Abstract
Oxygen is a potential biosignature for terrestrial Earth-like planets. The primary source of oxygen on Earth is oxygenic photosynthesis, which may be limited by the supply of riverine phosphorus. Therefore, phosphorus supply from the chemical weathering of continents is crucial for the evolution of pO2. Chemical weathering occurs on both the continents and seafloor and stabilizes the climate, but phosphorus is only supplied by continental weathering. The amount of continental weathering relative to seafloor weathering may be critical for primary productivity and pO2. The area of continents could change as a result of continental growth and the amount of ocean mass on the planetary surface, and these factors could be very different on extrasolar Earth-like planets. Here, we investigated the effects of continental and seafloor weathering on the atmospheric oxygen levels, in terms of the Earth-like phosphorus-limited marine biosphere. We used a simple biogeochemical model and investigated a possible relationship between continental growth and atmospheric oxygen levels. We found that the atmosphere could evolve totally different redox conditions (an abrupt rise of atmospheric oxygen levels or a reducing condition to form organic haze) caused by continental growth, which changes the relative contribution of silicate weathering feedback from seafloor to continent. We also found that conditions with lower solar luminosity and a larger land fraction provided a preferable condition for the phosphorus-limited marine biosphere to produce high levels of oxygen in the atmosphere. We also found that the atmospheric oxygen level is strongly affected by the activity of the anaerobic marine microbial ecosystem. Our results suggest that the area of land on the planetary surface may be crucial for achieving high oxygen levels in a phosphorus-limited marine biosphere. These results contribute to the fundamental understanding of the general behaviors of Earth-like planets with oceans and an Earth-like marine biosphere.
Highlights
Oxygen is produced from oxygenic photosynthesis on land and in the ocean, and comprises 21% of the present atmosphere
The phosphorus supply caused by continental weathering responds differently to CO2-induced or CH4-induced warming. This result suggests the importance of the competition between continental and seafloor weathering in controlling the phosphorus input rate and the global marine primary productivity
We showed that the changes in the relative contribution between land and seafloor weathering affects phosphorus input, and that the atmosphere could evolve totally different redox conditions depending on the structure of the marine ecosystem and the C O2 outgassing flux
Summary
Oxygen is produced from oxygenic photosynthesis on land and in the ocean, and comprises 21% of the present atmosphere. On the modern Earth, the net primary productivity of oxygenic photosynthesis in marine and land ecosystems is ~ 4 × 1015 mol C year–1 and ~ 5 × 1015 mol C year–1, respectively (Field et al 1998; Sarmiento and Gruber 2002). Global marine primary productivity in the present ocean may be limited by the supply of riverine phosphorus (Tyrrell 1999), so phosphorus supply from the weathering of continental phosphorus-hosted minerals, such as apatite, is crucial for the evolution of pO2 (Hao et al 2020a, b). The fundamental role of seafloor weathering for climate regulation has recently gained attention in understanding the stability of the climate on Earth and Earth-like exoplanets (e.g., Krissansen-Totton and Catling 2017; Krissansen-Totton et al 2018; Hao et al 2020b; Hayworth and Foley 2020). Changes in the land fraction and the climate should affect the supply rate of phosphorus to the ocean, which, in turn, affect marine primary productivity
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