[M-IS02] Astrobiology

[MIS02-08] Relationship among evolutions of continental crust, marine ecosystem, and atmosphere

*Yasuto Watanabe¹, Eiichi Tajika¹ (1.The University of Tokyo)

Keywords:Atmospheric Evolution, Biogeochemistry

The atmospheric oxygen level (pO2) has substantially increased from low (< 10^–5 PAL) to weakly oxidized condition (~10^–2 PAL) at around 2.4–2.1 Ga, which is called the Great Oxidation Event (GOE) (e.g. Lyons et al., 2014). Preceding the GOE, it is suggested that mass of continental crusts has increased during the Archean (4.0–2.5 Ga) (e.g. Hawkesworth et al., 2019), which would have contributed to accumulation of oxygen in the atmospheric through the increased burial of organic carbon at continental shelves (Godderis and Veizer, 2000) and through a change in the composition of continental crusts (Lee et al., 2016). Continents are also important in regulating the Earth’s climate through the carbonate-silicate geochemical cycle and in providing the phosphorus to the ocean through the surface weathering (e.g. Walker et al., 1981; Hao et al., 2020). However, the relationship among the evolutions of continents, marine microbial activity, and atmospheric oxygen has not been clearly understood. Here we conducted an equilibrium calculation using an ocean biogeochemical model with photochemical and climatic parameterization based on Harada et al. (2015) to demonstrate the relationship between crustal and atmospheric evolution with parameter sets assuming the early Earth and extrasolar Earth-like planets.
We show that the continental phosphorus input increases according to the continental growth (Hao et al., 2020). This is because the relative contribution of continental weathering against seafloor weathering increases with the continental growth. Once the rate of the continental weathering increases through continental growth, the equilibrium surface temperature is decreased due to the increased consumption of atmospheric CO₂. The seafloor weathering rate, on the other hand, decreases because of the decreased atmospheric CO₂ level and surface temperature. Thus, the relationship between the continental growth and the riverine phosphorus input is unique to the Earth-like planets with effective seafloor weathering. We found that, as a result of this relationship, the atmosphere evolves differently in response to the continental growth in the early Earth, depending on the structures of the marine microbial ecosystem. In the case for which the riverine phosphorus is primarily utilized by oxygenic photoautotrophs, evolution of the continental crust alone could drive the GOE by increasing the riverine phosphorus input when the CO₂ outgassing rate is sufficient. On the other hand, in the case for which the phosphorus is also utilized by anoxygenic photoautotrophs as in the late Archean ocean (e.g. Ozaki et al., 2019), the atmospheric oxygen level could not rise even with high CO₂ outgassing rate. In this case, the continental growth could drive the formation of hydrocarbon haze in the atmosphere when CO₂ outgassing rate is comparable to the present condition. This is because the atmospheric CO₂ level decreases with the increased CO₂ consumption from the atmosphere. The evolution of the atmosphere in the early Earth before and after the GOE and implications for extrasolar Earth-like planets will be discussed.