5:15 PM - 7:15 PM
[MIS22-P04] Simultaneous quantification on the emission flux and aerobic oxidation rate of methane in a crater lake by using the stable isotopes as tracers
Keywords:Hydrothermal, Stable Isotope, Methane Oxidation
Many living organisms on Earth use sunlight as the primary energy source. In environments where sunlight is not available, such as deep-water column or crust, however, we can find ecosystems consist of living organisms that use such CH4 as their energy source. While these ecosystems have played significant roles in life evolution and carbon cycle on earth, we have little knowledge on the scale. In this study, we quantified the scale of CH4-dependent ecosystem in a crater lake, Lake Towada in Japan (maximum depth: 300 m), where significant amount of CH4 is supplied from the bottom of the lake through volcanic activity. Such ecosystem that depends on CH4 supplied through volcanic activity may retain primitive characteristics. In addition, long residence time of water in the lake enables us to estimate accurate emission flux of CH4 from its bottom.
The water samplings were done in June and October 2024. Vertical distributions of water temperature and oxygen concentration were quantified using a CTD profiler. Water samples were collected from each layer using a Niskin sampler, and subsamples to measure CH4 were divided into glass vials. Then HgCl2 was added to each vial for sterilization to determine in situ concentration and isotopic composition of CH4 . In addition, samples for the incubation experiments to determine both CH4 oxidation rate and the isotope fractionation coefficient α were sampled and sealed in grass vial without adding HgCl2 for 1-3 days under the environment simulating in situ environment of sampling. The concentration and isotope ratios (δ13C, δ2H) of CH4 were determined using a continuous flow isotope ratio mass spectrometer.
The incubation experiments clarified that CH4 oxidation was active in the deep-water layer. In addition, the carbon isotope fractionation factor α associated with the microbial CH4 oxidation was 1.020±0.002 in the water column. Using the α, as well as CH4 concentration and δ13C at each depth, we estimated the “original” CH4 concentration (C0) from which removal of CH4 due to oxidation in the water column had been corrected. Then, the total methane quantity of CH4 in the deep water column (Ntotal) was estimated by using C0 at each depth. The Ntotal was 1.8x106 molCH4 in June and was 2.2x106 molCH4 in October. On the other hand, the Ntotal calculated from the in situ CH4 concentration were 0.4x106 molCH4 in June and 0.22x106 molCH4 in October, so the CH4 oxidation rate during the observation interval was estimated to be 8.4x103 molO2/day. On the other hand, total oxygen consumption rates in the deep-water layer, calculated from the changes in oxygen concentration in June and October, was 5.6x104 molO2/day. As a result, we estimated that the CH4 oxidation was responsible for 15% of the total oxygen consumption in the deep-water layer. Because the oxidations of organic carbon and nitrogen, that had been produced in the CH4-dependent ecosystems, were not included in the value, the actual oxygen consumption by the CH4-dependent ecosystems should be more than 15% in the deep-water layer. We concluded that CH4 oxidation played significant roles as the energy source for the ecosystems in the deep-water layer. In addition, we also found that most of methane emitted from the bottom was consumed in the lake water column. That is, the scale of the CH4-dependent ecosystems was controlled by the volcanic activity of the Towada volcano. Based the temporal changes in the temperature of the deep water layer from June to October, we estimated the heat flux to be 64 MW in the lake at present.
The water samplings were done in June and October 2024. Vertical distributions of water temperature and oxygen concentration were quantified using a CTD profiler. Water samples were collected from each layer using a Niskin sampler, and subsamples to measure CH4 were divided into glass vials. Then HgCl2 was added to each vial for sterilization to determine in situ concentration and isotopic composition of CH4 . In addition, samples for the incubation experiments to determine both CH4 oxidation rate and the isotope fractionation coefficient α were sampled and sealed in grass vial without adding HgCl2 for 1-3 days under the environment simulating in situ environment of sampling. The concentration and isotope ratios (δ13C, δ2H) of CH4 were determined using a continuous flow isotope ratio mass spectrometer.
The incubation experiments clarified that CH4 oxidation was active in the deep-water layer. In addition, the carbon isotope fractionation factor α associated with the microbial CH4 oxidation was 1.020±0.002 in the water column. Using the α, as well as CH4 concentration and δ13C at each depth, we estimated the “original” CH4 concentration (C0) from which removal of CH4 due to oxidation in the water column had been corrected. Then, the total methane quantity of CH4 in the deep water column (Ntotal) was estimated by using C0 at each depth. The Ntotal was 1.8x106 molCH4 in June and was 2.2x106 molCH4 in October. On the other hand, the Ntotal calculated from the in situ CH4 concentration were 0.4x106 molCH4 in June and 0.22x106 molCH4 in October, so the CH4 oxidation rate during the observation interval was estimated to be 8.4x103 molO2/day. On the other hand, total oxygen consumption rates in the deep-water layer, calculated from the changes in oxygen concentration in June and October, was 5.6x104 molO2/day. As a result, we estimated that the CH4 oxidation was responsible for 15% of the total oxygen consumption in the deep-water layer. Because the oxidations of organic carbon and nitrogen, that had been produced in the CH4-dependent ecosystems, were not included in the value, the actual oxygen consumption by the CH4-dependent ecosystems should be more than 15% in the deep-water layer. We concluded that CH4 oxidation played significant roles as the energy source for the ecosystems in the deep-water layer. In addition, we also found that most of methane emitted from the bottom was consumed in the lake water column. That is, the scale of the CH4-dependent ecosystems was controlled by the volcanic activity of the Towada volcano. Based the temporal changes in the temperature of the deep water layer from June to October, we estimated the heat flux to be 64 MW in the lake at present.