Sergio Gurrieri

Soil CO2 degassing on Mt Etna (Sicily) during the period 1989–1993: discrimination between climatic and volcanic influences

Wide variations were measured in the diffuse CO2 flux through the soils in three selected areas of Mt Etna between August 1989 and March 1993. Degassing of CO2 from the area of Zafferana Etnea-S. Venerina, on the eastern slope of the volcano, has been determined to be more strongly influenced by meteorological parameters than the other areas. The seasonal component found in the data from this area has been excluded using a filtering algorithm based on the best fitting equation calculated from the correlation between CO2 flux values and those of air temperature. The filtered data appear to have variations temporally coincident with those from the other areas, thus suggesting a common and probably deep source of gas. The highest fluxes measured in the two most peripheral areas may correlate well with other geophysical and volcanological anomalous signals that preceded the strong eruption of 1991–1993 and that were interpreted as deep pressure increases. Anomalous decreases in CO2 fluxes accompanied the onset and the evolution of that eruption and have been interpreted as a sign of upward migration of the gas source. The variations of CO2 flux at the 1989 SE fracture have also given interesting information on the timing of the magmatic intrusion that has then fed the 1991–1993 eruption.

Soil CO2 degassing along tectonic structures of Mount Etna (Sicily): the Pernicana fault

Abstract Carbon dioxide emissions from the soil have been investigated along lines of equally spaced sampling points perpendicular to the Pernicana fault on Mt Etna. Anomalous values of soil CO2 have been found not only along the fault plane, but also along directions parallel to it, both to the N and to the S of the main fault. The acquired data seem to reveal a shallow step-like geometry of the Pernicana fault system with parallel faults being generally not deeper than the interface between Etnas volcanic cover and its sedimentary basement (a few hundred meters). The distribution of the anomalous CO2 emissions has also revealed that the Pernicana fault continues at least as far as the Ionian sea, in an area where only sedimentary rocks crop out. This finding would suggest that the main structure is deeper than the base of the volcanic cover, thus cutting at least the uppermost portion of Etnas sedimentary basement. Isotopic analyses of C carried out in samples from locations of high CO2, seem to indicate that the emitted CO2 is a mixture of an organic shallow component and a minor deeper magmatic one. Both chemical and isotopic data on soil gases emitted in the easternmost part of the studied area distinguished another tectonic structure which seems to be much deeper than the Pernicana fault and is roughly directed NNE-SSW, this direction being coincident with an important structural trend of eastern Sicily.

New evidence for the form and extent of the Pernicana Fault System (Mt. Etna) from structural and soil–gas surveying

A multidisciplinary study based on structural and soil–gas surveys was carried out in order to investigate the relationship between soil CO2 degassing and the tectonic setting of the lower northeastern flank of Mt. Etna volcano. The results show that anomalous soil CO2 emissions occur mainly along faults trending WNW–ESE and also where these faults intersect the other main fault set (trending NE–SW) that displaces the study area. In particular, anomalies in CO2 degassing were revealed both along the Pernicana Fault and along another fault (Fiumefreddo Fault) which may represent the prolongation of the former towards the Ionian Sea coast. In the areas where these structures show evident surface faulting, they are all characterised by left-lateral displacements and aseismic creep behaviour. Furthermore, the geochemical survey revealed that these faults join in an area devoid of geological evidence of surface faulting and continue underneath an apparently unfaulted alluvial cover near the coastline. In the light of these findings, we suggest that the Pernicana and Fiumefreddo Faults are discrete segments of a near continuous left-lateral shear zone affecting the whole north-eastern flank of Mt. Etna as far as the Ionian coast.

Geochemical monitoring of groundwaters (1998–2001) at Vesuvius volcano (Italy)

Abstract This work presents the results of hydrogeochemical studies carried out at Vesuvius during the period May 1998–December 2001, mostly focusing on compositional time variations observed during this time. Based on their chemistry, groundwater samples are distinguished into two groups, 1 and 2, representative of water circulation in the southern and northern sectors of the volcano, respectively. Waters from group 1 are typically more acidic, warmer, and more saline than those of group 2. They also have higher CO 2 and CH 4 contents, attributed to enhanced input of deep-rising volatiles and prolonged water–rock interactions. Time-series highlight the fairly constant chemical composition of the entire aquifer. Groundwater temperature, pH, bicarbonate content and dissolved CO 2 display quite stable values in the study period, particularly in deep wells (piezometric level more than 100 m deep). Shallower water bodies present more evident temporal variations, related to seasonal and anthropogenic effects. This paper also describes some important variations in water chemistry which had occurred by the time of the seismic event in early October 1999, particularly in the Olivella spring located on the northern flank of the volcano. At this site, a great decrease in water pH and redox potential, and increased dissolved CO 2 contents and 3 He/ 4 He ratios were observed. These changes in chemical and isotope composition support the hypothesis of an input of magma-derived helium and carbon dioxide into the aquifer feeding the Olivella spring by the time of the earthquake.

Diffuse degassing of carbon dioxide at Somma-Vesuvius volcanic complex (Southern Italy) and its relation with regional tectonics

Abstract A systematic survey of soil CO 2 concentrations was carried out on the flanks of Somma–Vesuvius volcano in order to constrain possible pathways responsible of carbon dioxide diffuse degassing taking place during the present state of quiescence. Measurements were performed at 1162 sites in late winter–spring 2000, highlighting that soil CO 2 concentrations range from 50 to 10500 ppmV. A statistical analysis was developed in order to define the threshold value of anomaly and separate the biogenic CO 2 component, produced by soil respiration, from the inorganic component of deep provenance. A computer routine was also elaborated to interpret the grid of CO 2 anomalous concentration values and define the actual location, orientation and length of degassing structures. The results obtained by this procedure reveal a main control of the regional stress field on the patterns of gas migration. The identified degassing lineaments are typically oriented along the Apenninic (NW–SE) and anti-Apenninic (NE–SW) trends, which are known to govern the past geological and structural evolution of the Campanian Plain and present seismicity and deformation pattern of Mount Vesuvius. A main degassing area was recognized on the eastern and southern flanks of the volcano, which likely relates to the geometry of the underlying carbonate basement, reaching its top (500 m depth) in this sector of the volcano.

Geochemical investigations applied to active fault detection in a volcanic area: the North‐East Rift on Mt. Etna (Sicily, Italy)

Geochemical investigations were performed on the Northeast Rift of Mt. Etna, a prominent volcanic structure of this volcano. Low-temperature fumaroles were found on the upper part of this area and the isotopic compositions of C(CO2) and He suggest a likely magmatic origin of the emitted gases. On the contrary, very low degassing was found in the lower part of the NE-Rift, with CO2 concentrations generally very close to those in air. This pattern is probably due to sealing of the eruptive fissures by the repeated injections of magma solidified into dikes and by consequent shallow hydrothermal alteration of the fissured rocks due to residual magma degassing. High soil CO2 concentrations were also found in an elongated area parallel to the NE-Rift and located immediately to the east of it. The geometry of these anomalies suggests the existence of several hidden faults, sub-parallel to the NE-Rift and likely connected to the Pernicana fault system. This latter structure seems to merge with the NE-Rift at the altitude where the fumarole emissions are located. The hidden structures, together with the Pernicana system and the NE-Rift, represent the western and northern boundaries of a large eastward collapsing sector of Mt. Etna.