[B-BG01] Earth and Planetary Science Frontiers for Life and Global Environment

[BBG01-01] The coupled evolution of life and the atmosphere during the early Archean

★Invited Papers

*Kazumi Ozaki¹, Yasuto Watanabe², Edward W. Schwieterman³, Christopher T. Reinhard⁴, Peng Hong⁵, Eiichi Tajika² (1.Toho University, 2.The University of Tokyo, 3.University of California, 4.Georgia Institute of Technology, 5.Chiba Institute of Technology)

Keywords:Archean, Atmospheric chemistry, Biogeochemical cycles

The activity level of the biosphere is a critical factor controlling the chemical composition of the atmosphere, yet the mechanistic understanding of how primitive anaerobic life has affected the Earth's habitability remains unclear. This is especially true for the early Archean prior to the advent of oxygenic photosynthesis. Here, we revisit previous biogeochemical modeling of the global H, C, and Fe cycles in order to more precisely reconstruct global biological activity and its impacts on atmospheric chemistry. We employ a simplified photochemistry-ocean-ecosystem model to evaluate global biogeochemical cycles and biological productivity on the early Earth. The model includes a series of metabolic reactions in the primitive anoxic biosphere (e.g., anoxygenic photosynthesis, methanogens, and fermenters) and takes redox (H) balance in the ocean-atmosphere system into account. We explored the impact of varying key model parameters on global biogeochemistry using a stochastic approach and search for possible solutions which allow for habitable climate states lacking optically thick organic haze.
The results suggest that net primary production (NPP) was ~0.1% and 1% that of the modern Earth for the non-photosynthetic (chemotrophic) ecosystem and anoxygenic photosynthetic ecosystem, respectively. The extremely low NPP fluxes produced by our model imply that geological fluxes of reductants, rather than light or nutrients, would have been the limiting factor for biological productivity. Despite the low biological productivity of the primitive biosphere, atmospheric methane abundances would have been relatively high (fCH₄ = ~300 ppmv) because primitive biospheres efficiently recycle the material in the ocean-atmosphere system. Our results also demonstrate that the transition from a chemotrophic ecosystem to an anoxygenic photosynthetic ecosystem exerts minor impacts on atmospheric composition, making it difficult to diagnose both ecosystems based on atmospheric chemistry alone. While our results provide a comprehensive and statistically robust picture of the early-Archean Earth system that is consistent with available geologic records, further mechanistic understanding of the coupled H-C-Fe cycles is required to fully understand the stabilization mechanisms of atmospheric chemistry during the early-Archean. A better mechanistic understanding of the coupled evolution of primitive life and the atmosphere has great ramifications not only for the sustained habitability of Earth but for the search for life on Earth-like exoplanets with reducing atmospheres that may host primitive life.