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Referência Bibliográfica

ANDRADE, C., VIVEIROS, F., CRUZ, J.V., COUTINHO, R., BRANCO, R., (2019) - CO2 fluxes of two lakes in volcanic caves in the Azores, Portugal. Applied Geochemistry,​ 102: 218-228, doi:


This paper characterises diffuse CO2 degassing from lakes located inside volcanic caves on the islands of Terceira and Graciosa (in the Azores archipelago, Portugal). The Algar do Carvão lake is located inside a volcanic pit on Terceira at an altitude of 92 m and has a surface area of 11,500 m2 and a maximum depth of 8.2 m. The Furna do Enxofre lake is located inside a lava cave on Graciosa at an altitude of 500 m and has a surface area of approximately 300 m2 and a maximum depth of 9.3 m.

The water temperature of both lakes is low, with values ranging between 11 °C and 13 °C, in both the winter and summer periods, and no variations are observed along their water columns. The electrical conductivity ranges from 585 μS cm−1 to 687 μS cm−1 in Furna do Enxofre and from 117 μS cm−1 to 131 μS cm−1 in Algar do Carvão, reflecting a higher mineralisation in the former lake. The pH values in Furna do Enxofre range between 6.50 and 6.58, which reflects the effect of CO2 dissolution, while in Algar do Carvão, the pH values are more basic (7.42–8.53). The water types are Mg-HCO3 for Furna do Enxofre and Na-HCO3 for Algar do Carvão; the Mg-enrichment in Furna do Enxofre is associated with water–rock interactions, which are enhanced by the acidic environment.

Diffuse CO2 flux measurements were made using the accumulation chamber method, with a total of 37 measurements split into two surveys at Algar do Carvão and 71 measurements during a single survey at Furna do Enxofre. The total CO2 emitted from Furna do Enxofre was 6100 kg d−1, which was much lower than that emitted from Algar do Carvão (0.32–2.0 kg d−1). A volcanic origin was assigned to the lake on Graciosa due to the δ13C isotopic signature of the CO2; conversely, the CO2 released in Algar do Carvão is derived from a biogenic CO2 source.

Considering the surface areas of the studied lakes, the CO2 flux is in the range of 1.5–6.7 t km−2 d−1 for Algar do Carvão and is 508.3 t km−2 d−1 for Furna do Enxofre.