Radon surveys and real-time monitoring at Stromboli volcano: Influence of soil temperature, atmospheric pressure and tidal forces on ²²²Rn degassing

Abstract

We used a network of stations to perform systematic radon surveys at Stromboli volcano. The time series of periodic measurements show that monthly average ²²²Rn emissions reflect changes in volcanic activity and exhibit increasing trends prior and during the last major eruptive cycles. Maps of radon emissions indicate that diffuse degassing is operative at Stromboli volcano. Concentrated degassing essentially occurs in the summit area and within a sector proximal to the two major NE trending faults. These sites were chosen for deploying the two real-time stations that are currently operating at Stromboli. In these devices, the ²²²Rn electronic dosimeters are connected to a radiomodem for wireless data transfer to a receiving station at the volcano observatory. Radon activity, soil temperature and atmospheric pressure data are sampled and instantaneously transferred via web so that they can be checked remotely. Collected time series reveal an overall inverse correlation between radon emissions and seasonal temperature variations. Radon emissions in sectors of diffuse degassing are modulated by tidal forces as well. Radon activities recorded at the summit station, located along the fracture zone where the gas flux is concentrated, are positively correlated with changes in atmospheric pressure and confirm the occurrence of the “atmospheric stack effect”. We finally emphasize that real-time radon monitoring is an innovative technique that may be systematically applied in volcano surveillance.

Introduction

Stromboli volcano is a unique natural laboratory to investigate complex magmatic processes including gaseous transfer to the surface. When a magma batch is approaching the subvolcanic environment, the gas phase starts to exsolve generating a “two-phase system”. During magma ascent, gas expansion may produce variable explosive activities together with anomalous gas concentrations that may be detected and/or within the volcanic plume as a result of shallow magma degassing (e.g., Allard et al., 1994, Chiodini et al., 1996, Heiligmann et al., 1997, Ripepe et al., 2005, among others). Following the last two major eruptive cycles at Stromboli (2002–03 and 2007), a multidisciplinary effort was undertaken to integrate geophysical and geochemical data.

Among these, radon monitoring has established a basic role in recording variations in volcanic activity, as well as in detecting complex geodynamic processes associated with active tectonics. In nature, radon is mainly represented by the isotope ²²²Rn (with a half life of 3.82 days): it is an alpha emitting radioactive gas produced from the decay of ²²⁶Ra, in turn derived from uranium bearing materials. Marked radon anomalies may precede earthquakes (e.g., Fleischer and Mogro-Campero, 1985, Igarashi et al., 1995, Planicić et al., 2004) and volcanic eruptions (Chirkov, 1975, Connors et al., 1996, Cigolini et al., 2005, Alparone et al., 2005, Giammanco et al., 2007). In contrast to findings at Mount Etna, where a halo of magmatic CO₂ has been postulated to extend over much of the cone, Williams-Jones et al. (2000) have shown that Rn, CO₂ and δ¹³C values are higher on the lower flanks of Arenal, Poás and Galeras volcanoes, except near the fumaroles surrounding the active craters. In addition, Varley and Armienta (2001) pointed out that diffuse degassing seems to absent at Popocatépetl volcano. However, these features may derive from self-sealing processes that affect the fracture networks of the hydrothermal shells surrounding magmatic systems and may not be permanent in space and time (cf., Cigolini et al., 2001).

In recent papers, the importance of deploying radon networks to detect major radon anomalies on active volcanoes by means of periodic measurements has been stressed, thus identifying the sites of more efficient response to seismic transients and/or volcanic degassing (Cigolini et al., 2001, Cigolini et al., 2007, Pérez et al., 2008). These sites are the ones that could be successfully used in locating stations for continuous radon monitoring. Automatic alpha particle detectors play a key-role in volcano surveillance contributing to decode the interplay between seismic signals and other geochemical parameters. Indeed, these may reveal critical variations before and during the onset of volcanic eruptions. Thus, systematic time series analysis and signal processing give us the opportunity for better understanding the dynamic behaviour of volcanoes.

In this paper we summarize some of the data collected during our periodic surveys and present the recent results of “real time” radon monitoring at Stromboli volcano. We first introduce the methods for ²²²Rn measurements, and then discuss the time-series for radon emissions, soil temperatures, and atmospheric pressure.