The accumulation of oxygen in the atmosphere that occurred 2.5–2.45 billion years ago, when the atmospheric oxygen concentration rose from less than 0.1% of today’s value (and probably much less) to about 5% of today’s value. Photosynthesis by cyanobacteria is believed to have supplied the oxygen, but much of it would have been consumed by oxidizing methane as well as iron, sulphur, and other elements exposed to the air, and oxygen would have dissolved in surface water. These processes would have delayed its accumulation, but why it accumulated relatively suddenly is uncertain.