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 CH 4 enriched in coastal seawaters, we determined the distribution of both the concentrations and dual stable isotope compositions (δ 13 C and δ 2 H) of dissolved CH 4 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 CH 4 were found in the surface layer of Ise Bay, and their δ 13 C and δ 2 H values were close to those of CH 4 enriched in the major inflows, but deviated from those of CH 4 in the sedimentary layer at the bottom of Ise Bay. The oxidation rates of CH 4 in the water columns were negligibly small during the incubation experiments. In conclusion, the excess CH 4 in the surface layer of Ise Bay was derived from the inflows. The CH 4 enrichment in the freshwater sediments of the inflows showing up to four orders of magnitude higher CH 4 concentrations than those in the sediments of Ise Bay supported this conclusion. Similar results were obtained in Mikawa Bay. The total emission flux of CH 4 from the estuary area of Ise Bay was larger than the influx of CH 4 into Ise Bay via the inflows, suggesting that the CH 4 dissolved in the inflows was emitted into the atmosphere immediately after inflowing into the bay water.