[B-CG04] Decoding the history of Earth: From Hadean to the present

[BCG04-11] Upper Limit of the Activity of Anaerobic Microbial Ecosystem in the Early Earth

*Yasuto Watanabe¹, Eiichi Tajika¹, Kazumi Ozaki², Peng K Hong³ (1.The University of Tokyo, 2.Toho University, 3.Chiba Institute of Technology)

Keywords:Archean, Biogeochemistry

The global marine primary production in the modern ocean is suggested to be limited by the supply of phosphorus from rivers (e.g. Tyrrell, 1999). However, in the early Earth before the onset of oxygenic photosynthesis, the global marine primary production is suggested to have been limited by the production of electron donors, including hydrogen (H₂), ferrous iron, and carbon monoxide (CO) (e.g. Kharecha et al., 2005). Under the anoxic conditions in early Earth, it is suggested that an hydrocarbon haze layer is formed in the upper atmosphere when the supply of electron donors is large enough. Because such a haze layer absorbs the UV-flux from the sunlight effectively (Wolf and Toon, 2010), it should affect the atmospheric photochemistry, hence the activity of anaerobic marine microbial ecosystem. However, the relationship between the biological activity and the formation of haze has not been demonstrated. Here, we employed a coupled model of one-dimensional photochemical model “Atmos” (Arney et al., 2016) and a marine microbial ecosystem model to elucidate the responses of the biological activities in the early Earth in association with the formation of haze for wide ranges of parameters (atmospheric CO₂ level, H₂ outgassing rate, Fe(II) upwelling rate, and burial efficiency of organic carbon).
We found that the primary productivity of anaerobic biosphere under the hazy atmosphere is around 1.0 GtC/yr, independent of the thickness of the haze layer and the atmospheric CO₂ level. In the hazy condition, formation rates of electron donors (H₂ and CO) in the atmosphere decreases with development of haze, hence the biogenic methane flux and the haze formation rate is decreased. As a result, the maximum level of primary productivity due to anaerobic ecosystem is achieved at the clear-sky condition close to the boundary of the formation of haze. Interestingly, this maximum level is always around 1.5 GtC/yr (~3% of the present marine gross primary production), independent of values of parameters we considered. Thus, this maximum level corresponds to the upper limit of the activity level of anaerobic ecosystem in the early Earth. Once the supply of electron donors increases to allow the activity of anaerobic ecosystem to reach this critical level, haze is formed in the atmosphere and the activity level of anaerobic ecosystem decreases to ~1.0 GtC/yr. This upper limit is broadly consistent with the previous estimates of the primary production in the early Earth based on independent methods (Kharecha et al. 2005; Canfield, 2006; Hao et al., 2020). Even after the evolution of the oxygenic photosynthesis, the upper limit of the activity level of anaerobic ecosystem is almost the same. Based on these results, the evolution of the surface environments and the marine primary productions in the early Earth system will be discussed.