Volcanic forcing of early oxygen

The Archean Eon, spanning from approximately 4.0 to 2.5 billion years ago, was a time of atmospheric change on Earth. Prior to the Great Oxidation Event, the atmosphere generally had negligible molecular oxygen. Photoautotrophs, for example, cyanobacteria, likely evolved during this time and produced oxygen in small amounts through oxygenic photosynthesis, though this was then destroyed by reactions under a reducing atmosphere. The change in geochemical conditions that allowed this oxygen to persist and build up was a key event in the emergence and evolution of early life, especially complex animals.

However, a growing body of evidence indicates that the Earth experienced earlier transient oxygenation events — so-called whiffs of oxygen. A 2007 study (Science 317, 1903–1906; 2007) suggested that small amounts of oxygen may have been present in the atmosphere at least 50 million years before the start of the Great Oxidation Event, and persisted for several million years. Other episodes and locations of transient oxygenation have been identified since (see the Article by Chen et al. and the Article by Liang et al. in this issue).