14:45 〜 15:00
[BCG06-11] Conditions for high marine phosphate concentrations during the Archean
★Invited Papers
キーワード：太古代
The availability of phosphate (P), a limiting nutrient in the present ocean (Tyrrell, 1999), is critical for understanding marine productivity and the evolution of the atmospheric oxygen levels (pO2) in the history of the Earth. The atmospheric pO2 during the Archean (4.0–2.5 Ga) was more than 10–6 times smaller than the present atmospheric level (PAL). Despite this low atmospheric pO2, geological records of the transient oxygenation of the ocean–atmosphere system and phylogenetic analyses infer the emergence of oxygenic photoautotrophs by the middle Archean (e.g., Anbar et al. 2007; Jabłońska and Tawfik 2021). If this was true, the rate of biogenic O2 supply must have been overwhelmed by the removal flux of O2 for a sustained period of time. The suppression of the availability of P in the ocean has been proposed as one plausible reason for sustained anoxic conditions throughout the Archean, supported by the low P availability in iron formations and biogeochemical models (e.g., Reinhard et al., 2017; Watanabe et al., 2023c). However, an emerging proxy of the past marine P level, such as the carbonate-associated phosphate, indicates a P-enriched condition during the Archean (Ingalls et al., 2022; Rasmussen et al., 2021; 2023). These contradicting views of the marine P levels would highlight a lack of quantitative understanding of the P cycle in the Archean ocean.
In this study, we developed a biogeochemical box model of the marine P cycle. Using this model, we examine the possibility that oceanic P levels were higher than today during the Archean after the emergence of oxygenic photoautotrophs. We show that marine P concentrations higher than the present value would have been difficult to achieve under low atmospheric pO2 conditions during the Archean if oxygenic photoautotrophs had already evolved and their activity was limited by P availability. We further show that sustaining P-rich oceans while retaining the anoxic atmosphere requires the limitation by factors other than P, such as nitrogen, or a combination of ocean stagnation and an unrealistically high outgassing rate of reducing gases. This is because the P-rich oceans would have given rise to the oxygenation of the atmosphere unless the consumption rate of O2 is sufficiently high or the productivity is kept low. If these conditions were not met, the occurrence of oceanic P levels higher than present-day values during the Archean would require the absence of oxygenic photoautotrophs.
In this study, we developed a biogeochemical box model of the marine P cycle. Using this model, we examine the possibility that oceanic P levels were higher than today during the Archean after the emergence of oxygenic photoautotrophs. We show that marine P concentrations higher than the present value would have been difficult to achieve under low atmospheric pO2 conditions during the Archean if oxygenic photoautotrophs had already evolved and their activity was limited by P availability. We further show that sustaining P-rich oceans while retaining the anoxic atmosphere requires the limitation by factors other than P, such as nitrogen, or a combination of ocean stagnation and an unrealistically high outgassing rate of reducing gases. This is because the P-rich oceans would have given rise to the oxygenation of the atmosphere unless the consumption rate of O2 is sufficiently high or the productivity is kept low. If these conditions were not met, the occurrence of oceanic P levels higher than present-day values during the Archean would require the absence of oxygenic photoautotrophs.