Deep versus shallow sources of CO2 and Rn from a multi-parametric approach: the case of the Nisyros caldera (Aegean Arc, Greece)

Abstract

Estimating the quantity of CO₂ diffusively emitted from the Earth’s surface has important implications for volcanic surveillance and global atmospheric CO₂ budgets. However, the identification and quantification of non-hydrothermal contributions to CO₂ release can be ambiguous. Here, we describe a multi-parametric approach employed at the Nisyros caldera, Aegean Arc, Greece, to assess the relative influence of deep and shallow gases released from the soil. In April 2019, we measured diffuse soil surface CO₂ fluxes, together with their carbon isotope compositions, and at a depth of 80 cm, the CO₂ concentration, soil temperature, and the activities of radon and thoron. The contributions of deep CO₂ and biogenic CO₂ fluxes were distinguished on the basis of their carbon isotope compositions. A Principal Component Analysis (PCA), performed on the measured parameters, effectively discriminates between a deep- and a shallow degassing component. The total CO₂ output estimated from a relatively small testing area was two times higher with respect to that observed in a previous survey (October 2018). The difference is ascribed to variation in the soil biogenic CO₂ production, that was high in April 2019 (a wet period) and low or absent in October 2018 (a dry period). Accounting for seasonal biogenic activity is therefore critical in monitoring and quantifying CO₂ emissions in volcanic areas, because they can partially- or completely overwhelm the volcanic-hydrothermal signal.