[M-IS19] Aqua planetology

[MIS19-06] Fluctuations of the atmospheric oxygen and biogeochemical cycles on early Earth

★Invited Papers

*Yasuto Watanabe¹, Kazumi Ozaki², Mariko Harada², Hironao Matsumoto³, Eiichi Tajika⁴ (1.Meteorological Research Institute/University of Tokyo, 2.Tokyo Institute of Technology, 3.JAMSTEC, 4.University of Tokyo)

Keywords:Archean, Proterozoic

Unraveling the interactions and co-evolutions of the atmosphere, life, and environment on Earth is fundamentally important for understanding the long-term sustainability of the habitable environment on planetary surfaces. One of the most intriguing aspects of the evolution of the atmosphere–biosphere systems in the Earth’s history is the Great Oxidation Event (GOE), the first major rise of the atmospheric pO₂ (e.g., Lyons et al., 2014). Before the GOE, the Earth system was characterized by a prolonged anoxic atmosphere with low atmospheric pO₂ (< 10^–6 present atmospheric level; PAL), yet accumulated geologic records have suggested the possibility of atmospheric transient oxygenation events––whiff of oxygen—during the late Archean (3.0–2.5 Ga) (e.g., Anbar et al., 2007). After the GOE, on the other hand, the atmospheric pO₂ would have reached 10^–4–10^–1 PAL. It has been discussed, however, that the atmospheric pO₂ may have changed transiently, as inferred from the increases in the deposition of banded iron formations at 1.8–1.9 Ga (Rasmussen et al., 2012; Watanabe et al., 2023c). Although these records may infer the transient modulations of the atmospheric pO₂ and biogeochemical cycles on early Earth, the precise nature and causes of these events remain elusive.
In this study, we use a biogeochemical model that estimates the C–P–Fe–S–O₂ biogeochemical cycles on early Earth and behaviors of S and C stable isotopes. We simulate the transient response of the global biogeochemical cycle after the eruption of the LIPs for the pre- and post-GOE conditions. We found that the elevated riverine phosphorus supply to the ocean owing to climate warming caused by intense volcanism after the eruption of a LIP can lead to a transient increase in atmospheric pO₂ exceeding 10^–4 PAL under pre-GOE conditions. We further show that this transient oxygenation may last for several to ten million years. The conditions that affect this transient oxygenation are primarily the total carbon mass released from a LIP volcanism, the influx of reducing power from Earth’s interior to the ocean–atmosphere system, such as volcanic outgassing of reducing gases and the hydrothermal Fe(II) supply rate, and the continental area. Notably, the growth of the continental crust mitigates the conditions for transient oxygenations during the late Archean, primarily by enhancing nutrient supply to the ocean, supporting the occurrence of transient oxygenation events during the late Archean. Our model further demonstrates that when the eruption of a LIP occurs in post-GOE conditions, the atmospheric pO₂ would drop sharply if sufficient amounts of reducing gases were supplied by intense volcanism. We also show that the atmospheric pO₂ recovers to the background atmospheric pO₂ even within a short period (<1 Myr) after the cessation of the main interval of the eruption of the LIP owing to an accumulation of riverine phosphorus and an increase in O₂ production rates in the ocean. Our results infer a link between the eruption of LIPs and the fluctuations of the atmospheric pO₂ on early Earth. The eruption of LIPs may be a critical process that affects the long-term sustainability of the habitable conditions and the activity of life on early Earth.