The early Earth flipped back and forth between a hydrocarbon-free atmosphere and a hydrocarbon-rich one similar to that of Saturn's moon, Titan.
This 'see-sawing' atmosphere over 2.5 billion years ago was the result of intense microbial activity, say Newcastle University scientists, and would have had a profound effect on the climate of the Earth system.
"Models have previously suggested that the Earth's early atmosphere could have been warmed by a layer of organic haze. Our geochemical analyses of marine sediments from this time period provide the first evidence for such an atmosphere," says Dr Aubrey Zerkle of the School of Civil Engineering and Geosciences.
"However, instead of evidence for a continuously 'hazy' period we found the signal flipped on and off, in response to microbial activity. This provides us with insight into Earth's surface environment prior to oxygenation of the planet and confirms the importance of methane gas in regulating the early atmosphere."
Zerkle's team analyzed the geochemistry of marine sediments deposited between 2.65 and 2.5 billion years ago in what is now South Africa.
They found evidence of local production of oxygen by microbes in the oceans, but carbon and sulphur isotopes indicate that little of that oxygen entered the atmosphere.
Instead, the authors suggest, the atmosphere transitioned repeatedly between two states: one with a thin, hydrocarbon haze and the other haze-free. This view's borne out by models of the ancient atmosphere performed by colleagues at the NASA Astrobiology Institute.
The bi-stable organic haze stopped appearing when the atmosphere became oxygenated some 100 million years after the sediments were laid down.
"What is most surprising about this study is that our data seems to indicate the atmospheric events were discrete in nature, flip-flopping between one stable state into another," explains co-author Dr James Farquhar.
"This type of response is not all that different from the way scientists think climate operates today, and reminds us how delicate the balance between states can be."