09:15 〜 09:30
[U08-02] 炭化水素ヘイズ層を伴う還元的大気条件下での原始的海洋微生物生態系と大気一酸化炭素の挙動
キーワード：初期地球、大気光化学
An organic haze, an aerosol composed of hydrocarbons, forms in a highly reducing planetary atmosphere, originating from methane. It may have formed on anoxic early Earth, where the biogenic methane would have accumulated in the atmosphere (Pavlov et al., 2001). The covariation of C and S isotopes at ~2.65–2.5 Ga upper Nauga Formation, South Africa has been interpreted to reflect the repeated developments of an organic haze layer with a rapid transition between hazy and haze-free conditions (Izon et al., 2017). On early Earth, the primary production would have been conducted by anoxygenic photoautotrophs and chemoautotrophs. These primitive metabolisms require an external source of electron donors (e.g., H2, CO, Fe(II), and H2S) to assimilate carbon. Some of these electron donors, such as H2 and CO, are produced in the anoxic atmosphere and supplied to the ocean. Because the development of the organic haze layer should strongly affect the atmospheric photochemical reactions by shielding the UV flux, it would also affect the production rate of these electron donors, hence the productivity of the primitive marine biosphere and the supply rate of methane. However, the response of the marine biosphere to the development of a haze layer has not been demonstrated. For this reason, the biogeochemical conditions required for the development of an organic haze layer in the atmosphere and the mechanism for the rapid transitions between hazy and haze-free conditions were unclear. Here we investigated the response of the atmospheric composition, activity of primitive marine ecosystems, and climate under an organic haze layer, using a coupled model of atmospheric photochemistry and 1–D radiative-convective climate model, Atmos (Arney et al., 2016), and a coupled model of atmospheric photochemical model and marine microbial ecosystem model (Ozaki et al., 2018).
We show that the atmospheric H2 level decreases as the haze layer progresses, which decreases the primary productivity by suppressing the activity of H2-based photoautotrophs. This would have worked to stabilize the formation rate of hydrocarbon haze layers by suppressing the supply rate of biogenic CH4. The atmospheric CO level, on the other hand, increases as the haze layer progresses, so the CH4 production rate owing to CO-using chemoautotrophs increases. This would have worked as a positive feedback mechanism that enhances the activity of CO-using chemoautotrophs, a production rate of CH4, and the formation rate of haze particles when the haze layer starts to form in the atmosphere. The behaviors of both atmospheric H2 and CO under the haze layer are associated with the change in the reaction rate of CO and OH in the troposphere. This is driven by the changes in the tropospheric temperature and the photodissociation rate of water vapor. The accumulation of CO in the atmosphere may explain the occurrence of a rapid transition of the total organic carbon and δ13C in the geologic record at the onset of the development of an organic haze layer (Izon et al., 2017). This would proceed until the surface temperature decreases to limit the activity of decomposers (e.g. Domagal-Goldman et al., 2008). The stability of the hazy climate state would have been brought by the negative feedback for the formation rate of haze particles, which is driven by the changes in the activity of decomposers and H2-based photoautotrophs. The H2-based photoautotroph, CO-consuming chemoautotroph, and decomposers respond to the formation of the hydrocarbon haze layer differently, owing to their different responses to the surface temperature and the atmosphere photochemical reactions. The hazy climate state on early Earth would have been achieved by such tight interactions between atmospheric photochemistry and marine microbial ecosystem. This may indicate the fundamental role of CO- and H2-related metabolic pathways in affecting the atmospheric composition of early Earth and Earth-like exoplanets.
We show that the atmospheric H2 level decreases as the haze layer progresses, which decreases the primary productivity by suppressing the activity of H2-based photoautotrophs. This would have worked to stabilize the formation rate of hydrocarbon haze layers by suppressing the supply rate of biogenic CH4. The atmospheric CO level, on the other hand, increases as the haze layer progresses, so the CH4 production rate owing to CO-using chemoautotrophs increases. This would have worked as a positive feedback mechanism that enhances the activity of CO-using chemoautotrophs, a production rate of CH4, and the formation rate of haze particles when the haze layer starts to form in the atmosphere. The behaviors of both atmospheric H2 and CO under the haze layer are associated with the change in the reaction rate of CO and OH in the troposphere. This is driven by the changes in the tropospheric temperature and the photodissociation rate of water vapor. The accumulation of CO in the atmosphere may explain the occurrence of a rapid transition of the total organic carbon and δ13C in the geologic record at the onset of the development of an organic haze layer (Izon et al., 2017). This would proceed until the surface temperature decreases to limit the activity of decomposers (e.g. Domagal-Goldman et al., 2008). The stability of the hazy climate state would have been brought by the negative feedback for the formation rate of haze particles, which is driven by the changes in the activity of decomposers and H2-based photoautotrophs. The H2-based photoautotroph, CO-consuming chemoautotroph, and decomposers respond to the formation of the hydrocarbon haze layer differently, owing to their different responses to the surface temperature and the atmosphere photochemical reactions. The hazy climate state on early Earth would have been achieved by such tight interactions between atmospheric photochemistry and marine microbial ecosystem. This may indicate the fundamental role of CO- and H2-related metabolic pathways in affecting the atmospheric composition of early Earth and Earth-like exoplanets.