Wetlands are well-known as significant sources of CH4 to the atmosphere, whereas lakes are commonly considered as sinks for Cbearing volatiles produced within bottom sediments. In this work, we present the results of two surveys (summer 2020 and winter 2021, respectively) carried out at the 300×200 m wide Lake Astroni (Phlegraean Fields, southern Italy) to measure CH4 and CO2 diffusive fluxes (fCH4 and fCO2) from the lake surface. The fCH4 and fCO2 values, measured according to the floating chamber method, were compared to those computed according to water and dissolved gas chemical compositions. Both measured and computed data show that Lake Astroni is a significant CH4 and CO2 source to the atmosphere, with values of the same order of magnitude as those measured in highly-producing wetlands at this latitude. These results are quite surprising, considering that the depth of the lake (up to 5 m) is significantly higher than that of a typical wetland. Consistently, significant amounts of dissolved CH4 and CO2 were recognized at ≥1 m depths, where an anoxic hypolimnion occurs. According to the d13C-CH4 and d13C-CO2 values measured at the lake bottom, CH4 is totally biogenic, whereas CO2 is mostly of hydrothermal origin, as supported by the occurrence of hydrothermal gas emissions along the lake rim. Such a high CH4 release to the atmosphere is likely depending on the combination of different factors: (i) a large availability of labile organic matter provided by the forest surrounding the lake; (ii) the external (hydrothermal) CO2 source; (iii) the low exposure to wind due to the steep flanks of the Astroni caldera, preventing the vertical circulation of lake water and favoring the development of an anoxic layer. Microbiological analysis on lake water and sediment samples are planned to be carried out shortly to provide further insights into the biogeochemical processes at the origin of this extremely CH4-rich environment.
Measurements of CH4 fluxes from lakes: the case of Lake Astroni, Phlegraean Fields, southern Italy / Franco Tassi, Alice Repetto, Antonio Randazzo, Francesco Capecchiacci, Stefania Venturi, Jacopo Cabassi, Roberto Isaia, Stefano Caliro, Orlando Vaselli. - ELETTRONICO. - (2022), pp. 0-0. (Intervento presentato al convegno 1st Congress of Società Geochimica Italiana “From theoretical to applied geochemistry”).
Measurements of CH4 fluxes from lakes: the case of Lake Astroni, Phlegraean Fields, southern Italy
Franco Tassi;Antonio Randazzo;Stefania Venturi;Orlando Vaselli
2022
Abstract
Wetlands are well-known as significant sources of CH4 to the atmosphere, whereas lakes are commonly considered as sinks for Cbearing volatiles produced within bottom sediments. In this work, we present the results of two surveys (summer 2020 and winter 2021, respectively) carried out at the 300×200 m wide Lake Astroni (Phlegraean Fields, southern Italy) to measure CH4 and CO2 diffusive fluxes (fCH4 and fCO2) from the lake surface. The fCH4 and fCO2 values, measured according to the floating chamber method, were compared to those computed according to water and dissolved gas chemical compositions. Both measured and computed data show that Lake Astroni is a significant CH4 and CO2 source to the atmosphere, with values of the same order of magnitude as those measured in highly-producing wetlands at this latitude. These results are quite surprising, considering that the depth of the lake (up to 5 m) is significantly higher than that of a typical wetland. Consistently, significant amounts of dissolved CH4 and CO2 were recognized at ≥1 m depths, where an anoxic hypolimnion occurs. According to the d13C-CH4 and d13C-CO2 values measured at the lake bottom, CH4 is totally biogenic, whereas CO2 is mostly of hydrothermal origin, as supported by the occurrence of hydrothermal gas emissions along the lake rim. Such a high CH4 release to the atmosphere is likely depending on the combination of different factors: (i) a large availability of labile organic matter provided by the forest surrounding the lake; (ii) the external (hydrothermal) CO2 source; (iii) the low exposure to wind due to the steep flanks of the Astroni caldera, preventing the vertical circulation of lake water and favoring the development of an anoxic layer. Microbiological analysis on lake water and sediment samples are planned to be carried out shortly to provide further insights into the biogeochemical processes at the origin of this extremely CH4-rich environment.
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