Abstract
Several types of natural gas emissions (soil gas, low temperature fumaroles, gas bubbling in mud pools) were collected monthly on Mt. Etna volcano between July 2000 and July 2003 both from its summit and its flanks. Samples were analysed for the determination of the concentrations of CO2, CH4, He, H2, CO as well as the isotopic ratios of 13C/14C of CO2 (δ^13C) and He (R/Ra). The analysed gases were chemically divided into two groups: air-contaminated (from sites closer to the summit vents of Mt. Etna) and CO2-rich. Among the latter, samples from the lower SW flank of the volcano showed high contents of biogenic thermogenic and/or microbial CH4. Isotopic shift in the δ^13C values is caused by input of organic CO2 and/or by interaction between magmatic CO2 and shallow ground water as a function of water temperature and CO2 flux from depth. Based on a graphic method applied to δ^13C(subscript TDIC) of some ground water, the inferred isotopic composition of the pristine magmatic gas at Mt. Etna is characterised by δ^13C values ranging from -2 to -1‰. During the period July 2000-July 2003 significant variations were observed in many of the investigated parameters almost at all monitored sites. Seasonal influences were generally found to be negligible, with only a limited effect of air temperature changes on soil CO2 and ground temperature in only two of the air-contaminated sites. The largest chemical anomalies were observed in the air-contaminated sites, probably because of the strong buffering power of local ground water on gases released through the most peripheral areas where the CO2-rich sites are located. The anomalous changes observed during the study period can be explained in terms of progressive gas release from separate batches of magma that ascend towards the surface in a step-wise manner. Data relevant to the period following the 2002-03 eruption suggest that magma kept accumulating beneath the volcano, thus increasing the probability of a new large eruption at Mt. Etna.
Highlights
Mount Etna is the largest volcano in Europe (~1200 km2 of total surface with a maximum height of ~ 3300 m above sea level) and is one of the most active in the world
Isotope analyses on δ13C(CO2 ) and He in the gases sampled during the period of this study showed values ranging from -22 to +1.6 000 and from 1.2 to 7.5 Ra, respectively (Table 1)
Variations in the concentration and/or flux of gases issuing from the flanks of active or quiescent volcanoes can be due basically to changes in two different types of factors: environmental or volcanic
Summary
Mount Etna is the largest volcano in Europe (~1200 km of total surface with a maximum height of ~ 3300 m above sea level) and is one of the most active in the world. In this paper we present original data on the temporal evolution of the chemical and isotopic characteristics of gases collected simultaneously from low-temperature (T < 100°C) fumaroles, soil emissions, mofettes and mud volcanoes in and around the Mt. Etna area. Gas samples were collected from July 2000 to June 2003 in six sites in the Mt. Etna area (Fig. 1). Site P39 is located on the lower southwestern flank about 2 km southwest of the town of Paternò (altitude of about 115 m a.s.l) and approximately 20 km SSW of Etna’s summit craters It is a mofette characterised by huge emissions of CO2 and CH4 from the ground (Giammanco et al 1998a).
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