Evolution of a habitable planet: A biogeochemical modelling study of the rise of oxygen to modern levels

The question of whether rising oxygen levels through Earth history have driven the evolution of life, is a subject of considerable interest. A key investigative tool in this topic has been the geochemical analyses of sedimentary rocks, from which we have gained valuable insights into Earth’s changing redox landscape. Many of these geochemical proxies are marine in nature, thus only broad inferences about atmospheric oxygen levels can be made. However, some of this data can be utilised in Earth system models to generate quantitative predictions. While many such models exist for the Phanerozoic, none continuously chart atmospheric oxygen over billions of years, making it difficult to understand any link between oxygen and life. This research aims to redress that issue by estimating atmospheric oxygen levels for the last 3.4 Gyrs. This has required the redevelopment of a well-established Phanerozoic model: GEOCARBSULF, in three distinct phases. Phase one was to understand why GEOCARBSULF predicts high oxygen levels in the early Paleozoic, contrary to geochemical evidence. It was found that GEOCARBSULF’s simplified treatment of δ34S fractionation leads to oxygen being tightly regulated to near modern-day levels. Using an alternative approach to the sulfur cycle generates low oxygen in the early Paleozoic with a step change to modern levels starting in the Ordovician. Phase two extended this model back to investigate the Neoproterozoic Oxygenation Event, resulting in the suggestion that the first animals evolved against a backdrop of extreme variability in oxygen levels. Phase three encompasses the last 3.4 Gyrs, including quantitative estimates for the Great Oxidation Event and subsequent ‘oxygen overshoot’, finding that they were near equal in magnitude but only approximately one tenth of present-day levels, converse to other predictions. Overall, this work identifies that the Earth experienced three periods of atmospheric oxygen upheaval, with times of relative quiescence in between.