16:30 〜 16:45
[MIS02-09] 原始的生命圏を含む初期地球システムにおけるメタンフラックスの非線形的増幅機構
キーワード：太古代、大気光化学モデル、海洋微生物生態系、暗い太陽のパラドックス、系外地球型惑星
Geological records indicate that the climate of the Archean Earth was warm or even warmer than the modern Earth. On the other hand, the standard evolution model of the Sun suggests that early solar luminosity was 30~20% dimmer than the present value. The atmosphere containing high levels of greenhouse gases would be a key to explain the paradox. Methane (CH4) produced by methanogens had probably played an important role in the Archean Earth, in addition to carbon dioxide (CO2). Ozaki et al. (2018) developed a coupled atmosphere-ocean ecosystem model, and evaluated CH4 concentrations in the Archean atmosphere for two cases; one assumed ecosystem based on hydrogen (H2)-using anoxygenic photoautotrophs, and the other assumed ecosystem based on H2-using and iron (Fe)-using anoxygenic photoautotrophs, with carbon monoxide (CO)-consuming acetogens and methanogens. They found that the CH4 flux increases non-linearly in the latter case against volcanic H2 outgassing flux, which can explain the high CH4 levels in the atmosphere of the Archean Earth. The mechanism of non-linear amplification of CH4 flux, however, remains unclear.
Here, we investigate the mechanism for non-linear amplification of CH4 flux by detailed analysis with a coupled model of atmospheric photochemistry and marine ecosystem. We found that the difference between the two cases is derived from difference in redox balance at the same outgassing flux of H2. A net flux of reducing power increases with activity of Fe-using anoxygenic photoautotrophs. Two positive feedbacks for the production of H2 and CO, strongly impact on the CH4 flux and concentrations. Biogenic CH4 is converted back to H2 and CO by photolysis in the atmosphere, which are recycled by marine microbial ecosystem, forming organic matters followed by decomposition to produce CH4 again. That is, increase of H2 and CO concentrations owing to photolysis of CH4 in the atmosphere triggers positive feedback of CH4 flux. Increase of H2 concentration increases linearly, while CO concentration increases non-linearly with an increase in the CH4 flux. This result indicates that positive feedback with CO should contribute to amplification for CH4 flux more stronger than the positive feedback with H2.
We will also discuss on application of this model to Earth-like exoplanets orbiting around stars with different masses and spectral types.
Here, we investigate the mechanism for non-linear amplification of CH4 flux by detailed analysis with a coupled model of atmospheric photochemistry and marine ecosystem. We found that the difference between the two cases is derived from difference in redox balance at the same outgassing flux of H2. A net flux of reducing power increases with activity of Fe-using anoxygenic photoautotrophs. Two positive feedbacks for the production of H2 and CO, strongly impact on the CH4 flux and concentrations. Biogenic CH4 is converted back to H2 and CO by photolysis in the atmosphere, which are recycled by marine microbial ecosystem, forming organic matters followed by decomposition to produce CH4 again. That is, increase of H2 and CO concentrations owing to photolysis of CH4 in the atmosphere triggers positive feedback of CH4 flux. Increase of H2 concentration increases linearly, while CO concentration increases non-linearly with an increase in the CH4 flux. This result indicates that positive feedback with CO should contribute to amplification for CH4 flux more stronger than the positive feedback with H2.
We will also discuss on application of this model to Earth-like exoplanets orbiting around stars with different masses and spectral types.