Geoekologiya, 2020, Vol. 3, P. 23-35


V. V. Ershova,# and D. D. Bondarenkoa,##

a Institute of Marine Geology and Geophysics, Far East Branch, Russian Academy of Sciences,
ul. Nauki, 1B, Yuzhno-Sakhalinsk, 693022 Russia

#E-mail: valery_ershov@mail.ru,

In this paper, we have generalized and analyzed the data on isotopic and chemical composition of gases eject­ed by terrestrial mud volcanoes (about 700 samples from more than 270 volcanoes). It is shown that for most mud volcanoes methane prevails in the composition of ejected gases - its median concentration is about 92.5vol. %. However, there are also mud volcanoes that eject mainly carbon dioxide or nitrogen. Usually ni­trogen in mud volcanic gases is present as an impurity - its median concentration is about 2 vol. %. For some gas samples (about 150) both nitrogen concentration and oxygen concentration are available. There is a strong positive linear correlation between the concentrations of these gases (r = 0.64). The angular coefficient of this linear dependence is 3.5, which is similar to the ratio of nitrogen/oxygen in atmospheric air (3.7). Therefore, we believe that nitrogen in samples of mud volcanic gases is predominantly atmogenic. The CO2/CH4 ratio in mud volcanic gases suggests that the carbon dioxide flux from mud volcanoes can be 1.1-10.5 million ton per year. We estimate that mud volcanic reservoirs contain about 10 billion tons of carbon (as CO2 and CH4). Based on the isotopic composition of carbon, methane in most cases (about 92% of all gas samples) is ther­mogenic. Isotopic data also suggest that some mud volcanoes (about 10% of all gas samples) may release car­bon dioxide from the upper mantle. More than a third of all gas samples contain carbon dioxide, which is formed during anaerobic biodegradation of petroleum. It was found that the isotopic and chemical composi­tion of mud volcanic gases vary among different regions of the world. This may be related to different tectonic setting. In regions, belonging to subduction zones and manifestations of modern activity of magmatic volca­noes, methane in mud volcanic gases, as a rule, has heavier isotopic composition (§13C-CH4 values range from -35 to -25%o PDB). Here are also mud volcanoes that release mantle or metamorphic carbon dioxide. This is accompanied by a high concentration of carbon dioxide in the mud volcanic gases. It is shown that in terms of the carbon isotopic composition, methane from mud volcanoes is largely identical to anthropogenic methane released into the atmosphere during the extraction of fossil fuels. This contributes to great uncer­tainty for estimates of anthropogenic emissions in the analysis of the global methane budget.

Keywords: mud volcanoes, methane, carbon dioxide, gas geochemistry, stable carbon isotopes, origin of mud vol­canic gases, geographical distribution

DOI: 10.31857/S0869780920030029


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