THE ITALIAN MEROMICTIC VOLCANIC LAKES OF MONTICCHIO (MT. VULTURE) AND ALBANO (COLLI ALBANI) J. Cabassi 1 , F. Tassi 1,2 , O. Vaselli 1,2 , J. Fiebig 3 , A. Delgado Huertas 4 1 Department of Earth Sciences, Via G. La Pira, 4, 50121 Florence, Italy 2 CNR – Institute Geosciences and Earth Resources, Via G. La Pira, 4, 50121 Florence, Italy 3 Institut für Geowissenschaften, Goethe-Universität, Altenhöferallee 1, 60438 Frankfurt am Main, Germany 4 CSIC – Estacion Experimental de Zaidin, Prof. Albareda 1, 18008 Granada, Spain 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) (Fig. 1) volcanic lakes are the only meromictic volcanic lakes in Italy, being characterized by a stable chemical and thermal stratification and the 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 . Water and gas sampling at depth was carried out through 25 m long Rilsan tubes connected by steel valves. Water was collected every 10 m and 5 m for Albano and Monticchio Lakes, respectively, by means of a 250 ml syringe after rinsing the upper water column. Gas samples were collected by exsolving the gas-phase into 100 mL pre-evacuated gas flasks (Fig. 2). Water geochemistry is dominated by Na-HCO 3 (Fig. 3) due to both interaction of lake water with volcanic rocks and the presence of dissolved CO 2 . The vertical profiles of temperature, pH, dissolved O 2 (Fig. 4) and anionic and cationic species (HCO 3 , Ca, SO 4 , NH 4 ) (Fig. 5) 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 (Fig. 6). The total gas pressure of dissolved gases is lower than the hydrostatic pressure (Fig. 7), indicating that a gas release able to produce “limnic eruptions”, 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 (Fig. 6) 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 according to 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) a serious hazard for limnic eruptions can presently be ruled out for these lakes. Nevertheless, the vertical patterns of the CO 2 /CH 4 ratio and the δ 13 C-CO 2 and δ 13 C-CH 4 values may represent promising tools to evaluate the recharge rate of CO 2 -rich fluids into these lakes. Moreover, the comparison between the measured and calculated values of δ 13 C DIC , based on the values of δ 13 C-CO 2 and concentration of CO 2 and HCO 3 (Fig. 8), shows that the latter suffer more dramatically the processes occurring in the lake system. The calculated value of δ 13 C DIC can therefore be considered a useful tool to trace the processes regulating the carbon cycle in the lake waters. -60 -55 -50 -7 -6 -5 -4 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 Depth (m) ‰ V-PDB  13 C-CO 2  13 C-CH 4 ALBANO (CH 4 ->CO 2 ) oxidation reduction (CO 2 ->CH 4 ) -60 -55 -50 -10 -8 -6 -4 -2 0 -40 -35 -30 -25 -20 -15 -10 -5 0 Depth (m) ‰ V-PDB  13 C-CO 2  13 C-CH 4 MONTICCHIO GRANDE -60 -55 -50 -10 -8 -6 -4 -2 0 ‰ V-PDB  13 C-CO 2  13 C-CH 4 MONTICCHIO PICCOLO (CH 4 ->CO 2 ) oxidation reduction (CO 2 ->CH 4 ) 180 160 140 120 100 80 60 40 20 0 -6 -4 -2 0 2 4 ‰ V-PDB Depth (m)  13 C CO2  13 C (DIC measured)  13 C (DIC calculated) ALBANO -6 -4 -2 0 2 4 ‰ V-PDB  13 C CO2  13 C (DIC measured)  13 C (DIC calculated) MONTICCHIO PICCOLO 40 35 30 25 20 15 10 5 0 -6 -4 -2 0 2 ‰ V-PDB  13 C CO2  13 C (DIC measured)  13 C (DIC calculated) MONTICCHIO GRANDE Depth (m) Monticchio Grande Monticchio Piccolo Albano Figure 1 Water 10 50 100 Dissolved gas Battery Syringe 25m 50m 75m 100m 125m 170m 150m 1m Rilsan tube 6 mm Thorion valve Vacuum flask Eco-sonar: reconstruction of bathymetry pHmeter: pH determination Figure 2 0.0 0.5 1.0 1.5 2.0 2.5 0 2 4 6 8 10 12 14 16 ALBANO MONTICCHIO GRANDE MONTICCHIO PICCOLO HCO 3 (meq) Ca (meq) Depth 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 2 4 6 8 10 12 14 16 ALBANO MONT. GRANDE MONT. PICCOLO CO 2 0.04 % (15 °C) CO 2 1 % (25 °C) HCO - 3 (meq) pH Depth 180 160 140 120 100 80 60 40 20 0 Temperature (°C) Depth (m) ALBANO 8 10 12 14 16 18 20 22 24 26 28 T °C 180 160 140 120 100 80 60 40 20 0 pH Depth (m) 6.8 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 ALBANO pH 40 35 30 25 20 15 10 5 0 Depth (m) pH MONTICCHIO PICCOLO MONTICCHIO GRANDE 6.0 6.5 7.0 7.5 8.0 8.5 pH 40 35 30 25 20 15 10 5 0 40 35 30 25 20 15 10 5 0 MONTICCHIO GRANDE Temperature (°C) Depth (m) 10 12 14 16 18 20 22 24 MONTICCHIO PICCOLO 10 12 14 16 18 20 22 24 T °C 40 35 30 25 20 15 10 5 O 2 (mol/l) Depth (m) MONTICCHIO PICCOLO MONTICCHIO GRANDE 0 50 100 150 200 250 O 2 epilimnion hypolimnion 180 160 140 120 100 80 60 40 20 0 O 2 (mol/l) Depth (m) 0 20 40 60 80 100 ALBANO O 2 epilimnion metalimnion hypolimnion Figure 3 Figure 4 160 140 120 100 80 60 40 20 0 0 1 2 3 4 5 6 7 Concentration (mg/l) Depth (m) SO 2- 4 HS - ALBANO 160 140 120 100 80 60 40 20 0 ALBANO Concentration (mg/l) Depth (m) Ca Mg Na K 15 20 25 30 35 40 45 50 55 60 160 140 120 100 80 60 40 20 0 HCO - 3 Depth (m) 240 260 280 300 320 340 ALBANO Concentration (mg/l) MONTICCHIO PICCOLO Concentration (mg/l) Ca Mg Na K 5 10 15 20 25 30 35 40 45 40 35 30 25 20 15 10 5 0 MONTICCHIO GRANDE Concentration (mg/l) Depth (m) 5 10 15 20 25 30 35 40 0 2 4 6 8 10 12 14 16 18 20 HS (GRANDE) HS (PICCOLO) SO 4 (PICCOLO) SO 2- 4 HS - Concentration (mg/l) SO 4 (GRANDE) HCO - 3 MONTICCHIO PICCOLO HCO - 3 MONTICCHIO GRANDE Concentration (mg/l) 200 400 600 800 1000 -40 -35 -30 -25 -20 -15 -10 -5 0 0.01 0.1 1 Concentration (mg/l) Depth (m) MONTICCHIO GRANDE P NO - 3 NH + 4 -40 -35 -30 -25 -20 -15 -10 -5 0 0.01 0.1 1 10 100 P NO - 3 NH + 4 Concentration (mg/l) Depth (m) MONTICCHIO PICCOLO -160 -140 -120 -100 -80 -60 -40 -20 0 Depth (m) ALBANO P NO - 3 NH + 4 0.01 0.1 1 Concentration (mg/l) Figure 5 Figure 6 Figure 8 180 160 140 120 100 80 60 40 20 0 Depth (m) 1E-4 1E-3 0.01 0.1 1 ALBANO O 2 O 2 Ar Ar N 2 N 2 CH 4 CH 4 CO 2 CO 2 Partial Pression (atm) Pression (atm) Hydrostatic Pression Total Gas Pression 0 2 4 6 8 10 12 14 16 18 40 35 30 25 20 15 10 5 Depth (m) 1E-3 0.01 0.1 1 10 MONTICCHIO PICCOLO N 2 N 2 O 2 O 2 Ar Ar CO 2 CO 2 CH 4 CH 4 Partial Pression (atm) 1E-4 1E-3 0.01 0.1 1 CH 4 CH 4 Partial Pression (atm) MONTICCHIO GRANDE N 2 N 2 O 2 O 2 Ar Ar CO 2 CO 2 1 2 3 4 5 Hydrostatic Pression Total Gas Pression Monticchio Piccolo Pression (atm) Total Gas Pression Monticchio Grande 1 2 3 4 5 Figure 7 View publication stats View publication stats