Organic and inorganic geochemistry of low temperature gas discharges at the Baia di Levante beach, Vulcano Island, Italy

Discharge from subaereal and submarine gas vents of the Baia di Levante beach gases from the Vulcano Island were sampled for major and trace gas components in May and November 1995. Chemical compositions and equilibrium calculations suggest three different groups of CO sub(2)-rich gas emissions depending on their distance from the La Fossa crater: (1) gas vents close to the Faraglione area are characterised by high H sub(2)S contents, high calculated equilibrium temperatures based on inorganic species and relatively high proportion of alkene compounds; (2) gas vents close to Vulcanello are characterised by low calculated equilibrium temperatures and low amounts of alkenes; and (3) Pontile sample has the highest equilibrium CO sub(2) pressure (up to 68 bars) which may account for the observed absence of benzene. The relative large variability of H sub(2)S in the Baia di Levante beach gas discharge may be attributed to either different interactions between iron sulphides and weakly acid waters or catalytic effect of elemental sulphur on the de-hydrogenation of cyclo-hexane. Thermodynamic calculations suggest that the main inorganic species and CH sub(4) may have re-equilibrated at relatively shallow depth (10-200 m b.s.l. and 30-600 m b.s.l. for a lithostatic and hydrostatic pressure, respectively). The slow kinetics of reactions in the C sub(n)H sub(2n) /C sub(n)H sub(2n+2) systems, with respect to that of CH sub(4)-CO-CO sub(2), may explain the observed propene/propane ratios, which can only be reached at reaction temperatures of 300-350 degree C. This low speed of reactions can also explain the observed disequilibrium of C1-C4 alkanes. copyright 2001 Elsevier Science B.V. All rights reserved.