About 2.4 billion years ago there was a dramatic increase in the level of free oxygen in the Earth’s atmosphere, which led to the widespread extinction of many species of bacteria which had evolved when the atmosphere was anaerobic. Previously it has been thought that this oxygen was released by blue-green bacteria, but in an article entitled “Large oxygen excess in the primitive mantle could be the source of the Great Oxygenation Event” published in January 2018, D. Andrault et al state that:

Before the Archean to Proterozoic Transition (APT) the tectonic regime was dominated by microplates floating on a low viscosity mantle. Such a regime restricted chemical exchange between the shallow and deeper mantle reservoirs. After the APT, a more global convection regime led to deep subduction of slabs.

They go on to propose that:

the improved vertical mixing of the mantle favoured the release to the Earth’s surface of an oxygen excess initially trapped in the deep mantle. This excess built up when the primordial lower mantle was left with a high Fe3+/(Fe2++Fe3+) ratio (#Fe3+), after metallic iron segregated down into the core. Our synchrotron-based in situ experiments suggest a primordial Fe3+excess of ~20 % for the mantle iron. By comparison with the #Fe3+ of the present mantle, this Fe3+excess would correspond to 500–1000 times the O2 content in the Earth’s atmosphere. The tectonic transition greatly facilitated the ascent of oxidised lower mantle material towards the Earth’s surface, inducing a continuous arrival of O2 at the Earth’s surface and into the atmosphere.

In other words, the oxygen was released when the previously floating plates began to be subducted into the mantle, creating convection currents which brought oxygen-rich rocks to the surface, where the oxygen was released.

But the date when subduction began is controversial. As S. Turner noted in his article “Heading down early on? Start of subduction on Earth” published in 2014:

Many workers suggest that subduction may have only commenced toward the end of the Archean or later [but] some form of subduction may have been operating as early as the Hadean or Eoarchean.