ALBERT

Description: We report the first detailed study on the types and distributions of active subaqueous fumaroles and surface diffuse CO 2 degassing in the three main volcanic lakes of São Miguel Island (Sete Cidades, Fogo and Furnas), Azores archipelago, Portugal. The results of the surveys, carried out in May 2011 using a floating accumulation chamber and a dual beam 50 and 200 kHz echo sounder, revealed a very low surface CO 2 degassing at the three lakes, in the range of 32–608 kg d –1 . However, dense subaqueous degassing plumes were found in the north of the Furnas crater lake (7.5–9 plumes per 100 m 2 ), and moderate-density degassing in the Fogo (1.5–2 plumes per 100 m 2 ) and Sete Cidades crater lakes (1–1.5 plumes per 100 m 2 ). The echo sounder detected hydroacoustic signatures interpreted as acoustic flares, ‘puffing’ bubble plumes or walls of bubbles associated with numerous subaqueous fumaroles. The recorded echograms show that the bubbles rise at average speeds of between 19 and 30 cm s –1 at the bottom, with frequencies of release from 1–2 to 31 s. Most subaqueous fumaroles disappear due to the dissolution of CO 2 before reaching the lake surface. These dissolution processes are enhanced by the pH range observed in the three volcanic lakes ( c. 7–9). Observed dissolved CO 2 values indicate that the pressure of this gas in the three lakes remained much lower than the hydrostatic pressure and the risk of a limnic eruption is therefore negligible. We suggest that the rising levels of CO 2 from the subaqueous bubbles could constitute a critical fuel for subsurface phytoplankton layers, interpreted as horizontal acoustic layers with high backscattering values. The highest density of subaqueous bubbling correlates with recent submerged secondary craters formed around the caldera rims of the three Late Quaternary stratovolcano complexes of São Miguel Island. Our results emphasize the need to perform regular surface degassing studies as an important volcanic surveillance tool in the Azores archipelago.