VIVEIROS, F., CHIODINI, G., CARDELLINI, C., CALIRO, S., ZANON, V., SILVA, C., RIZZO, A., HIPÓLITO, A., MORENO, L. (2020) - Deep CO₂ emitted at Furnas do Enxofre geothermal area (Terceira Island, Azores archipelago). An approach for determining CO₂ sources and total emissions using carbon isotopic data. Journal of Volcanology and Geothermal Research, 401, 106968, doi: 10.1016/j.jvolgeores.2020.106968.

​Quantification of the CO₂ released by the volcanoes to the atmosphere is relevant for the evaluation of the balance between deep-derived, biogenic and anthropogenic contributions. The current study estimates the CO₂ released from Furnas do Enxofre degassing area (Terceira Island, Azores archipelago) by applying an approach that integrates the flux of CO₂ from the soil with the δ¹³C-CO₂ values. A deep-derived CO₂ output of 2.54 t d⁻¹ is estimated for an area of ~23,715 m². High biogenic-derived CO₂ flux values (~45 g m⁻² d⁻¹) associated with light carbon isotopic content (δ¹³C=−28‰±1.1‰) are detected and explained by the type of vegetation that characterizes the study site. Carbon isotopic compositions of the CO₂ (−6.4‰±1.2‰) measured in olivine-hosted fluid inclusions of the Terceira basalts are presented for the first time and contribute to defining the mantle-CO₂ signature. Differences between these values and heavier carbon isotope values from gas in fumaroles at Furnas do Enxofre (−4.66‰to−4.27‰) are explained by the carbon isotopic fractionation occurring when CO₂ reacts to form calcite in the geothermal reservoir at temperatures >180°C. A clear correlation between the soil temperature and deep CO₂ fluxes is observed and the integration of the diffuse degassing information with the composition of the fumarolic emissions allows estimating a thermal energy flux of 1.1 MW.