Earth Planet. Sci. Lett. 366, 17–26 (2013)

During the Great Oxidation Event 2.4 billion years ago, oxygen levels in the Earth's atmosphere rose dramatically. Sulphur isotope data suggest a flux of oxygen into the atmosphere some 300 million years earlier.

Florian Kurzweil of Westfälische Wilhelms-Universität Münster and colleagues measured multiple sulphur isotopes in shales found in Ontario, Canada, that formed 2.71 billion years ago. A photochemical model of the Archaean atmosphere including these sulphur isotope values simulates a transition from a hazy, methane-rich atmosphere to an increasingly oxidative one 2.7 billion years ago, as oxygen production outpaced the creation of methane. This allowed oxygen to persist in the atmosphere, albeit at levels far below those seen during the Great Oxidation Event some 300 million years later.

The authors attribute the production of oxygen to a pronounced expansion of bacteria capable of producing oxygen through photosynthesis. Furthermore, weathering of fresh volcanic rock was enhanced at this time, which would have delivered nutrients to the oceans to fuel photosynthesis.