Biogeochemical processes involving dissolved CO sub(2) and CH sub(4) at Albano, Averno, and Monticchio meromictic volcanic lakes (Central-Southern Italy)

• Published in: Bulletin of volcanology Vol. 75; no. 1; pp. 1 - 19
• Main Authors: Cabassi, Jacopo, Tassi, Franco, Vaselli, Orlando, Fiebig, Jens, Nocentini, Matteo, Capecchiacci, Francesco, Rouwet, Dmitri, Bicocchi, Gabriele
• Format: Journal Article
• Language: English
• Published: 01.01.2013
• Subjects: Italy; Italy, Albano L
• ISSN: 0258-8900; 1432-0819
• DOI: 10.1007/s00445-012-0683-0

This paper focuses on the chemical and isotopic features of dissolved gases (CH sub(4) and CO sub(2)) 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 sub(4) and CO sub(2). The delta super(13)C-CH sub(4) and delta D-CH sub(4) values of dissolved gas samples from the maximum depths of the investigated lakes (from -66.8 to -55.6ppt V-PDB and from -279 to -195ppt V-SMOW, respectively) suggest that CH sub(4) is mainly produced by microbial activity. The delta super(13)C-CO sub(2) values of Lake Grande, Lake Piccolo, and Lake Albano (ranging from -5.8 to -0.4ppt V-PDB) indicate a significant CO sub(2) contribution from sublacustrine vents originating from (1) mantle degassing and (2) thermometamorphic reactions involving limestone, i.e., the same CO sub(2) source feeding the regional thermal and cold CO sub(2)-rich fluid emissions. In contrast, the relatively low delta super(13)C-CO sub(2) values (from -13.4 to -8.2ppt V-PDB) of Lake Averno indicate a prevalent organic CO sub(2). Chemical and isotopic compositions of dissolved CO sub(2) and CH sub(4) at different depths are mainly depending on (1) CO sub(2) inputs from external sources (hydrothermal and/or anthropogenic); (2) CO sub(2)-CH sub(4) 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 sub(2) and CH sub(4) concentrations. In the hypolimnion, where the delta super(13)C-CO sub(2) values progressively increase with depth and the delta super(13)C-CH sub(4) values show an opposite trend, biogenic CO sub(2) production from CH sub(4) 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 delta super(13)C-CO sub(2) values, and measured delta super(13)C sub(TDIC) values are not consistent, indicating that CO sub(2) and the main carbon-bearing ion species (HCO sub(3) super(-)) are not in isotopic equilibrium, likely due to the fast kinetics of biochemical processes involving both CO sub(2) and CH sub(4). This study demonstrates that the vertical patterns of the CO sub(2)/CH sub(4) ratio and of delta super(13)C-CO sub(2) and delta super(13)C-CH sub(4) are to be regarded as promising tools to detect perturbations, related to different causes, such as changes in the CO sub(2) input from sublacustrine springs, that may affect aerobic and anaerobic layers of meromictic volcanic lakes.