Mefite d'Ansanto, southern Apennines (Italy): the natural CO2 seep which emits the largest quantity of non-volcanic CO2 on Earth

Mefite d'Ansanto, located in the southern Apennines (Italy), emits the largest quantity of low-temperature CO2-rich gases ever measured in non-volcanic environments worldwide (Italiano et al. 2000; Rogie et al. 2000; Chiodini et al. 2010). Italy is a region of widespread and intense natural earth degassing, being about 300 CO2 seeps, which exhibit a wide range of surface expressions, flux, and temperatures (Roberts et al. 2011); among the hundreds of documented seeps, Mefite d'Ansanto is unique in the quantity of CO2 that it emits compared to the other sites, releasing about 2000 tons per day of CO2, by venting and diffuse degassing, over an area of 4000 m2 (Chiodini et al. 2010). The degassing area, where vegetation is absent or intensely damaged, covers the flank of a steep-sloping hill (Figure 1); due to its density, the huge amount of gas flows along a narrow valley, toward west, forming a dangerous and invisible gas river. Other smaller CO2 vents are located not far from Mefite d'Ansanto, e.g., the Mefitiniella (which means 'little Mefite'), a vigorous vent emission inside a depression, ~4 km NW from Mefite d'Ansanto (MaGa database, available at: http://www.magadb.net). The gas emission consists of CO2 (98 vol%) with minor amounts of non-atmospheric N2 (1.3 vol%, N2/Ar ~ 1000), H2S (0.33 vol%) and CH4 (0.23 vol%); the isotopic signatures (?13C = 0.43? VPDB, (3He/4He) /(3He/4Heatm )= 2.27) suggest a deep origin of the fluids (Chiodini et al. 2010). The degassing area is located above the structural high point of the NW-SE-trending Frigento Antiform; the emission is likely to be fed by a reservoir located in the underlying anticline, made up of permeable limestones (Piattaforma Apula Interna; Mesozoic) and covered by clayey sediments (Unità Lagonegresi; Miocene) (Chiodini et al. 2010; Pischiutta et al. 2013; Roberts et al. 2017). The CO2 gas zone, found from a depth of 1128 to 1600 m, rests above an accumulation of brackish water in the central and upper part of the culmination of the deep carbonatic acquifer, and a saline water along the flank of the buried anticline (Amoresano et al. 2015). Results from wavefield polarisation indicate the presence of a NE-SW orientated transfer fault which intersects the regional NW-SE normal faults close to the site of degassing (Pischiutta et al. 2013). Although the low-temperature of water associated to gas emission (15-21°C; Duchi et al. 1995; Amoresano et al. 2015), medium enthalpy geothermal resources have been estimated, in the framework of the project ''VIGOR-GEOTERMIA'', with a reservoir fluid temperature up to 120-125°C at the depth of about 3000 m below the ground level (Inversi et al. 2013; Amoresano et al. 2015). The gas emission is known since the Roman age, when the area was identified as the den of the harmful goddess of fertility and death Mephitis (Sinno 1969); the danger associated to gas emission is also known since long time, as suggested by name 'Ansanto', which means 'that breathes with difficulty, panting'. At the present-day, the site is considered very dangerous, because CO2 is a colorless and odorless gas, undetectable by the human senses, and lethal concentration (> 30%; Rogie et al. 2000) can be present up to 2-3 m of height above the ground surface. Unlike CO2, the co-released H2S, also present in dangerous concentration, is easily identifiable due to its strong "rotten-egg" odor. Several fatal accidents involving humans and animals have occurred (Chiodini et al. 2010). Simulations of gas dispersion using a physical numerical model suggested that lethal air CO2 concentration (>15% at heights of 1.5 m) can affect the first 200 m of the valley, where carcasses of animals were systematically found; air CO2 concentration of 5%, which causes breathing increases to twice the normal rate, a weak narcotic effect and headache to humans, can affect all the topographic depressions around the vent (Chiodini et al. 2010). Nevertheless, the access to the area is not currently restricted.