Constraints on Paleoproterozoic atmospheric oxygen levels.

The oxygenation of Earth's surface environment dramatically altered key biological and geochemical cycles and ultimately ushered in the rise of an ecologically diverse biosphere. However, atmospheric oxygen partial pressures (pO2) estimates for large swaths of the Precambrian remain intensely debated. Here we evaluate and explore the use of carbonate cerium (Ce) anomalies (Ce/Ce*) as a quantitative atmospheric pO2 proxy and provide estimates of Proterozoic pO2 using marine carbonates from a unique Precambrian carbonate succession-the Paleoproterozoic Pethei Group. In contrast to most previous work, we measure Ce/Ce* on marine carbonate precipitates that formed in situ across a depth gradient, building on previous detailed sedimentology and stratigraphy to constrain the paleo-depth of each sample. Measuring Ce/Ce* across a full platform to basin depth gradient, we found only minor depleted Ce anomalies restricted to the platform and upper slope facies. We combine these results with a Ce oxidation model to provide a quantitative constraint on atmospheric pO2 1.87 billion years ago (Ga). Our results suggest Paleoproterozoic atmospheric oxygen concentrations were low, near 0.1% of the present atmospheric level. This work provides another crucial line of empirical evidence that atmospheric oxygen levels returned to low concentrations following the Lomagundi Event, and remained low enough for large portions of the Proterozoic to have impacted the ecology of the earliest complex organisms.