Temporal analysis of d13C CO2 and CO2 efflux in soil gas emissions at Mt. Etna: a new tool for volcano monitoring

Abstract

We monitored the soil gas emission of CO₂ from selected sites of Mt. Etna volcano during the period February 2009 to December 2010 by measuring periodically the soil CO₂ efflux together with the associated stable carbon isotope composition of CO₂ from February 2009 to December 2010. Correlation between the two parameters showed distinct behaviors depending on the sites as a reflection of the different interactions between crustal and sub-crustal fluids. Where deep CO₂ interacted with shallow cold ground water and/or with shallow biogenic CO₂, a positive correlation between soil CO₂ effluxes and carbon isotopes was evident and it depended strongly on the velocity of gas through the soil. In these cases, the highest CO₂ effluxes corresponded to d¹³C_CO2 values similar to those of the deep magmatic CO₂ emitted from the crater and peri-crateric gas emissions at the summit. In areas where a shallow hydrothermal system was presumed, then a similar correlation was less evident or even absent, suggesting strong control on C isotopes arising from the interactions between CO₂ gas and dissolved HCO₃^- that occur in aquifers at T>120 °C. Marked temporal variations were observed in both parameters at all sites. No significant effect of meteorological parameters was found, so the observed changes were reasonably attributed to variations in volcanic activity of Mt. Etna. In particular, the variations were attributed to increased degassing of CO₂ from incoming new magma, possibly coupled with increased hydrothermal activity in at least some of the shallow aquifers of the volcano. The largest anomalies in the monitored parameters preceded the opening of the New Southeast crater in late 2009 and therefore they could represent a key to unveiling the dynamics of the volcano.