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The carbon isotopic composition of dissolved C-bearing species is a powerful tool to discriminate the origin of carbon in thermal waters from volcanic and hydrothermal systems. However, the δ 13 C values of dissolved CO 2 and TDIC (Total Dissolved Inorganic Carbon) are often different with respect to the isotopic signature that characterizes the potential carbon primary sources, i.e. deep hydrothermal reservoirs, magmatic gases and organic activity. The most commonly invoked explanation for such isotopic values is related to mixing processes between deep and shallow end-members. Nevertheless, experimental and empirical investigations demonstrated that isotopic fractionation due to secondary processes acting on the uprising fluids from the hydrothermal reservoirs is able to reproduce the measured isotopic values. In this paper, we investigated the chemistry of thermal waters, collected at Campi Flegrei and Vulcano Island (southern Italy), whose origin is related to interaction processes among magmatic gases, meteoric water, seawater and hosting rocks. A special focus was dedicated to the δ 13 C values of dissolved CO 2 (δ 13 C CO2(aq) ) and total dissolved inorganic carbon (δ 13 C TDIC ). The δ 13 C CO2(aq) and δ 13 C TDIC values in the water samples from both these systems ranged from (i) those measured in fumarolic gases, likely directly related to the deep hydrothermal-magmatic reservoir, and (ii) those typically characterizing biogenic CO 2 , i.e. produced by microbially-driven degradation of organic matter. A simple mixing model of the two end-members, apparently explaining these intermediate carbon isotopic values, contrasts with the chemical composition of the dissolved gases. On the contrary, isotopic fractionation due to secondary processes, such as calcite precipitation, affecting hydrothermal fluids during their underground circulation, seems to exhaustively justify both the chemical and isotopic data. If not recognized, these processes, which frequently occur in volcanic and hydrothermal systems, may lead to an erroneous interpretation of the carbon source, causing an underestimation of the contribution of the hydrothermal/magmatic fluids to the dissolved carbon species. These results pose extreme caution in the interpretation of intermediate δ 13 C CO2(aq) and δ 13 C TDIC values for the assessment of the carbon budget of hydrothermal-volcanic systems. [ABSTRACT FROM AUTHOR]

Hule and Río Cuarto are maar lakes located 11 and 18 km N of Poás volcano along a 27 km long fracture zone, in the Central Volcanic Range of Costa Rica. Both lakes are characterized by a stable thermic and chemical stratification and recently they were affected by fish killing events likely related to the uprising of deep anoxic waters to the surface caused by rollover phenomena. The vertical profiles of temperature, pH, redox potential, chemical and isotopic compositions of water and dissolved gases, as well as prokaryotic diversity estimated by DNA fingerprinting and massive 16S rRNA pyrosequencing along the water column of the two lakes, have highlighted that different bio-geochemical processes occur in these meromictic lakes. Although the two lakes host different bacterial and archaeal phylogenetic groups, water and gas chemistry in both lakes is controlled by the same prokaryotic functions, especially regarding the CO2-CH4 cycle. Addition of hydrothermal CO2 through the bottom of the lakes plays a fundamental priming role in developing a stable water stratification and fuelling anoxic bacterial and archaeal populations. Methanogens and methane oxidizers as well as autotrophic and heterotrophic aerobic bacteria responsible of organic carbon recycling resulted to be stratified with depth and strictly related to the chemical-physical conditions and availability of free oxygen, affecting both the CO2 and CH4 chemical concentrations and their isotopic compositions along the water column. Hule and Río Cuarto lakes were demonstrated to contain a CO2 (CH4, N2)-rich gas reservoir mainly controlled by the interactions occurring between geosphere and biosphere. Thus, we introduced the term of bio-activity volcanic lakes to distinguish these lakes, which have analogues worldwide (e.g. Kivu: D.R.C.-Rwanda; Albano, Monticchio and Averno: Italy; Pavin: France) from volcanic lakes only characterized by geogenic CO2 reservoir such as Nyos and Monoun (Cameroon). [ABSTRACT FROM AUTHOR]

Abstract: The chemical and isotopic compositions of (i) CO2-rich fluids exploited from the ~5,000m deep Pieve Santo Stefano 1 (PSS1) borehole located in the Upper Tiber Basin (Northern Apennines, Italy) and (ii) natural gas seeps located in its surroundings, mainly in the Mt. Fungaia ophiolitic-bearing complex (Ligurian Unit), are presented and discussed. Deep seismic profiles have allowed to identify a thrust-related antiform (Caprese Antiform), a regional geological structure trapping the pressurized deep fluids. Fluids from the Caprese Reservoir (CR) consist of a CO2-, N2-rich gas phase sourced by both mantle degassing and thermal degradation of carbonate rocks and organic-rich sediments of the Umbro–Tuscan sedimentary series and a Na–Cl saline (up to 82g/L of TDS) brine. Gases naturally discharging in the study area are related to those present in the CR. These gases, during their uprising toward the surface, mix with shallow aquifers. Addition of (i) H2 and H2S, derived by the interaction with ophiolite-bearing Ligurian Units, and (ii) thermogenic hydrocarbons, originated from the degradation of organic matter contained in the turbiditic (sandstone-rich) formations (Cervarola–Falterona Unit) was also evidenced. Fluid inclusions, trapped in calcite and quartz crystals from the PSS1 drill-core at the depth of 3864 to 3867m, contain CO2–N2 and H2O–NaCl phases. The different paleo CO2–N2 contents and densities found with respect to the fluids currently exploited from the CR have been related to the evolution of the reservoir throughout the time. Compositional data of fluids exploited from PSS1 borehole provide a unique opportunity to better characterize chemical–physical processes and source regions of past and present deep fluids circulating in Northern Apennines. [Copyright &y& Elsevier]