Volcanic release of carbon dioxide into Earth’s atmosphere occurs not only during eruptive periods, but also during quiescent periods. This sustained release is the case for some soil CO2 degassing areas in the Azores archipelago (Portugal), where CO2 emitted to the atmosphere is estimated to be about 1,200 tons per day .
The Azores archipelago comprises nine volcanic islands located at the triple junction of the Eurasian, North American, and African tectonic plates. Due to this particular geodynamic setting, the islands are frequently affected by earthquakes and volcanism. The last volcanic eruption in the archipelago was submarine, and occurred in the period 1998-2001 close to the Terceira Island. Since human settlement in the 15th Century, this island was also affected by a subaerial volcanic event in 1761. Nowadays, most hydrothermal activity is associated with a fumarolic field (Furnas do Enxofre fumaroles) located in the central part of the island (Pico Alto volcano).
At the end of 2002, researchers installed a permanent soil CO2 flux station in Furnas do Enxofre fumarolic field to perform hourly measurements. The station recorded long-term (seasonal) and short-term CO2 flux variations, generally as a result of environmental interferences (e.g., barometric pressures changes and rainfall events).
Between 25 and 31 August 2014, researchers from the Centre for Volcanology and Geological Risks Assessment (CVARG, University of the Azores, Portugal) and the University of Perugia (Italy) carried out a detailed CO2 flux survey at Terceira Island to evaluate diffuse CO2 emissions not only from the area around the fumaroles, but also at sites associated with important tectonic structures. They established a regular grid and took measurements based on the accumulation chamber method. Considering the important vegetation that covers the Azorean soils, simultaneously to the CO2 flux measurements researchers also collected gases to assess carbon isotopic composition to discriminate between different sources (biogenic vs. hydrothermal) of CO2. The INGV-Osservatorio Vesuviano (Italy) laboratories performed these analyses. During the last week of May two researchers from CVARG also collected rock samples to study the fluid inclusions present in mineral phases, and to estimate the carbon isotopic composition of deep phases.
This work is part of TERCO2 (Soil CO2 released at Terceira volcanic island) and partially funded by the DCO’s DECADE initiative. This is an interdisciplinary project that joins volcanologists, geochemists, petrologists, and structural geologists, and allows, for the first time, quantification of CO2 emissions from the Furnas do Enxofre soils and thus contributes to estimates of Earth’s carbon budget. Evaluation of CO2 sources, as well as integration of the carbon isotopes released by soils and the composition of the crystal phase, will provide important information about the degassing path.
Report and images provided by Fátima Viveiros, CVARG, Portugal.