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Oxygen Surprise in Earth’s Ancient Atmosphere

An analysis of fossilised space dust collected form the Pilbara has challenged the accepted view that the Earth’s atmosphere was oxygen-poor 2.7 billion years ago.

The study, published in Nature (http://tinyurl.com/hk6msxw), found that the ancient Earth’s upper atmosphere contained about the same amount of oxygen as today, and that a layer of methane haze separated this oxygen-rich upper layer from the oxygen-starved lower atmosphere.

The international team extracted micrometeorites from samples of ancient limestone collected in the Pilbara region in Western Australia, and examined them at the Monash Centre for Electron Microscopy and the Australian Synchrotron.

“We found that most of the micrometeorites had once been particles of metallic iron – common in meteorites – that had been turned into iron oxide minerals in the upper atmosphere, indicating higher concentrations of oxygen than expected,” said Dr Andrew Tomkins of Monash University. “This was an exciting result because it is the first time anyone has found a way to sample the chemistry of the ancient Earth’s upper atmosphere,” he said.

Imperial College researcher Dr Matthew Genge calculated that oxygen concentrations in the upper atmosphere would need to be close to modern-day levels to explain the observations. “This was a surprise because it has been firmly established that the Earth’s lower atmosphere was very poor in oxygen 2.7 billion years ago,” Genge said. “How the upper atmosphere could contain so much oxygen before the appearance of photosynthetic organisms was a real puzzle.”

Tomkins explained that the new results suggest the Earth at this time may have had a layered atmosphere with little vertical mixing, and higher levels of oxygen in the upper atmosphere produced by the breakdown of CO2 by ultraviolet light.

“A possible explanation for this layered atmosphere might have involved a methane haze layer at middle levels of the atmosphere. The methane in such a layer would absorb UV light, releasing heat and creating a warm zone in the atmosphere that would inhibit vertical mixing,” Tomkins said.

“It is incredible to think that by studying fossilised particles of space dust the width of a human hair, we can gain new insights into the chemical make-up of Earth’s upper atmosphere billions of years ago,” Tomkins said.

“The next stage of our research will be to extract micrometeorites from a series of rocks covering over a billion years of Earth’s history in order to learn more about changes in atmospheric chemistry and structure across geological time. We will focus particularly on the great oxidation event, which happened 2.4 billion years ago when there was a sudden jump in oxygen concentration in the lower atmosphere.”