Beschreibung:
<jats:title>Abstract</jats:title><jats:p>To investigate unexplored areas and apply methodologies suitable for monitoring volcanic activity, a pioneer study was undertaken in the crater lagoon of Vila Franca do Campo Islet to identify possible CO<jats:sub>2</jats:sub> degassing zones in the marine environment. This study was carried out using a floating accumulation chamber, which to the best of our knowledge, has only been applied to lacustrine environments.</jats:p><jats:p>A survey was carried out in the crater lagoon in August 2017, resulting in a total of 143 CO<jats:sub>2</jats:sub> flux measurements. Other parameters, such as water temperature, pH, electrical conductivity, and depth, were also determined at each measuring point, covering a study area of 0.02 km<jats:sup>2</jats:sup>. At one of the deepest sites with higher CO<jats:sub>2</jats:sub> flux values, water samples were collected at depth for determination of stable isotopic composition.</jats:p><jats:p>Water temperatures ranged between 21.0 and 27.0 °C, pH from 8.24 to 8.89, and electrical conductivity from 52.5 to 53.9 mS/cm. As expected, due to the marine water composition, sampled waters are of the Na-Cl type. CO<jats:sub>2</jats:sub> flux varied between 0.581 and 1.0 g m<jats:sup>-2</jats:sup> d<jats:sup>-1</jats:sup> (average = 2.967 g m<jats:sup>-2</jats:sup> d<jats:sup>-1</jats:sup>). A single CO<jats:sub>2</jats:sub> population, characterized by low CO<jats:sub>2</jats:sub> fluxes, points to a biogenic CO<jats:sub>2</jats:sub> origin (δ<jats:sup>13</jats:sup>C = −9.62‰). The estimated value for the total CO<jats:sub>2</jats:sub> emitted from the water surface is 0.05 t d<jats:sup>-1</jats:sup>. CO<jats:sub>2</jats:sub> flux measurements were also compared with the structural features that cross the tuff cone, but no clear relation was observed with fractures/faults.</jats:p><jats:p>This methodology should be applied to other coastal marine areas, especially where anomalous gas emissions have been reported.</jats:p>