Chemical and isotopic features of dissolved gases in Albano, Averno and Monticchio volcanic lakes (central-southern Italy): new tools for Nyos-type lake monitoring

Lakes hosted in non-active volcanoes may store large amounts of gases, mainly CO2 and CH4, when these are: i) added from sub-lacustrine vents or ii) generated by microbial activity. Such gas accumulation at depth may be favouring a pronounced vertical thermal, chemical and isotopic stratification of these water bodies. External perturbations (i.e. earthquakes, landslides, heavy rains) able to affect the stability of deep lake strata may trigger massive release of dissolved gases (“limnic eruptions”) similar to those occurred at the Cameroonian Lakes Monoun and Nyos in 1984 and 1986, respectively. These events can pose a severe hazard if occurring in densely populated areas. Several lakes are hosted in the central and southern volcanic systems of the Italian Peninsula, although those characterized by chemical and thermal stratification with significant amounts of nonatmospheric dissolved gases at depth are: Albano (Alban Hills), Averno (Phlegrean Fields) and Monticchio Grande and Monticchio Piccolo (Mt. Vulture). The distribution of the dissolved gas composition along the vertical profiles of these lakes is similar, being dominated by N2 in the oxic epilimnion, whereas CO2 is the main gas species in the anoxic hypolimnion. The vertical pattern of CH4 concentration intimately mimics that of CO2, both showing significant increases from surface to bottom. The 13C-CO2 values of Monticchio Grande, Monticchio Piccolo and Albano lakes (ranging between -5.8 and -0.4 ‰ V-PDB) are consistent with those of mantle-derived CO2. Conversely, at Averno lake the 13C-CO2 values are between -13.4 and -8.2 ‰ V-PDB, supporting the occurrence of prevalent organic CO2. The 13C-CH4 and D-CH4 values of all the investigated lakes, as low as -67 ‰ V-PDB and -283 ‰ V-SMOW, respectively, suggest that the CH4 production is mainly due to bacterial activity. The 13C-CO2 values progressively increase with depth, whereas the carbon isotopes in methane have an opposite trend. This contrasting carbon isotopic distribution of the two main gaseous species can be related to: 1) CO2-CH4 isotopic exchange; 2) reduction of CO2 to CH4 in the anoxic hypolimnion; 3) oxidation of CH4 to CO2 in the oxic epilimnion; 4) direct production of CH4 and CO2 by bacterial activity. The morphological features (water volumes of Monticchio Grande, Monticchio Piccolo, Averno and Albano are 3.3x106, 4x106, 6x106 and 450x106 m3, respectively) and the relatively low total gas concentrations (max. 19.4 mmol/L at -39 m in Monticchio Piccolo) suggest that the gas reservoirs of these lakes don’t currently represent a serious hazard for limnic eruptions. Nevertheless, this study demonstrates that the vertical patterns of the CO2/CH4 ratio and the 13C-CO2 and 13C-CH4 values are important geochemical tracers able to record any change in depth of the oxic and anoxic layers, thus representing new and very promising monitoring tools for evaluating the recharge rate of CO2-rich extra-lacustrine fluids.