9:00 AM - 9:20 AM
[BBG02-01] Primitive microbial ecosystem and the faint young Sun paradox
★Invited Papers
Keywords:Archean, faint young Sun paradox, methane
Climate of the Earth in the Archean is thought to have been warm or even much warmer than it is today, although the Sun was much (about 20%) dimmer. This "faint young Sun paradox" is solved if the level of carbon dioxide (CO2) in the atmosphere was much higher in the Archean. It is however revealed that pCO2 in the late Archean was not as high as the levels predicted from the climate models. It is therefore considered that concentration of methane (CH4) in the atmosphere should have been higher, which compensated for the deficit of greenhouse effect of CO2. However, because CH4 is photochemically unstable, it is uncertain that such a high level of CH4 could have been maintained in the Archean atmosphere.
Primary productivity in the Archean ocean is important to estimate CH4 flux to the atmosphere, because CH4 was produced from activity of methanogen. Primitive photosynthetic bacteria, which did not produce oxygen, were probably primary producers in the Archean oceans. They probably used H2 and Fe2+ as a electron donor for photosynthesis. We therefore try to estimate CH4 flux and concentration in the atmosphere with a coupled model of primitive microbial ecosystem, photochemical reactions, biogeochemical cycle, and climate.
We found that, the CH4 flux to the atmosphere is too low to form warm climate when only one photosynthesizer (H2-based or Fe-based anoxygenic photoautotroph) is considered in the ecosystem, but the CH4 flux becomes enough to create warm climate when hybrid ecosystem of H2-based and Fe-based anoxygenic photoautotrophs is considered. This is because of a nonlinear amplification of methane cycle due to nonlinear increases of CH4 and H2 concentrations in the atmosphere against increase of CH4 flux.
We conclude that diversity of primitive anoxygenic photoautotrophs was important for stabilization of warm climate in the Archean. It also implies that microbial activity and CH4 are important to understand environment of young Earth-like habitable planets in the exoplanetary systems.
Primary productivity in the Archean ocean is important to estimate CH4 flux to the atmosphere, because CH4 was produced from activity of methanogen. Primitive photosynthetic bacteria, which did not produce oxygen, were probably primary producers in the Archean oceans. They probably used H2 and Fe2+ as a electron donor for photosynthesis. We therefore try to estimate CH4 flux and concentration in the atmosphere with a coupled model of primitive microbial ecosystem, photochemical reactions, biogeochemical cycle, and climate.
We found that, the CH4 flux to the atmosphere is too low to form warm climate when only one photosynthesizer (H2-based or Fe-based anoxygenic photoautotroph) is considered in the ecosystem, but the CH4 flux becomes enough to create warm climate when hybrid ecosystem of H2-based and Fe-based anoxygenic photoautotrophs is considered. This is because of a nonlinear amplification of methane cycle due to nonlinear increases of CH4 and H2 concentrations in the atmosphere against increase of CH4 flux.
We conclude that diversity of primitive anoxygenic photoautotrophs was important for stabilization of warm climate in the Archean. It also implies that microbial activity and CH4 are important to understand environment of young Earth-like habitable planets in the exoplanetary systems.