Atmospheric oxygenation caused by a change in volcanic degassing pressure

Around two and a half billion years ago (following the end of the Archaean eon), the atmosphere turned from anoxic to weakly oxic in what is known as the Great Oxidation Event. Using a model of volcanic degassing, Gaillard et al. demonstrate that a preceding period of continental crust formation may have been the trigger. They propose that as continents emerged and volcanoes became increasingly subaerial rather than submarine, magmatic volatiles were degassed at lower pressures, leading to a progressive oxidation of the gases released. This shift to a release of sulphur as sulphur dioxide rather than as hydrogen sulphide could then have fed marine sulphate reduction and the eventual oxygenation of Earth's atmosphere. The Precambrian history of our planet is marked by two major events: a pulse of continental crust formation at the end of the Archaean eon and a weak oxygenation of the atmosphere (the Great Oxidation Event) that followed, at 2.45 billion years ago. This oxygenation has been linked to the emergence of oxygenic cyanobacteria1,2 and to changes in the compositions of volcanic gases3,4, but not to the composition of erupting lavas—geochemical constraints indicate that the oxidation state of basalts and their mantle sources has remained constant since 3.5 billion years ago5,6. Here we propose that a decrease in the average pressure of volcanic degassing changed the oxidation state of sulphur in volcanic gases, initiating the modern biogeochemical sulphur cycle and triggering atmospheric oxygenation. Using thermodynamic calculations simulating gas–melt equilibria in erupting magmas, we suggest that mostly submarine Archaean volcanoes produced gases with SO2/H2S