The Azores archipelago (Portugal) contains 88 lakes, two located inside caves spread across six of the nine volcanic islands of the archipelago; the formation of the lakes is associated with various types of volcanic structures. Surveys were performed on 45 water bodies corresponding to a representative data set and omitting only small ephemeral ponds to extensively research CO
2 emissions from the volcanic lakes.
The modified accumulation chamber method was used to measure the CO2 flux at the surface of the lakes; 16,119 measurements were performed with flux values ranging from 0 to 20,960 g m-2 d-1. Graphical statistical approaches identified different geochemical populations from the CO2 flux data. Multiple populations associated with limnological and biogenic processes or volcanic/hydrothermal sources were identified in some cases. The influence of these sources was also confirmed by the determined δ13C content of the lakes.
The lakes of the Azores emit approximately ~171 × 103 t yr-1 of CO2 into the atmosphere; much of this is released by the Furnas and Furna do Enxofre lakes, both showing mantle-derived CO2 origin. Deep-seated CO2 accounts for approximately 42% of the total degassing.