Origins of methane discharging from volcanic-hydrothermal, geothermal and cold emissions in Italy

We have addressed the origins of methane in fumarolic, thermal and cold emissions discharging from a diversity of geological settings spread all over the Italian Peninsula and at Panarea and Pantelleria islands. Fluid discharges from the main geothermal systems of the peri-Tyrrhenian area (including Mt. Amiata, Larderello, Latera and Manziana) show delta C-13-CH4 and delta D-CH4 values >-28 parts per thousand. V-PDB and >-160 parts per thousand. V-SMOW, respectively, and C-1/C2+ concentration ratios >1000. These patterns suggest that CH4 discharged from these systems is primarily generated by abiogenic reduction of CO2 and/or CO. Methane seems to have, at least partially, a different origin than the higher hydrocarbons that might be entirely related to the thermal decomposition of organic matter or, alternatively, to methane polymerization. Fumaroles from the volcanic systems of Solfatara, Vesuvio, Panarea and Pantelleria have C-1/C2+ concentration ratios in a wide range (from 17 to 4300), but delta C-13-CH4 and delta D-CH4 values >-21 parts per thousand V-PDB and >-120 parts per thousand V-SMOW, respectively. Such isotopic values are very similar to those reported for CH4 discharging from ultramafic hydrothermal systems. In agreement with recent evidence provided for Solfatara and Pantelleria we, therefore, infer that the CH4 in the volcanic fumaroles is mainly abiogenic, too. The delta C-13-CH4 (from -21.6 to -36.9 parts per thousand V-PDB) and delta D-CH4 (from -113 to - 164 parts per thousand V-SMOW) values, as well as the C-1/C2+ concentration ratios (from 19 to 2240), in the CO2-rich fluid discharges located between the geothermal-volcanic systems and the Apennine sedimentary chain tend to decrease eastwards as the temperatures of the fluid reservoirs progressively diminish, suggesting that the CH4 production is mainly related to thermogenic processes. According to this general trend, the CH4-dominated fluids from mud volcanoes located eastwards of the CO2-rich emissions are fed by an almost pure thermogenic source. The easternmost fluid discharges from the Po valley and the Adriatic coast, where organic matter evolves at relatively shallow depths in the presence of a low geothermal gradient, record the lowest delta C-13-CH4 and delta D-CH4 values, i.e. as low as - 69 parts per thousand. V-PDB and <-190 parts per thousand. V-SMOW, respectively, and C-1/C2+ concentration ratios up to 5500, suggesting that the CH4 production is mainly from microbially driven processes. (C) 2012 Elsevier B.V. All rights reserved.