Carbon isotopic composition of soil CO 2 efflux, a powerful method to discriminate different sources feeding soil CO 2 degassing in volcanic-hydrothermal areas

Abstract

A new method combining measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux was developed in order to qualitatively and quantitatively characterise the CO 2 source feeding the soil CO 2 diffuse degassing. The method was tested in March 2007 at the Solfatara of Pozzuoli volcano degassing area (Naples, Italy) where more than 300 measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux were performed, surveying Solfatara crater and its surroundings. The wide range of CO 2 flux and CO 2 isotopic composition values (from 8.4 g m − 2 d − 1 to 28,834 g m − 2 d − 1 , and from 0.73‰ to − 33.54‰, respectively), together with their statistical distributions suggests the occurrence of multiple CO 2 sources feeding soil degassing. The combined interpretation of flux and isotopic data allows us to identify and characterise two distinct gas sources: a hydrothermal and a biogenic source. The soil CO 2 from the hydrothermal source is characterised by a mean δ 13C CO 2 of − 2.3‰ ± 0.9‰, hence close to the isotopic composition of the fumarolic CO 2 ( δ 13C CO 2 = − 1.48‰ ± 0.22‰) and by a mean CO 2 flux of 2875 g m − 2 d − 1 . The CO 2 from the biogenic source is characterised by a mean δ 13C CO 2 of − 19.4‰ ± 2.1‰, and by a mean CO 2 flux of 26 g m − 2 d − 1 , which are both in the range of the typical values for biologic CO 2 soil degassing. This reliable characterisation of the biogenic CO 2 flux would not have been possible by solely applying a statistical analysis of the CO 2 flux values, which is commonly applied in volcanological studies for the partitioning between background fluxes and anomalous CO 2 fluxes. A map of the Solfatara diffuse degassing structure was derived from the estimated threshold for the biogenic CO 2 flux, highlighting that soil degassing of hydrothermal CO 2 mixed in different proportion with biogenic CO 2 occurs over a large area (~ 0.8 km 2), which extends over the inner part of the Solfatara crater as well as the eastern periphery, corresponding with a NW–SE fault system. The presented method and data analysis are important means of surveillance of the volcanic activity.