What Controls the Evolution of Earth's Surface O2 and CO2 Levels?

Over Earth history, atmospheric and marine oxygenation appears to have proceeded in a stepwise fashion, with major transitions during the Paleoproterozoic, Neoproterozoic and Paleozoic Eras. In contrast, CO₂ concentration has most likely declined gradually over Earth history, but with a superimposed ~100 Myr cycle, leading to 'icehouse' and 'greenhouse' climate shifts. In this talk I summarise the latest results from our biogeochemical modelling efforts, which aim to assess what has driven these changes. We conclude that Earth's observed stepwise oxygenation can be explained by the internal dynamics of global biogeochemical cycles, and does not necessarily require 'trigger' events. We find that changes in CO₂ concentration and associated icehouse/greenhouse variability can largely be explained by changes to plate tectonics, which influence both the sources and sinks of CO₂.