Origin of methane and light hydrocarbons in the gas manifestations of Greece.

Abstract

The geologic emissions of greenhouse gases (CO2 and CH4) give an important natural contribution to the global carbon budget. However, the contribution of these emissions to the global carbon cycle and their possible role on the climate change remain still poorly quantified (Guliyev and Feizullayev, 1997; Milkov, 2000; Etiope et al., 2015 and references therein). Methane, the most abundant organic compound in Earth's atmosphere, may be created either from existing organic matter or synthesized from inorganic molecules. Accordingly, it can be differentiated in two main classes: a) biotic (either microbial or thermogenic) and b) abiotic. For this study, 115 gas samples of fumarolic, thermal and cold discharges from all over the Hellenic territory were collected and both chemical (CO2, H2S, CH4, N2, O2, Ar, H2 and light hydrocarbons) and isotopic (13C-CO2, 13C-CH4, D-CH4) analyses were performed, in order to investigate the genetic processes that produced CH4 in fluids related with the complex geodynamic setting of Greece. On the basis of the spatial distribution of the gas discharges and their type of emission, the whole dataset was subdivided into 3 main “domains”, as follows: 1) Volcanic Arc (VA) - 34 samples; 2) External Hellenides (EH) - 23 samples of cold emissions and of hyperalkaline aqueous solutions; 3) Internal Hellenides (IH) - 62 samples of cold and geothermal emissions. Almost each group is characterized, as long as subdivided in 3 groups based on the type of emission (on-land free or dissolved gases and subaqueous gases) and a 4th group includes literature data.