Archaean haze

The Earth’s atmosphere periodically transitioned between a hydrocarbon haze and haze-free conditions 2.65-2.5 billion years ago, reports a paper published online this week in Nature Geoscience. The haze permanently retreated following the oxygenation of the atmosphere approximately 100 million years later. Aubrey Zerkle and colleagues analysed 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, but carbon and sulphur isotopes indicate that little of that oxygen entered the atmosphere. Instead, the authors suggest that the atmosphere transitioned repeatedly between two states: one with a thick, hydrocarbon haze and the other haze-free. The team attributes the transitions to changes in the rate of methane production by microbes.