Methane flux of miniseepage in mud volcanoes of SW Taiwan: Comparison with the data from Europe and Azerbaijan

This is an author's peer-reviewed final manuscript draft, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/journal-of-asian-earth-sciences/.Mud volcanoes (MVs) are considered as important methane (CH₄) sources for the\ud atmosphere; gas is not only released from macroseepage, i.e., from craters and visible gas bubbling\ud manifestations, but also from invisible and pervasive exhalation from the ground, named miniseepage.\ud CH₄ flux related to miniseepage was measured only in a few MVs, in Azerbaijan, Italy, Japan, Romania\ud and Taiwan.\ud This study examines in detail the flux data acquired in 5 MVs and 1 "dry" seep in SW Taiwan, and\ud further compares with other 23 MVs in Italy, Romania and Azerbaijan. Miniseepage in SW Taiwan MVs\ud and seeps annually contribute at least 110 tons of methane directly to the atmosphere, and represents\ud about ~80 % of total degassing during a quiescent period. Combining miniseepage flux and geoelectrical\ud data from the Wu-shan-ding MV revealed a possible link between gas flux and electrical\ud resistivity of the vadose zone. This suggests that unsaturated subsoil is a preferential zone for shallow\ud gas accumulation and seepage to the atmosphere. Besides, miniseepage flux in Chu-huo everlasting\ud decreases by increasing the distance from the main gas channelling [channeling] zone and molecular fractionation\ud (methane/ethane ratio) is higher for lower flux seepage, consistently with what observed in other MVs\ud worldwide. Measurements from Azerbaijan, Italy, Romania, and Taiwan converge to indicate that\ud miniseepage is directly proportional to the vent output and it is a significant component of the total\ud methane budget of a MV. A miniseepage vs. macroseepage flux equation has been statistically assessed\ud and it can be used to estimate theoretically the flux of miniseepage for MVs of which only the flux from\ud vents was evaluated, or will be evaluated in future, in case miniseepage detection systems were not\ud available. This will allow a more complete and objective quantification of gas emission in MVs, thus\ud also refining the estimation of global methane emission