DID VOLCANISM CAUSE THE WHIFF OF OXYGEN 2.5 BILLION YEARS AGO?

The Great Oxidation Event (GOE) at ~2.35 Ga brought about the irreversible rise of oxygen in the Earth’s atmosphere. However, multiple lines of evidence suggest that transient oxygenation may have occurred for short periods of time hundreds of millions of years before the GOE. Most notably, clues from multiple redox-sensitive elements in the Mt. McRae Shale led numerous authors to propose that one such whiff of oxygen occurred ~2.5 Ga. Nevertheless, it has so far not been established what caused this temporary oxygen spike. One possible trigger may have been a pulse of phosphorus delivery to the ocean, which in turn stimulated primary productivity of photosynthetic organisms and increased subsequent carbon burial. Phosphorus is derived from weathering of continental crust. Several mechanisms can enhance phosphorus weathering, such as tectonics or volcanism. Indeed, the volcanic emplacement of subaerial Large Igneous Provinces, thought to have been active at around ~2.75 and ~2.5 Ga, would have exposed extensive regions of fresh phosphorus-rich flood basalt to chemical weathering. Moreover, volcanic ash might have locally fertilized the adjacent ocean. We tested this hypothesis by combining new and existing elemental abundance and isotopic data across the stratigraphic profile of the Mt. McRae Shale. In addition, we included mercury abundances as a novel proxy for volcanism that has been successfully used in several studies of the Phanerozoic. Our data reveal a distinct mercury enrichment before the whiff of oxygen, coincidental with a spike in typically volcanism-sourced trace metals and immediately preceding one of the largest positive Δ³³S excursions in the rock record. This mercury excursion is followed by an increase in phosphorus concentrations leading into the oxygen whiff. Based on this series of events, we hypothesize that a volcanic eruption could have provided the trigger for the temporary rise of oxygen 150 million years before the GOE.