[ACG59-01] Tracing sources and behaviors of excess methane in the Ise Bay water by using stable isotopes as tracers
Keywords:coastal marine areas, methane, material circulation, biochemistry
In this study, we measured the concentration of dissolved CH4 and stable isotope ratios (δ13C and δD) of the water column, the interstitial water of sediments in the Ise Bay and Mikawa Bay and the inflowing rivers (Kiso, Nagara and Yahagi River). Using the stable isotope ratio as an indicator, we tried to clarify the origins of CH4 and its behaviors in the aquatic environments. Sampling was performed in each of 2012, 2013, 2016, 2017 and 2019.
As a result, the maximum concentration of CH4 (60 to 299 nmol/kg) was observed in the surface water near the estuaries of the main inflow rivers in the Ise Bay, and the supersaturation rate could reach 2700% to 13000%. The isotope ratio of CH4 (δ13C: -60 to -56 ‰, δD: -215 to -190 ‰) in the surface water of Ise Bay is almost as the same as the isotope ratio of CH4 (δ13C: -57 ‰, δD: -206 ‰) in the inflow rivers. Therefore, the maximum concentration of CH4 which was observed in the surface layer of the Ise Bay is derived from the inflowing rivers, and the oxidation of CH4 in the water column is negligible. Consequently, the concentration of supersaturated CH4 that has flowed in from the river has decreased due to either dilution or release to the atmosphere. Since the CH4 emission flux to the atmosphere of the Ise Bay was calculated to be 3.2 to 52.3 × 103 mol / d, which is almost consistent with the CH4 inflow flux from the inflowing river to Ise Bay (4.3 to 73.4 × 103 mol / d). That is to say, the most of the CH4 flowing from the river into Ise Bay was released to the atmosphere immediately after flowing. In conclusion, it is important to clarify the origin and behavior of CH4 in inflowing river water in considering the amount of CH4 released into the atmosphere in coastal areas.