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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Tracing the sources of excess methane in Ise and Mikawa bays using dual stable isotopes as tracers

Hui Lan, Urumu Tsunogai, Fumiko Nakagawa, Masanori Ito, Ryo Shingubara, Yuko Miyoshi, Shuichi Hara
Geochemical Journal, Vol. 57, No. 2, P. 59-72, 2023


To clarify the sources and fate of CH4 enriched in coastal seawaters, we determined the distribution of both the concentrations and dual stable isotope compositions (δ13C and δ2H) of dissolved CH4 in the bays of Ise and Mikawa in Japan during five sampling campaigns from 2013 to 2020, together with those in the major inflows of the Kiso, Nagara, and Yahagi Rivers. Excess CH4 were found in the surface layer of Ise Bay, and their δ13C and δ2H values were close to those of CH4 enriched in the major inflows, but deviated from those of CH4 in the sedimentary layer at the bottom of Ise Bay. The oxidation rates of CH4 in the water columns were negligibly small during the incubation experiments. In conclusion, the excess CH4 in the surface layer of Ise Bay was derived from the inflows. The CH4 enrichment in the freshwater sediments of the inflows showing up to four orders of magnitude higher CH4 concentrations than those in the sediments of Ise Bay supported this conclusion. Similar results were obtained in Mikawa Bay. The total emission flux of CH4 from the estuary area of Ise Bay was larger than the influx of CH4 into Ise Bay via the inflows, suggesting that the CH4 dissolved in the inflows was emitted into the atmosphere immediately after inflowing into the bay water.


Keywords: excess methane, stable carbon and hydrogen isotopes, emission flux, sediments, estuaries

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