A bistable organic-rich atmosphere on the Neoarchaean Earth

It has been hypothesized that, before widespread oxygenation about 2.45 billion years ago, the Earth's atmosphere contained an organic haze similar to that on Titan. However, these theoretical predictions have not been substantiated by geological evidence. Here we use multiproxy geochemical analyses of sediments from the 2.65-2.5-billion-year-old Ghaap Group, in South Africa, to reconstruct ocean and atmospheric chemistry during this time. We find evidence for oxygen production in microbial mats and localized oxygenation of surface waters. Carbon and sulphur isotopes indicate that this oxygen production occurred under a reduced atmosphere that was periodically rich in methane, consistent with the prediction of a hydrocarbon haze. We use a photochemical model to corroborate our geochemical data. Our simulations predict transitions between two stable atmospheric states, one with organic haze and the other haze-free. The transitions are presumably governed by variations in the amount of biological methane production during the Archaean eon. We find that the isotopic signatures we observe are evident in other data sets from this period and conclude that methane was an important component of the atmosphere throughout the Archaean.