What is the name of the event about 2.4 billion years ago that dramatically increased oxygen in Earth's atmosphere?

The event about 2.4 billion years ago that dramatically increased oxygen in Earth's atmosphere was the Great Oxidation Event. Also called the Great Oxygenation Event, it marked the first major accumulation of free molecular oxygen in Earth's atmosphere and shallow oceans.

The oxygen was produced mainly by cyanobacteria through oxygenic photosynthesis. For hundreds of millions of years, oxygen reacted with dissolved iron, sulfur, and other materials instead of accumulating freely. Once these chemical “oxygen sinks” became increasingly saturated, oxygen began entering the atmosphere in significant amounts.

The change transformed Earth. Oxygen oxidized methane, a powerful greenhouse gas, and may have contributed to the Huronian glaciations. It also made new, energy-rich forms of metabolism possible and altered the chemistry of rocks, oceans, and soils.

A common mix-up is the Neoproterozoic Oxygenation Event, which occurred much later, roughly 850–540 million years ago. The Great Oxidation Event belongs to the Paleoproterozoic Era and is generally dated to about 2.46–2.06 billion years ago.