Combined monitoring of CO2 efflux, 222-Rn and 220-Rn in soil gas on Mt. Etna: a new geochemical tool for volcano surveillance.

Abstract

Since 2002, measurements of 222Rn, 220Rn activity and of CO2 efflux in soil and fumaroles were carried out at several locations on Mt. Etna volcano. An empirical relationship links the 222Rn/220Rn ratio to the CO2 efflux: deep sources of gas are characterized by high 222Rn activity and high CO2 efflux, whereas shallow sources are indicated by high 220Rn activity and relatively low CO2 efflux. This relationship is more constraining on the type and depth of the gas source than using the 222Rn/220Rn ratio alone. Since June 2006, periodical measurements of these parameters were carried out in 10 sites located over a surface of about 7 km2 on the east flank of Mt. Etna (Zafferana village). The chosen area is characterized by anomalous diffuse degassing produced by fault-driven leakage of volatiles from a magma source whose depth is inferred at about 4-7 km below the surface. The sampling frequency varied between once a month to once every ten days. We studied the temporal variation of the ratio between CO2 efflux and (222Rn/220Rn), that we define as a Soil Gas Disequilibrium Index (SGDI). Increases of this parameter occurred just before and during the 2006 eruptive period (July to December 2006), and at the onset of the March-May 2007 sequence of summit paroxysmal episodes. Furthermore, a slow increasing trend preceded by a few months the August-September 2007 summit activity of Etna, culminated with the September 4th 2007 paroxysmal episode. Remarkable spike-like increases not associated with eruptions occurred on January 10th, 2007, correlated with anomalous increases in volcanic tremor, and on June 20th, 2007, linked with marked short-lived anomalies both in the ground deformation and in the gravimetric signals recorded by the INGV-Catania monitoring networks. The last increase in this geochemical index was recorded in late March 2008, correlated with a marked increase both in the volcanic tremor and in the plume SO2 flux. This index looks very promising as a new tool for volcano monitoring, as it seem very sensitive to volcanic unrest.