Gas seepage from Tokamachi mud volcanoes, onshore Niigata Basin (Japan): Origin, post-genetic alterations and CH4–CO2 fluxes

Abstract

Methane and CO2 emissions from the two most active mud volcanoes in central Japan, Murono and Kamou (Tokamachi City, Niigata Basin), were measured in from both craters or vents (macro-seepage) and invisible exhalation from the soil (mini- and microseepage). Molecular and isotopic compositions of the released gases were also determined. Gas is thermogenic (d13CCH4 from 32.9‰ to 36.2‰), likely associated with oil, and enrichments of 13C in CO2 (d13CCO2 up to +28.3‰) and propane (d13CC3H8 up to 8.6‰) suggest subsurface petroleum biodegradation. Gas source and post-genetic alteration processes did not change from 2004 to 2010. Methane flux ranged within the orders of magnitude of 101–104 gmˉ2 dˉ1 in macro-seeps, and up to 446 g mˉ2 dˉ1 from diffuse seepage. Positive CH4 fluxes from dry soil were widespread throughout the investigated areas. Total CH4 emission from Murono and Kamou were estimated to be at least 20 and 3.7 ton aˉ1, respectively, of which more than half was from invisible seepage surrounding the mud volcano vents. At the macro-seeps, CO2 fluxes were directly proportional to CH4 fluxes, and the volumetric ratios between CH4 flux and CO2 flux were similar to the compositional CH4/CO2 volume ratio. Macro-seep flux data, in addition to those of other 13 mud volcanoes, supported the hypothesis that molecular fractionation (increase of the ‘‘Bernard ratio’’ C1/(C2 + C3)) is inversely proportional to gas migration fluxes. The CH4 ‘‘emission factor’’ (total measured output divided by investigated seepage area) was similar to that derived in other mud volcanoes of the same size and activity. The updated global ‘‘emission-factor’’ data-set, now including 27 mud volcanoes from different countries, suggests that previous estimates of global CH4 emission from mud volcanoes may be significantly underestimated.