Haze clears on ancient Earth's early atmosphere

London's famous smogs were nothing compared with the thick haze that enveloped Earth 2.5 billion years ago. The atmosphere at the time was rich in hydrocarbons, similar to that of Saturn's moon, Titan, today. Just before the atmosphere ditched methane and began accumulating oxygen, though, it appears to have flipped back and forth between its hydrocarbon haze and clear skies.

About 2.4 billion years ago, photosynthesising microbes had at last made enough oxygen for it to accumulate in the atmosphere. But they took their sweet time: photosynthesis had been around for nearly 1 billion years before this "great oxygenation event" happened. The reason for that lag, says biogeochemist Aubrey Zerkle of Newcastle University, UK, is one of the great mysteries of Earth's history.

Now Zerkle and colleagues have dissected the composition of Earth's atmosphere before that oxygenation event. They worked out the ancient atmospheric conditions by analysing the chemical make-up of a core of ocean sediment deposited on a region of South Africa which was flooded between 2.65 and 2.5 billion years ago. This was during a period when microbes ruled a methane-bound world and Earth's continents began forming in earnest.

At certain points during that 150-million-year interval, the researchers found that the oxygen produced by photosynthetic organisms was staying in the ground rather than entering the atmosphere. During these times, methane dominated the atmosphere, resulting in the thick hydrocarbon fog. But at other points, the amount of methane in the atmosphere decreased and the haze probably cleared. It finally disappeared once and for all when the Earth's atmosphere turned permanently to oxygen.

It seems that the Earth was cycling between the two states every few million years ? not a terribly long time in geological terms, Zerkle points out. The reason, her group proposes, is that organisms were making different amounts of methane during this interval, possibly because different amounts of nutrients were available in the ocean.

A sunlight-blocking haze, even one that lasted only a few million years at a time, could certainly have affected the evolution of microbes that depend on light to photosynthesise and contributed to the delay before the final oxygenation of the atmosphere, says biogeochemist Jay Kaufman of the University of Maryland in College Park, who was not involved in the current study. He calls the paper "the best modelling effort that's been done" to understand methane clouds during this period. To be truly convinced, though, he and Zerkle agree that researchers need to sample cores from other areas of the world to ensure that the same patterns are seen everywhere.

Journal reference: Nature Geosciences, DOI: 10.1038/ngeo1425

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