Carbon degassing from the Solid Earth: light hydrocarbons and CO2 emissions into the atmosphere

Dr. Giuseppe Etiope

Istituto Nazionale di Geofisica e Vulcanologia

One of the fundamental questions related to Earth’s subsurface deep carbon reservoirs and fluxes regards the role played by solid Earth’s degassing on climate and environment. Research carried out in the last ten years provided the first estimates of global emissions into the atmosphere of endogenous CO2 (Morner and Etiope, 2002) and CH4 (e.g., Etiope et al., 2008), which are the main greenhouse gases produced in the Earth interior. The amount of carbon dioxide released into the atmosphere from volcanic and non-volcanic areas was conservatively estimated to be at least 600 Tg/y. This value is two and three orders of magnitude lower than the global CO2 amounts emitted from anthropogenic and natural biologic activity, respectively. Recent estimates of methane emissions from geologic sources are about 53 Tg/y, which is only 7 times lower than anthropogenic sources and more than 1/3 of other natural sources (i.e., about 10% of total anthropogenic methane sources). Thus, geological sources of methane, although neglected in the past, represent the second largest natural emission of this gas to the troposphere, after wetlands. They are now considered as a new class of greenhouse-gas source for the European (EEA/EMEP) and US (EPA) emission inventories. Global geo-methane emission estimates have recently been supported by top-down analysis based on the newly derived atmospheric fossil methane (radiocarbon-free) fraction of ~30% (Etiope et al., 2008). Six categories of source (from the atmospheric perspective) or pathways (from the geologic perspective) must be considered: mud volcanoes, gas seeps (independent of mud volcanism), microseepage (diffuse exhalation from soil in petroleum basins), submarine seepage, geothermal (non-volcanic) manifestations and volcanoes. Marine seepage and microseepage are the largest sources (~20 and 10-25 Tg/y, respectively) and their uncertainties need to be reduced. A global data-set of more than 200 onshore gas seeps from 17 countries shows that in about 80% of cases the gas is thermogenic (produced in deeper sedimentary rocks), less than 4% purely microbial and ~16% mixed (Etiope et al., 2009). Seeps releasing abiogenic (inorganic) methane are very rare, and most of them are related to serpentinization processes. However, the recent discovery of a gas seep in Turkey (Chimera), which release huge amounts of abiogenic methane (Hosgormez et al., 2008), could re-open the discussions on the degassing of inorganic hydrocarbons with deep-origin carbon. More recently, a robust data-set consisting of methane, ethane and propane concentration in surface gas manifestations from 238 sites throughout the world, allowed to estimate a global geologic output of ethane and propane in the order of 2-4 Tg y-1 and 1-2.4 Tg y-1, respectively (Etiope and Ciccioli, 2009), i.e., Earth’s degassing accounts for at least 17% and 10% of total ethane and propane emissions. Therefore, in relation to the impact on the atmosphere and climate, geologic seepage of light hydrocarbons is more important than CO2 degassing, not only for the greenhouse gas budget but also for emission of ozone precursors and photochemical pollutants. The atmospheric budget of hydrocarbons is not independent of geophysical processes of the solid Earth and planet degassing.

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Hosgormez H., Etiope G., Yalçın M.N. (2008). Geofluids, 8, 263-275.
Morner N.A., Etiope G. (2002). Global and Planet. Change., 33, 1-2, 185-203.