Dissolved CO 2 and CH 4 from sub-lacustrine fluid vents and/or bacterial-related processes commonly constitute relevant reservoirs in crater lakes hosted in non-active volcanoes. In these systems, an external perturbation (i.e. earthquakes, landslides, heavy rains) affecting the stability of deep lake strata may produce the abrupt release of a relevant amount of gases, as occurred at Monoun and Nyos lakes (Cameroon) in 1984 and 1986, respectively [Sigurdsson et al. 1987, Evans et al. 1993, 1994]. Such events, the so-called "limnic eruptions" [Kusakabe 1996 and references therein], may represent a severe hazard, especially when occurring in the proximity of densely populated areas. In central-southern Italy, four volcanic lakes, i.e. Albano (Alban Hills volcanic complex; Central Italy), Averno (Phlegrean Fields; southern Italy), Monticchio Grande and Monticchio Piccolo (Vulture volcano; southern Italy), show a marked chemical and thermal stratification and presence of non-atmospheric gas species at depth. Water rollover episodes have occasionally affected these lakes in historical times. This study investigates the chemical-physical features of water and dissolved gases along the lake vertical profiles, with a special focus on the distribution of the δ 13 C values in CO 2 and CH 4 with depth. Generally speaking, all the investigated lakes display a significant chemical stratification, mainly consisting of an increase of the HCO 3 -and Ca 2+ concentrations. The dissolved gas composition is dominated by N 2 in the oxic epilimnion, whereas CO 2 is the main gas compound in the anoxic hypolimnion. The vertical patterns of CH 4 concentrations resemble those of CO 2 , since both compounds show an increase from the surface to the bottom of 3-4 orders of magnitude. The δ 13 C-CH 4 and δ D-CH 4 values (down to -67 ‰ V-PDB and -283 V-SMOW, respectively) suggest that the bacterial activity is basically the main responsible of CH 4 production. On the contrary, the δ 13 C-CO 2 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 CO 2 , whereas, at Averno, where the δ 13 C-CO 2 values range between -13.4 and -8.2 ‰ V-PDB, CO 2 seems to be prevalently organic. 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 δ 13 C-CO 2 values are indeed progressively more positive at increasing depth, whereas an opposite trend is shown by the δ 13 C-CH 4 values. These results show 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) 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 δ 13 C-CO 2 and  δ 13 C-CH 4 values are to be considered useful monitoring tools to control the rate of fluids discharged from the lake bottoms.

Chemical and Isotopic Features of Waters and Dissolved Gases along Vertical Profiles at Albano, Averno and Monticchio Volcanic Lakes (Italy) / Franco Tassi; Orlando Vaselli; Jens Fiebig; Jacopo Cabassi; Matteo Nocentini; Antonio Delgado Huertas. - STAMPA. - (2010), pp. 0-0. (Intervento presentato al convegno Second International Workshop on RESEARCH IN SHALLOW MARINE AND FRESH WATER SYSTEMS tenutosi a Milazzo).

Chemical and Isotopic Features of Waters and Dissolved Gases along Vertical Profiles at Albano, Averno and Monticchio Volcanic Lakes (Italy)

TASSI, FRANCO;VASELLI, ORLANDO;CABASSI, JACOPO;
2010

Abstract

Dissolved CO 2 and CH 4 from sub-lacustrine fluid vents and/or bacterial-related processes commonly constitute relevant reservoirs in crater lakes hosted in non-active volcanoes. In these systems, an external perturbation (i.e. earthquakes, landslides, heavy rains) affecting the stability of deep lake strata may produce the abrupt release of a relevant amount of gases, as occurred at Monoun and Nyos lakes (Cameroon) in 1984 and 1986, respectively [Sigurdsson et al. 1987, Evans et al. 1993, 1994]. Such events, the so-called "limnic eruptions" [Kusakabe 1996 and references therein], may represent a severe hazard, especially when occurring in the proximity of densely populated areas. In central-southern Italy, four volcanic lakes, i.e. Albano (Alban Hills volcanic complex; Central Italy), Averno (Phlegrean Fields; southern Italy), Monticchio Grande and Monticchio Piccolo (Vulture volcano; southern Italy), show a marked chemical and thermal stratification and presence of non-atmospheric gas species at depth. Water rollover episodes have occasionally affected these lakes in historical times. This study investigates the chemical-physical features of water and dissolved gases along the lake vertical profiles, with a special focus on the distribution of the δ 13 C values in CO 2 and CH 4 with depth. Generally speaking, all the investigated lakes display a significant chemical stratification, mainly consisting of an increase of the HCO 3 -and Ca 2+ concentrations. The dissolved gas composition is dominated by N 2 in the oxic epilimnion, whereas CO 2 is the main gas compound in the anoxic hypolimnion. The vertical patterns of CH 4 concentrations resemble those of CO 2 , since both compounds show an increase from the surface to the bottom of 3-4 orders of magnitude. The δ 13 C-CH 4 and δ D-CH 4 values (down to -67 ‰ V-PDB and -283 V-SMOW, respectively) suggest that the bacterial activity is basically the main responsible of CH 4 production. On the contrary, the δ 13 C-CO 2 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 CO 2 , whereas, at Averno, where the δ 13 C-CO 2 values range between -13.4 and -8.2 ‰ V-PDB, CO 2 seems to be prevalently organic. 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 δ 13 C-CO 2 values are indeed progressively more positive at increasing depth, whereas an opposite trend is shown by the δ 13 C-CH 4 values. These results show 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) 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 δ 13 C-CO 2 and  δ 13 C-CH 4 values are to be considered useful monitoring tools to control the rate of fluids discharged from the lake bottoms.
2010
Abstracts Book Second International Workshop on RESEARCH IN SHALLOW MARINE AND FRESH WATER SYSTEMS
Second International Workshop on RESEARCH IN SHALLOW MARINE AND FRESH WATER SYSTEMS
Milazzo
Franco Tassi; Orlando Vaselli; Jens Fiebig; Jacopo Cabassi; Matteo Nocentini; Antonio Delgado Huertas
File in questo prodotto:
Non ci sono file associati a questo prodotto.

I documenti in FLORE sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificatore per citare o creare un link a questa risorsa: https://hdl.handle.net/2158/955151
Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact