Preliminary investigation on soil CO2 and soil CH4 effluxes from a geothermal area near Palagonia (Sicily, Italy)

Abstract

The central part of eastern Sicily is characterized by several areas of strong crustal degassing. These gases are mostly carbon dioxide and methane and are particularly concentrated in some focused emissions (gas vents, mud volcanoes) or as diffuse degassing along major faults. In this study, the extent and orientation of soil CO2 and soil CH4 effluxes were investigated on a wide area (about 40 km2) located about 50 km southwest of Mt. Etna (Fig. 1). From a structural point of view, this area lays on a typical foredeep - foreland system that marks the boundary between the southern part of the Eurasian plate and the northern part of the African plate in the central Mediterranean. The situation was further complicated by the tectonic subsidence of the foredeep to the north of the northern Hyblean margin (YELLIN-DROR et alii, 1997; CARBONE et al., 1982) and the formation of a series of ENE-WSW oriented grabens and horsts, the Lentini Graben being the most important of these structures. This area was characterized by both submarine and subaerial volcanism until 1.7-2 Ma (BARBERI et alii, 1974; GRASSO et alii, 1983; GURENKO AND SCHMINCKE, 2002; SCHMINCKE et alii, 1997; TRUA et alii, 1997), and it was affected by strong seismicity in historical times. The surface geology consists of recent alluvial deposits and Plio-Pleistocene clays and sands interbedded with coeval basaltic lavas and pyroclastics of alkaline affinity (CARBONE et alii, 1987). Strong gas emissions in this area have been known since pre-Greek times, as reported by FERRARA (1805). PONTE (1919, 1934) showed that the gas that erupted through Naftìa Lake (located just southwest of the village of Palagonia, see Fig. 1) was pure carbon dioxide and calculated total emissions at about 80,000 t d-1. Today, the main gas emission is exploited for commercial use (Mofeta dei Palici plant, CO2 output estimated as about 250 t d-1, R. Romano pers. comm., 1998). The area is also characterized by several water well with warm (20-50 °C) water. Recent geochemical studies on this area (DE GREGORIO et alii, 2002; CARACAUSI et alii, 2003a, 2003b; GIAMMANCO et alii, 2007) showed that anomalous CO2 degassing has a hydrothermal or magmatic origin and it is linked to the dynamics of Mt. Etna’s plumbing system.