First combined flux chamber survey of mercury and CO2 emissions from soil diffuse degassing at Solfatara of Pozzuoli crater, Campi Flegrei (Italy): Mapping and quantification of gas release

Abstract

There have been limited studies to date targeting gaseous elemental mercury (GEM) flux from soil emission in enriched volcanic substrates and its relation with CO2 release and tectonic structures. In order to evaluate and understand the processes of soil–air exchanges involved at Solfatara of Pozzuoli volcano, the most active zone of Campi Flegrei caldera (Italy), an intensive field measurement survey has been achieved in September 2013 by using high-time resolution techniques. Soil–air exchange fluxes of GEM and CO2 have been measured simultaneously at 116 points, widely distributed within the crater. Quantification of gas flux has been assessed by using field accumulation chamber method in conjunction with a Lumex®-RA 915 + portable mercury vapor analyzer and a LICOR for CO2 determination, respectively. The spatial distribution of GEM and CO2 emissions correlated quite closely with the hydrothermal and geological features of the studied area. The highest GEM fluxes (from 4.04 to 5.9 × 10− 5 g m− 2 d− 1) were encountered close to the southern part of the crater interested by an intense fumarolic activity and along the SE–SW tectonic fracture (1.26 × 10− 6–6.91 × 10− 5 g GEM m− 2 d− 1). Conversely, the lowest values have been detected all along the western rim of the crater, characterized by a weak gas flux and a lush vegetation on a very sealed clay soil, which likely inhibited mercury emission (range: 1.5 × 10− 7–7.18 × 10− 6 g GEM m− 2 d− 1). Results indicate that the GEM exchange between soil and air inside the Solfatara crater is about 2–3 orders of magnitude stronger than that in the background areas (10− 8–10− 7 g m− 2 d− 1). CO2 soil diffuse degassing exhibited an analogous spatial pattern to the GEM fluxes, with emission rates ranging from about 15 to ~ 20,000 g CO2 m− 2 d− 1, from the outermost western zones to the south-eastern sector of the crater. The observed significant correlation between GEM and CO2 suggested that in volcanic system GEM volatilizes from substrate in a similar manner to the release of CO2. The quantitative estimation of the total amount of CO2 and GEM released from the Solfatara crater gave values of about 304 ± 13 and 3.7 ± 0.2 × 10− 6 t d− 1, respectively. Finally, based on our dataset and previous work, we propose that an average GEM/CO2 molar ratio of ~ 2 × 10− 8 (n = 9) is best representative of hydrothermal degassing. Taking into account the uncertainty in global hydrothermal CO2 emissions from sub-aerial environments (~ 1012 Mol yr− 1), we infer a global volcanic GEM flux from hydrothermal environments of ~ about 8.5 t yr− 1. Although this value has to be considered as a lower limit for the global emission of GEM from these sources, we suggest that on a local scale hydrothermal activity can be regarded as a significant source of GEM than previously recognized to the atmospheric pool.