Chemical and isotopic features of gas reservoirs in Albano, Averno and Monticchio crater lakes (central-southern Italy)

Deep waters of crater lakes hosted in non-active volcanoes are able to store huge amounts of gases, mainly CO 2 and CH 4 , that can be added from sub-lacustrine vents and/or produced by processes related to bacterial activity. Destabilization of deep lake strata may trigger massive release of dissolved gases producing "limnic eruption", like those occurred at the Cameroonian Lakes Monoun and Nyos in 1984 and 1986, respectively. Volcanic systems of the central and southern Italian peninsula host several crater lakes. Among them, the Albano (Alban Hills volcanic complex; Central Italy), Averno (Phlegrean Fields; southern Italy), Monticchio Grande and Monticchio Piccolo (Vulture volcano; southern Italy) lakes are the only ones showing chemical and thermal stratification and presence of significant amounts of dissolved gases at depth. The distribution of the dissolved gas composition along the vertical profiles of these lakes is similar, being characterized by dominating N 2 in the oxic epilimnion, while CO 2 is the main gas species in the anoxic hypolimnion. The vertical patterns of CH 4 concentrations resemble those of CO 2 , since both these compounds show an increase from the surface to the bottom of 3-4 orders of magnitude. The  13 C-CO 2 values of Monticchio Grande, Monticchio Piccolo and Albano lakes (ranging between -0.4 and -5.8 ‰ V-PDB) are consistent with those of mantle-derived CO 2 . Conversely, at the Averno lake the  13 C-CO 2 values range between -8.2 and -13.4 ‰ V-PDB, supporting the occurrence of prevalent organic CO 2 .  13 C-CH 4 and D-CH 4 values of all the investigated lakes (down to -67 ‰ V-PDB and -283 V-SMOW, respectively) suggest that bacterial activity is basically the main responsible of CH 4 production. The carbon isotopic signature of the two main dissolved gas species along the vertical profiles seems to depend, besides of their origin, on 1) CO 2 -CH 4 isotopic exchange, 2) CO 2 reduction to CH 4 at reducing conditions, 2) CH 4 oxidation to CO 2 at oxidizing conditions. The  13 C-CO 2 values are indeed progressively more positive at increasing depth, whereas an opposite trend is shown by the 13 C-CH 4 values. In conclusion, these results have shown that, although the morphometric features (water volumes of Monticchio Grande, Monticchio Piccolo, Averno and Albano lakes are 3.3 x10 6 , 4 x10 6 , 6 x10 6 and 450 x10 6 m 3 , respectively) and the relatively low gas concentrations (max 19.4 mmol/L at a depth of 39 m in the Monticchio Piccolo lake) suggest that the gas reservoirs of these lakes cannot presently represent a serious hazard for limnic eruptions, the vertical patterns of the CO 2 /CH 4 ratio and the  13 C-CO 2 and  13 C-CH 4 values may represent useful monitoring tools to control the rate of fluids discharged from the lake bottoms.