Biogeochemical processes involving dissolved CO and CH at Albano, Averno, and Monticchio meromictic volcanic lakes (Central-Southern Italy).

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Title:	Biogeochemical processes involving dissolved CO and CH at Albano, Averno, and Monticchio meromictic volcanic lakes (Central-Southern Italy).
Authors:	Cabassi, Jacopo¹ jacopo.cabassi@unifi.it, Tassi, Franco franco.tassi@unifi.it, Vaselli, Orlando orlando.vaselli@unifi.it, Fiebig, Jens² Jens.Fiebig@em.uni-frankfurt.de, Nocentini, Matteo matteo.nocentini@unifi.it, Capecchiacci, Francesco³ francesco.capecchiacci@unifi.it, Rouwet, Dmitri⁴ dmitri.rouwet@bo.ingv.it, Bicocchi, Gabriele¹ gabriele.bicocchi@unifi.it
Source:	Bulletin of Volcanology. Jan2013, Vol. 75 Issue 1, p1-19. 19p.
Subject Terms:	Crater lakes, Evaporation (Meteorology), Heat transfer, Magmas, *Calderas
Geographic Terms:	Italy
Abstract:	This paper focuses on the chemical and isotopic features of dissolved gases (CH and CO) from four meromictic lakes hosted in volcanic systems of Central-Southern Italy: Lake Albano (Alban Hills), Lake Averno (Phlegrean Fields), and Monticchio Grande and Piccolo lakes (Mt. Vulture). Deep waters in these lakes are characterized by the presence of a significant reservoir of extra-atmospheric dissolved gases mainly consisting of CH and CO. The δC-CH and δD-CH values of dissolved gas samples from the maximum depths of the investigated lakes (from −66.8 to −55.6 ‰ V-PDB and from −279 to −195 ‰ V-SMOW, respectively) suggest that CH is mainly produced by microbial activity. The δC-CO values of Lake Grande, Lake Piccolo, and Lake Albano (ranging from −5.8 to −0.4 ‰ V-PDB) indicate a significant CO contribution from sublacustrine vents originating from (1) mantle degassing and (2) thermometamorphic reactions involving limestone, i.e., the same CO source feeding the regional thermal and cold CO-rich fluid emissions. In contrast, the relatively low δC-CO values (from −13.4 to −8.2 ‰ V-PDB) of Lake Averno indicate a prevalent organic CO. Chemical and isotopic compositions of dissolved CO and CH at different depths are mainly depending on (1) CO inputs from external sources (hydrothermal and/or anthropogenic); (2) CO-CH isotopic exchange; and (3) methanogenic and methanotrophic activity. In the epilimnion, vertical water mixing, free oxygen availability, and photosynthesis cause the dramatic decrease of both CO and CH concentrations. In the hypolimnion, where the δC-CO values progressively increase with depth and the δC-CH values show an opposite trend, biogenic CO production from CH using different electron donor species, such as sulfate, tend to counteract the methanogenesis process whose efficiency achieves its climax at the water-bottom sediment interface. Theoretical values, calculated on the basis of δC-CO values, and measured δC values are not consistent, indicating that CO and the main carbon-bearing ion species (HCO) are not in isotopic equilibrium, likely due to the fast kinetics of biochemical processes involving both CO and CH. This study demonstrates that the vertical patterns of the CO/CH ratio and of δC-CO and δC-CH are to be regarded as promising tools to detect perturbations, related to different causes, such as changes in the CO input from sublacustrine springs, that may affect aerobic and anaerobic layers of meromictic volcanic lakes. [ABSTRACT FROM AUTHOR]
Database:	Energy & Power Source

Description
Abstract:	This paper focuses on the chemical and isotopic features of dissolved gases (CH and CO) from four meromictic lakes hosted in volcanic systems of Central-Southern Italy: Lake Albano (Alban Hills), Lake Averno (Phlegrean Fields), and Monticchio Grande and Piccolo lakes (Mt. Vulture). Deep waters in these lakes are characterized by the presence of a significant reservoir of extra-atmospheric dissolved gases mainly consisting of CH and CO. The δC-CH and δD-CH values of dissolved gas samples from the maximum depths of the investigated lakes (from −66.8 to −55.6 ‰ V-PDB and from −279 to −195 ‰ V-SMOW, respectively) suggest that CH is mainly produced by microbial activity. The δC-CO values of Lake Grande, Lake Piccolo, and Lake Albano (ranging from −5.8 to −0.4 ‰ V-PDB) indicate a significant CO contribution from sublacustrine vents originating from (1) mantle degassing and (2) thermometamorphic reactions involving limestone, i.e., the same CO source feeding the regional thermal and cold CO-rich fluid emissions. In contrast, the relatively low δC-CO values (from −13.4 to −8.2 ‰ V-PDB) of Lake Averno indicate a prevalent organic CO. Chemical and isotopic compositions of dissolved CO and CH at different depths are mainly depending on (1) CO inputs from external sources (hydrothermal and/or anthropogenic); (2) CO-CH isotopic exchange; and (3) methanogenic and methanotrophic activity. In the epilimnion, vertical water mixing, free oxygen availability, and photosynthesis cause the dramatic decrease of both CO and CH concentrations. In the hypolimnion, where the δC-CO values progressively increase with depth and the δC-CH values show an opposite trend, biogenic CO production from CH using different electron donor species, such as sulfate, tend to counteract the methanogenesis process whose efficiency achieves its climax at the water-bottom sediment interface. Theoretical values, calculated on the basis of δC-CO values, and measured δC values are not consistent, indicating that CO and the main carbon-bearing ion species (HCO) are not in isotopic equilibrium, likely due to the fast kinetics of biochemical processes involving both CO and CH. This study demonstrates that the vertical patterns of the CO/CH ratio and of δC-CO and δC-CH are to be regarded as promising tools to detect perturbations, related to different causes, such as changes in the CO input from sublacustrine springs, that may affect aerobic and anaerobic layers of meromictic volcanic lakes. [ABSTRACT FROM AUTHOR]
ISSN:	02588900
DOI:	10.1007/s00445-012-0683-0