Proceedings of GIA


Giuseppe Etiope


New gas geochemical surveys in Europe and Asia have recently allowed to extend the worldwide gas dataset on gas origin and flux from terrestrial mud volcanoes. Molecular and isotopic composition of gas is now available for more than 140 onshore mud volcanoes from 13 countries: global statistics shows that at least 76% of mud volcanoes release a thermogenic gas, 20% mixed and only 4% microbial. Some postgenetic secondary processes may alter the original molecular and isotopic composition of the gas; however only some mixing, molecular fractionation and secondary methanogenesis related to subsurface anaerobic biodegradation seem to be significant. There is minimal evidence of isotopic fractionation due to bacterial oxidation or gas diffusion. 

Detailed gas flux measurements are available from a much lower number of mud volcanoes, from Italy, Romania, Azerbaijan and Taiwan. It was evidenced, however, that in all surveyed mud volcanoes gas is released not only from the vents (craters, gryphons or bubbling pools) but also, and in large amounts, from the mud cover and surrounding soil, through an invisible and diffuse microseepage which may range from units to thousands of mg CH4 per square meter per day. Accordingly, thorough emission factors have been assessed and elaborated to derive regional and global estimates of methane emission. The more constrained estimate suggests that terrestrial mud volcanoes globally release at least 6-9 Mtons of CH4 per year. Further research is needed in order to provide more reliable estimates of gas emissions concerning mud volcano eruptions.