CO2 emissions from Fogo intracaldera volcanic lakes (Sao Miguel Island, Acores): A tool for volcanic monitoring

A geochemical study was carried out at Fogo Lake during June 2016, in one of the deepest lakes on Sao Miguel Island, as well as on small associated pond, in order to identify possible anomalous degassing areas. Fogo Lake is located inside the summit caldera of logo Volcano, one of the three active central volcanoes that dominate Sao Miguel geology. The hydrogeochemical profile carried out along the water column of the logo lake suggests that the lake was thermally stratified, corresponding to poorly mineralized and cold Na-Cl type waters. The same composition is found in the small pond, but with a slight Ca-enrichment. To measure the CO2 flux at the lake surface, the modified accumulation chamber method was used, and a total of 1696 measurements were accomplished, covering an area of approximately 1.56 km(2). Measured CO2 flux values are relatively low, ranging from 0 to 28.4 g m(-2) d(-1) (mean - 2.6 g m(-2) d(-1)) and from 2.1 to 15.0 g m(-2) d(-1) (mean - 6.3 g m(-2) d(-1)), respectively at Fogo Lake and at the small pond. The very low values measured in the central area of Fogo Lake are associated with the monomictic character of the lake, as the CO2 is not able to ascend into the surface when the water column is stratified during the warmer period. Statistical analyses suggest that the CO2 emission is associated with a single source of CO2, probably biogenic; a similar signature is pointed by the water delta C-13 isotopic content. The estimated total CO2 emissions from the Fogo Lake was approximately 4.04 t d(-1), much higher than the value observed at the small pond (0.19 t d(-1)). (C) 2020 Elsevier B.V. All rights reserved.