Geochemistry of C-bearing gas compounds in natural fluids under crustal conditions: insights into deep and shallow processes

The PhD research project was aimed to improve the scientific knowledge of the origin and fate of C-bearing gas compounds released from active volcanoes, hydrothermal systems and tectonically active sedimentary basins. The first goal was to investigate the primary source(s) of CH4 and light hydrocarbons in volcanic-hydrothermal gases under crustal conditions. This objective was achieved by comparing the composition of low molecular weight organic fraction (C1-C4) and associated CO2 and H2O in fumarolic gases and geothermal wells from different study areas around the world. We demonstrated that these hydrocarbons derive from biotic sources, i.e., predominantly from the thermal decomposition of organic matter. Meteoric waters and seawater circulating through the crust shuttle organic matter from Earth’s surface into the reservoir rocks. There, high temperature pyrolysis of organic matter and open system degassing generates n-alkanes with isotopic compositions previously classified as being indicative for abiogenesis. These results led us to question the dogma of crustal production of abiotic hydrocarbons and highlighted the potential of n-alkanes to become sensitive indicators of life on habitable (exo)planets. The second goal was to study the secondary processes affecting the composition of CO2, CH4 and light hydrocarbons in natural fluids during their uprising from the deep reservoirs to the surface in different geologic setting, ranging from active volcanoes to sedimentary basins. Under magmatic-hydrothermal conditions, catalytic organic reactions may strongly affect volatile organic compounds, drastically changing alkanes-alkenes-aromatics relative abundances and isotopic composition of C1–C4 hydrocarbons. At peripheral areas of volcanic systems and tectonically active sedimentary basins, composition of CO2 and CH4 in interstitial soil gases and dissolved gases in groundwater are mainly controlled by supergene mechanisms, such as calcite precipitation and microbial-driven processes. These secondary processes likely play a major role in regulating the ultimate release of C-bearing gas compounds into the atmosphere.