THE ITALIAN MEROMICTIC VOLCANIC LAKES OF MONTICCHIO (MT. VULTURE) AND ALBANO (COLLI ALBANI)

The Albano (Colli Albani, Lazio, max. depth 167 m), Monticchio Grande (max. depth 35 m) and Monticchio Piccolo (max. depth 38 m) (Monte Vulture, Basilicata) volcanic lakes are the only meromictic volcanic lakes in Italy, being characterized by stable chemical and thermal stratification of water and presence of significant amounts of dissolved gases at depth. In the present study, the chemical, physical and biological processes that control the water isotopic and compositional stratification in these three systems were investigated. The main, minor and trace compounds were determined in the aqueous phase along with the isotopic ratios of oxygen, hydrogen and carbon (inorganic) in water molecule and DIC (Dissolved Inorganic Carbon), respectively, and those of carbon in CO 2 and carbon and hydrogen in CH 4 . The Na-HCO 3 composition depends on both interaction of lake water with volcanic rocks and the presence of dissolved CO 2 . The vertical profiles of temperature, pH, dissolved O 2 and anionic and cationic species (HCO 3 , Ca, SO 4 , NH 4) indicate a clear separation between epilimnion, where oxygen is available as dissolved phase, and hypolimnion, dominated by reducing conditions. This is also confirmed by the vertical distribution of concentrations and isotopic characteristics of CO 2 and CH 4 . The total gas pressure (P tot) of dissolved gases is lower than the hydrostatic pressure, indicating that a gas release able to produce "limnic eruption", like those occurred at the Cameroonian Lakes of Monoun and Nyos in 1984 and 1986, respectively, has to be considered unlike. The δ 13 C CO2 values are consistent with a deep-seated source of dissolved CO 2 that is likely produced by i) thermometamorphism of carbonate formations and ii) mantle degassing. The δ 13 C CH4 values are consistent with a biogenic source of CH 4 , being similar to those typically related to carbonate reduction by bacterial activity. The distribution along the lake vertical profile of the isotopic composition of two main gaseous carbon species can be related to: 1) CO 2 -CH 4 isotopic exchange; 2) reduction of CO 2 to CH 4 at reducing conditions; 3) oxidation of CH 4 to CO 2 at oxidizing conditions; 4) direct CH 4 and CO 2 production by bacterial activity. In conclusion, these results have shown that, although the morphometric features (water volumes of Monticchio Grande, Monticchio Piccolo and Albano lakes are 3.3 x10 6 , 4 x10 6 and 450 x10 6 m 3 , respectively) and the relatively low gas concentrations (max 19.4 mmol/L at a depth of 38 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 promising tools to evaluate the recharge rate of CO 2 -rich fluids into these lakes. The calculated value of δ 13 C DIC can also be considered a useful parameter to trace the processes regulating the carbon cycle within the lake waters.