Oxygen is crucial for life on Earth today, with oxygen’s first major rise ~2.4-2.0 billion years ago during the Great Oxidation Event (GOE) argued to have been a turning point for planetary habitability. Furthermore, the subsequent trajectories of atmospheric and marine oxygen after this initial rise are critical for clarifying linkages between oxygenation and the evolution of complex life. Importantly, massive iron formations have long been recognized as uniquely sensitive to marine oxygen levels and so form critical archives of early Earth’s oceans. Yet their relationship to marine oxygenation is complicated by uncertainties in the triggers for deposition, which are particularly striking when examining massive iron deposits after the GOE. To help address these issues, I present new geologic and geochronologic results from the Ironwood Iron Formation and Emperor Volcanics near Lake Superior. Synthesis of these new regional constraints with a revised compilation of existing data creates a global perspective that allows critical assessment of the marine conditions that trigger post-GOE massive iron deposition and the factors influencing the trajectory of marine oxygen.