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

Abstract

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.