Estimation of soil CO2 flux emissions at Fogo Volcano (São Miguel Island, Azores)

Secondary manifestations of volcanism found out at Fogo Volcano (São Miguel Island, Azores) comprise essentially hydrothermal fumaroles, thermal and cold-CO₂ rich springs, as well as diffuse degassing areas. Visible emissions are essentially located on the north flank of the volcano and are associated to the general NW-SE faults of the Ribeira Grande graben. Diffuse degassing surveys aiming at measuring the soil CO₂ concentration at about 50 cm depth were carried out in the north flank of the volcano in the last 20 years and anomalous CO₂ was associated to NW-SE trends, coincident with the graben faults. Soil CO₂ flux surveys, through the accumulation chamber method, were carried out during summer-autumn 2022 in the areas surrounding the three main fumarolic fields: Caldeira Velha, Caldeiras da Ribeira Grande, and Pico Vermelho. A total of 1207 sites were sampled in an area with about 1 km². Interpolated data (sequential Gaussian simulation) together with the Graphical Statistical Approach estimated a value around 95.5 t/d for the CO₂ emitted to the atmosphere. Main anomalous CO₂ fluxes are correlated with temperature anomalies and DDS (Diffuse Degassing Structures) show general NW-SE orientations, suggesting the correlation between degassing and tectonic structures in the study site, similarly to the observed in previous studies. This work contributes not only to the estimation on the total carbon-budget, but may also be a valuable tool to identify potential anomalies related with unrest of the volcanic system.

This work was partially funded by FCT – Fundação para a Ciência e Tecnologia, under project MAGAT - From MAGma to the ATmosphere - uma contribuição para desenvolver a próxima geração de sensores geoquímicos para a monitorização em tempo real do movimento do magma em profundidade (CIRCNA/OCT/0016/2019).