USING IRON-RICH ROCKS TO RECONSTRUCT PROTEROZOIC OXYGEN LEVELS

The oxygenation of the atmosphere—one of the most fundamental transformations in Earth’s history—dramatically altered the chemical composition of the oceans and provides a compelling example of how life can reshape the planetary surface environments. Further, it is commonly proposed that surface oxygen levels played a key role in controlling the timing and tempo of the origin and early diversification of animals. Although oxygen levels were likely more dynamic than previously imagined, here we make a case that emerging records provide evidence for low atmospheric oxygen levels for the majority of Earth’s history. Specifically, we review records from Proterozoic iron-rich rocks and present a conceptual framework that suggests background oxygen levels were below 1% of the present atmospheric level (PAL) through the billon years leading up to the rise of animals. Evidence for low background oxygen levels throughout the Proterozoic bolsters the case that environmental conditions were a critical factor in controlling the structure of ecosystems through Earth’s history.