Journal of the Virtual Explorer

Italy is the site of extensive CO₂ degassing by active volcanoes and by direct soil emission: active Italian volcanoes (e.g., Etna, Vesuvio, Campi Flegrei, Ischia, Vulcano, Stromboli, and Pantelleria) represent a relevant CO₂ source, distributed along Central-Southern peninsular Italy, and in Sicily; in addition, hundreds of zones of CO₂ soil degassing are located in Tuscany, Latium, Campania, the Apennines, Sicily, and Sardinia (Fig. 1; e.g., Allard et al., 1991; Chiodini et al., 1999, 2004; Rogie et al., 2000; Chiodini and Frondini, 2001). Present day Earth’s degassing in Italy amounts to 35 - 60 Mt CO₂/year (Mörner and Etiope, 2002, and references therein), about 7 – 12 % of the national anthropogenic CO₂ emissions (457 Mt CO₂/year in 2004; Bassani et al., 2009). Thus, secular variations of Earth’s degassing, at the Quaternary scale, may bear influence the present-day atmospheric natural carbon budget.

Cycling of carbon within the solid Earth represents the fundamental basis to understand the long-term (millions to billions of years) global carbon cycle, and influences the solid Earth’s CO₂ contribution to the atmosphere. Global estimates of volcanic mantle degassing, based on computations - involving C cycle modeling and the mass flux of volcanic material - indicate 100 -370 Mt CO₂/y (cf., Table 1, and references therein). More difficult appears an evaluation of the non-volcanic CO₂ direct degassing from the lithosphere at the global scale: conservative estimates are in the order of 100 to 600 Mt/y (Table 1; Kerrick et al., 1995; Mörner and Etiope, 2002).

The mantle represents the largest Earth’s carbon reservoir (about 80 - 200 x 106 Gt), containing greatly much more carbon than the atmosphere (600 Gt), oceans (39,000 Gt), and other near-surface reservoirs combined (cf., Shcheka et al., 2006, and references therein). Thus, the starting point to understand the deep carbon cycle resides in the theory of plate tectonics: in order to balance the continuous natural CO₂ emission, carbon must be recycled in the mantle (Zhang and Zindler, 1993). Carbon directly deposited on the ocean floor (i.e., pelagic carbonates and organic carbon) and carbonates formed by sea-water alteration of basalts may account for an yearly CO₂ flux of about 100 - 150 Mt into the mantle by subduction (Alt and Teagle, 1999; Coltice et al., 2004).

Experimental petrology has demonstrated how melting of subducted lithosphere may extract carbon from the subducting crust, whereas refractory carbonate minerals could allow transport of carbon to the 400 km and 600 km transition zones or to the lower mantle (e.g., Kerrick and Connolly, 2001a and b; Dasgupta et al., 2004; Thomsen and Smith, 2008). When melting occurs, return of subducted carbonated rocks by mobile carbonate melts through the mantle is geologically rapid, less than a few million years (Hammouda and Laporte, 2000). A first quantitative evaluation of deep CO₂ fluxes from the deep upper mantle beneath mid ocean ridges (MOR) has been presented by Dasgupta et al. (2006), based on petrological modeling. Estimated mantle CO₂ fluxes are between 120 - 3,400 Mt /y, and although quite variable, are compatible with estimates of MOR-volcanism CO₂ emission (Table 1; Cartigny et al., 2008).

Figure 1. Earth’s CO₂ emission in Italy

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Earth’s CO₂ emission in Italy, as derived from the on-line catalogue of Italian gas emissions (blue area; http://googas.ov.ingv.it; Chiodini and Valenza, 2007), and from the distribution of the main Plio-Quaternary volcanoes (Peccerillo, 2005). Active volcanoes in red. Open symbols refer to outcrops below the sea level). Volcanic centers marked by white circle bear peridotite xenoliths.

At the regional scale, deep cycling of Earth’s carbon - i.e. how decarbonation or melting of carbonate-rich lithologies from a subducted lithosphere may affect the efficiency of carbon release in the lithosphere-asthenosphere system - was recently investigated in the western Mediterranean region by Frezzotti et al. (2009), who presented an integrated petrological and geophysical model that explains the present-day CO₂ degassing in Italy. In this paper, we summarize the main data on Earth’s CO₂ emission in Italy, and review the mechanical properties of the lithosphere-asthenosphere system along some key sections in the western Mediterranean region to discuss the possible origin of deep CO₂ emission in volcanic and non-volcanic areas, in the light of the main geophysical and geochemical features of the upper mantle in the Italian region.