GEM—Geologic Emissions of Methane, the missing source in the atmospheric methane budget

Central to any study of climate change is the
development of an inventory that identifies and quantifies natural and anthropogenic sources and sinks of greenhouse gases (GHG). Recent studies have demonstrated
that geologic emissions of methane (GEM),
although not considered in the inventories of the IntergovernmentalPanel on Climate Change (IPCC), are an important GHG source. Etiope and Klusman (2002, Chemosphere 49, 777–789) documented that significant amounts of methane, produced within the Earth crust, are released naturally into the atmosphere
through faults and fractured rocks. Major GEMs are related to hydrocarbon production in sedimentary basins (biogenic and thermogenic methane), through
continuous exhalation and eruptions from more than 1200 onshore and offshore mud volcanoes (MVs), through diffuse soil microseepage, and shallow marine
seeps; secondarily, methane is released from geothermal and volcano-magmatic systems. Minor geologic sources are those related to natural exhalation from coal-bearing
rocks (influenced by mining activities), degassing from crystalline basement and mantle. While marine seeps have been studied for decades, methane flux from MVs
has been the object of detailed measurements only since 2001, when hundreds of gas flux measurements were performed from vents and soilin the main terrestrial
MVs of Europe, in Romania and Italy (Etiope et al.,2003, Geophysical Research Letters 30, 1094, doi:10.1029/2002GL016287; and references therein). In 2003 gas flux was measured in Azerbaijan, which hosts
the world’s biggest MVs and densest MV population (Etiope et al., 2004, Geology, in press). In all areas
investigated around 102–103 tons of methane per km2 are annually injected into the atmosphere. The global
estimates of GEM from MVs range from 5 to 13Tg yr-1 (Etiope and Milkov, 2004, Environmental Geology, in press).