Smith Lecture - Andrew Heard, Woods Hole Oceanographic Institute

The rise of oxygen (O2) in Earth’s surface environment during the ‘Great Oxidation Event’ (GOE) occurred about 2.3 billion years ago and set Earth on the path to becoming hospitable for complex life with high metabolic O2 requirements. While proxy records have allowed us to quantify rising atmospheric O2 levels quite effectively during this time period, we have far fewer constraints on the timing and extent of ocean oxygenation beneath this evolving atmosphere right as the GOE was taking place. In this talk I will present new constraints on the coupled oxygenation of the ocean and atmosphere during the GOE using measurements of stable thallium and vanadium isotopes in Paleoproterozoic shales. These two global ocean paleoredox proxies exhibit subtly distinct responses to the impacts of ocean oxygenation that together allow us to build a nuanced picture of syn-GOE marine redox dynamics. I will present thallium isotopic data that demonstrates an instantaneous coupling between the initial oxygenation of the atmosphere above a canonical threshold value, and the onset of significant manganese oxide burial in the oceans. Then, I will show how vanadium isotope constraints indicate marine surface O2 reached levels persistently above quantifiable thresholds within as little as a few million years of the onset of permanent atmospheric oxygenation.