Climatic, tectonic, and biological factors affecting the oxidation state of the atmosphere and oceans: Implications for Phanerozoic O2 evolution

Abstract:

The Earth’s atmosphere and oceans have seen fundamental changes in its oxidation state in response to the climatic, tectonic and geochemical variations. Over the past decade, several geochemical proxies have led to significant progress in understanding the paleredox states of ancient oceans. However, a quantitative interpretation of these data for atmospheric O₂ levels remain unclear because the relationship between atmospheric O₂ levels (pO₂) and oceanic redox state depends on several environmental factors, such as terrestrial weathering rate, sea-level stands, and sinking rate of particulate organic matter (POM) in the water column and so on. It is widely thought that the redox-dependent P cycling also plays a crucial role in regulating pO₂ because it acts as a negative feedback on a geological timescale. It is important that strength of this feedback for a given pO₂ is also modulated by environmental factors, affecting not only O₂ levels at steady state but also its susceptibility to environmental changes. In this study, a quantitative role of environmental factors in the oxidation state of Earth’s surface environment is evaluated with an oceanic biogeochemical cycle model (CANOPS) coupled with global C cycle model, which enables us to understand the ancient CO₂ and O₂ evolution. Our results demonstrate that atmospheric O₂ level at steady state is affected by CO₂ input flux from Earth’s interior via changes in biogeochemical cycles, but its response is modulated by several internal factors such as shelf area and POM sinking rate. We also found that early Paleozoic atmospheric O₂ levels before the advent of land plant would be determined so that oceans may locate at the “edge of anoxia (EoA)” where the redox-dependency of marine P cycle plays a crucial role in regulating O₂ cycle, and that POM sinking rate has a great impact on the EoA. Our findings provide insights into the O₂ cycle over the Phanerozoic in response to the climatic and tectonic variations and also shed light on the causal linkage between the critical biological evolution (such as an establishment of biological pump and advent of land plant) and the oxidation state of the atmosphere and oceans.