Estimation of the CO2 flux from Furnas volcanic Lake (São Miguel, Azores)

•Furnas lake waters are Na–HCO3 fluids with a neutral-diluted character.•Anomalous CO2 degassing areas identified at Furnas volcanic Lake are associated with tectonic structures.•Stratification of Furnas lake influences the CO2 amount that is emitted to the atmosphere.•The total amount of CO2 degassing to the atmosphere was estimated as ~ 52 t d− 1 and ~ 600 t d− 1

A study on diffuse CO2 degassing was undertaken at Furnas lake (São Miguel island, Azores) in order to estimate the total diffuse CO2 output and identify anomalous degassing areas over the lake. Furnas lake is located in Furnas Volcano, the easternmost of the three active central volcanoes of the São Miguel island. The lake has an area of 1.87 km2 and a maximum length and width equal to 2025 and 1600 m, respectively. The maximum depth of the water column is 15 m and the estimated water storage is 14 × 106 m3. Lake water temperature is cold, with temperature values between 13 °C and 15 °C in the winter period and 18.9 °C to 19.3 °C in early autumn, and the variation along the water column suggests a monomictic character. The major-ion relative composition is in decreasing order Na+ > K+ > Ca2 + > Mg2 + for cations and HCO3− > Cl− > SO42− for anions, and conductivity and pH measurements, respectively in the range of 152 to 165 μS cm− 1 and 5.3 to 8.7, suggests that Furnas has neutral-diluted waters and can be classified as a non-active lake. Diffuse CO2 flux measurements were made using the accumulation chamber method with a total of 1537 and 2577 measurements performed in two different sampling campaigns. The total amount of diffuse CO2 emitted to the atmosphere was estimated between 28 and 321 t km− 2 d− 1, respectively, in the second and first sampling campaigns, corresponding to ~ 52 and ~ 600 t d− 1. The main anomalous degassing area identified over the Furnas lake during both surveys is probably associated to a WNW–ESE trending tectonic structure. Other secondary areas are also suggested to be tectonically influenced. Identified anomalous areas showed similarities to the ones observed during previous soil CO2 degassing studies.