Abstract
We monitored the soil gas emission of CO 2 from selected sites of Mt. Etna volcano during the period February 2009 to December 2010 by measuring periodically the soil CO 2 efflux together with the associated stable carbon isotope composition of CO 2 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 CO 2 interacted with shallow cold ground water and/or with shallow biogenic CO 2 , a positive correlation between soil CO 2 effluxes and carbon isotopes was evident and it depended strongly on the velocity of gas through the soil. In these cases, the highest CO 2 effluxes corresponded to d 13 C CO2 values similar to those of the deep magmatic CO 2 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 CO 2 gas and dissolved HCO 3 - 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 CO 2 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.
Highlights
Etna showed that the central eastern flank and the lower southwestern flank of the volcano are characterised by the strongest anomalies in both soil gas concentration and soil CO2 efflux and by the highest content of magmatic CO2, which dissolves into local groundwater [Anzà et al 1989, Giammanco et al 1995, 1996, Allard et al 1997, Brusca et al 2001, Aiuppa et al 2004, Giammanco and Bonfanti 2009]
The isotopic shift of carbon observed at this site was explained as being due to strong interaction between deep magmatic fluids and a hydrothermal aquifer at T° > 120 °C because, in this case, the fractionation factor in water between gaseous CO2 and dissolved HCO3- favors enrichment of the heavier carbon isotope in the residual gas phase that passes through the ground water [Mook et al 1974]
If we assume a pristine C-isotope composition in the range from -2.5 to -1‰ as representative of the magmatic CO2 emitted from Etna [Giammanco et al 1998, Pecoraino and Giammanco 2005], the range of values that we observed would be compatible with an isotopic shift of +3 to +4‰
Summary
Most of the release of CO2 occurs through the active summit vents of the volcano, but a remarkable fraction of this gas In this case, CO2 emissions at the surface occur in diffuse form through volcano-tectonic faults [e.g., Giammanco et al 1998, Aiuppa et al 2004] and they are produced by magma outgassing from reservoirs located at intermediate to great depths beneath the volcano [Bruno et al 2001, Aiuppa et al 2004, Giammanco et al 2013]. Etna showed that the central eastern flank and the lower southwestern flank of the volcano are characterised by the strongest anomalies in both soil gas concentration and soil CO2 efflux and by the highest content of magmatic CO2, which dissolves into local groundwater [Anzà et al 1989, Giammanco et al 1995, 1996, Allard et al 1997, Brusca et al 2001, Aiuppa et al 2004, Giammanco and Bonfanti 2009]
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