Early Earth Hazy One Day, Clear the Next

THE GIST

Earth's early atmosphere fluctuated between an oxygen-rich environment and a thick methane haze.

The transitions may have been caused by changes in the rate of methane production by microbes.

A new study of ancient South African rocks indicates Earth may have experienced huge swings in the composition of its early atmosphere.

The study in the journal Nature Geoscience indicates Earth's atmosphere transitioned between an oxygen-rich environment and a thick methane hydrocarbon haze similar to what is now seen on the Saturnian moon Titan.

The work by scientists including Dr Aubrey Zerkle from the University of Newcastle in the United Kingdom, analyzed marine sediments deposited in the Campbellrand-Malmani carbonate platform in South Africa's Ghaap Group.

The platform is one of the oldest on Earth with rocks dating back to between 2.65 and 2.5 billion years ago.

The analyses allowed Zerkle and colleagues to reconstruct the ocean and atmospheric chemistry of the period, finding evidence of oxygen production by microbes.

They also found carbon and sulfur isotopes indicating the oxygen was made in a reduced atmosphere that was periodically rich in methane.

Zerkle and colleagues believe the findings are consistent with previous theories of Earth's early atmosphere having a thick organic haze similar to that on Titan.

However their simulations suggests Earth's atmosphere repeatedly transitioned between two main atmospheric states, one haze free, the other thick in hydrocarbons.

Zerkle and colleagues attribute the transitions to changes in the rate of methane production by microbes.

They say the hydrocarbon haze didn't permanently retreat until the oxygenation of the atmosphere some 100 million years later.

Professor Malcolm Walter from the Australian Center for astro-biology at the University of New South Wales says the paper confirms a very large change in the chemistry of the Earth's surface about 2.65 billion years ago.

"We've known about the basic elements of this for a while, but they've refined the processes that took place at the time," says Walter. "It's hard to read the record in rocks this old."

Walter says the findings are consistent with previous research into 2.7 billion year old stromatolite fossils from the Pilbara region of Western Australia conducted by his team.

"They've done a good job of trying to pin down exactly when things happened, just what happened, how long it happened for and what the chemical processes were."

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