Verifying Estimates of the Amount of Methane Carried by a Methane Plume in the Joetsu Basin, Eastern Margin of the Sea of Japan

An initial estimate of the amount of methane carried by a single methane plume was calculated to be 4 x 10(9) g(4,000 ton CH4) to 2 x 10(9) g (2,000 ton CH4) per year (Aoyama and Matsumoto, 2009), based on quantitative echo sounder measurements of the methane plume and bubble capture and release experiments. The estimate generated considerable interest because it suggested the potential importance of plumes as natural gas resources. However, a critical mistake in the calculations was found in converting mole amounts to weight of methane. Revised and corrected estimates of annual methane transported by a single plume are between 2.63 x 10(6) g (2.63 ton CH4) to 1.60 x 10(6) g (1.60 ton CH4), which are only 0.07% to 0.08% of the original estimates. For comparison, the revised amount of methane discharged from an individual methane seep is estimated based on direct measurements of gas bubbles from seep sites at Joetsu Knoll and Umitaka Spur, Joetsu basin. A total of 200 ml to 1,150 ml of bubbles were captured within 642 to 481 seconds. Total gas flux depends on the composition of the bubbles. Assuming pure gas, the annual discharge is estimated to be 0.71 ton to 4.84 ton CH4. If the bubbles consist of pure hydrate, the seepage is slightly higher at 1.15 ton to 8.83 ton CH4 per year.