at the scale of the concentrations back to background values may take a few entire karst system. days, especially after a severe increase caused by a combination of natural and anthropogenic factors

(Menichetti et al. 1994). The CO2 records close the cave entrance in the Frasassi Gorge show a variation between 350 and 450 ppm that can be related to weather conditions and seasonal vegetation production (Fig. 4).

From July to November of 1997 the CO2 record above the sulfidic lake shows different cycles that can be correlated with the tectonic activity affecting the area (Fig. 4). CO2 values reaching 1,400 ppm in August are most likely due to seasonal cycles, while the lower values in September may be related to seismic activity with the occurance of a few earthquakes of M > 5 and epicenters at a distance of 40 km from Frasassi. A frequency distribution diagram of the CO2 concentrations in the underground sulfuric stream shows an ++ asymmetric distribution (Fig. 4a), while the distribution of Figure 5. [HCO3] vs. [Ca ] (mg ⁄ L) binary diagram for the - Frasassi waters. Solid line represents the [HCO3 ] ⁄ [Ca2+] values epigenic CO2 concentrations is much more symmetrical. expected for calcite and dolomite dissolution based on a Removing the seasonal trend places the endogenic CO2 ++ heterogeneous equilibrium: CaMg(CO3)2 + 2CO2 + 2H2O = Ca release, which could be related to pre/co-seismic crustal ++ + Mg + 4HCO3. Dashed curves represent equilibrium conditions at different PCO2. Open circles: runoff waters; fill circles: mineralized waters; fill diamonds: dripping waters (from Menichetti et al., 2008).

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