A PERSISTENTLY LOW LEVEL OF ATMOSPHERIC OXYGEN IN EARTH’S MIDDLE AGE (Invited Presentation)

Resolving how Earth’s surface redox conditions evolved through the Proterozoic Eon is fundamental to understanding how biogeochemical cycles have changed through time. Previous studies mostly focused on individual stratigraphic units and do not provide long term estimates of atmospheric oxygen level through time. Moreover, carbonate rocks offer a better time-averaged view of the upper oceans and even the atmosphere. The redox sensitivity of cerium relative to other rare earth elements and its uptake in carbonate minerals make the Ce anomaly (Ce/Ce*) a particularly useful proxy for capturing redox conditions in the local marine environment. Here, we report Ce/Ce* data in marine carbonate rocks through 3.5 billion years (Ga) of Earth’s history, particularly focusing on the mid-Proterozoic (i.e., 1.8 – 0.8 Ga). To better understand the role of atmospheric oxygenation, we use Ce/Ce* data to estimate the partial pressure of atmospheric oxygen (pO₂) through this time. Our thermodynamics-based modeling supports a major rise in atmospheric oxygen level in the aftermath of the Great Oxidation Event (~2.4 Ga), followed by relatively invariant pO₂ of about 1% of present atmospheric level through most of the Proterozoic Eon (2.4 to 0.65 Ga).