ABRUPT ONSET OF SUBAERIAL VOLCANISM AND THE RISE OF ATMOSPHERIC OXYGEN

The simple hypothesis that the establishment of an oxygen-rich atmosphere ~2.4 billion years ago occurred when oxygen-producing cyanobacteria evolved is arguably contradicted by abundant biomarker evidence for their presence in rocks 300-million years older. Given the event's coincidence with the transition from the Archean to the Proterozoic Eons, a cause-and-effect relationship with the defining tectonic change (the stabilization of continental cratons) seems likely. A shift in the locus of volcanism from largely submarine to a mix of submarine and subaerial is here argued to be the primary cause. This shift would have been accompanied by a significant reduction in the sink for oxygen provided by reduced volcanic gases: Archean submarine volcanoes emitted molecular hydrogen in excess, allowing H₂ to accumulate in the atmosphere and prevent the buildup of oxygen, whereas Proterozoic subaerial/submarine volcanoes did not, so oxygen came to dominate atmospheric composition.