AsianScientist (May 19, 2016) – Using the oldest fossil micrometeorites, or space dust, ever found, researchers in Australia have made a surprising discovery about the chemistry of Earth’s atmosphere 2.7 billion years ago. Their findings have been published in Nature.
The team from Monash University have found that ancient Earth’s upper atmosphere contained about the same amount of oxygen as today—challenging the accepted view that Earth’s ancient atmosphere was oxygen-poor. Furthermore, a methane haze layer separated this oxygen-rich upper layer from the oxygen-starved lower atmosphere.
According to lead scientist, Dr. Andrew Tomkins from the School of Earth, Atmosphere and Environment, the team extracted micrometeorites from samples of ancient limestone collected in the Pilbara region in Western Australia.
“Using cutting-edge microscopes, 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,” Tomkins said.
“This was an exciting result because it is the first time anyone has found a way to sample the chemistry of ancient Earth’s upper atmosphere.”
According to calculations by Imperial College researcher Dr. Matthew Genge, one of the scientists in the study, 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; how the upper atmosphere could contain so much oxygen before the appearance of photosynthetic organisms was a real puzzle,” Genge said.
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 ultraviolet 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 fossilized particles of space dust the width of a human hair, we can gain new insights into the chemical makeup of Earth’s upper atmosphere billions of years ago.”
The next steps for the team, according to Tomkins, will involve extracting micrometeorites from a series of rocks in order to learn more about changes in atmospheric chemistry and structure across geological time. They 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.
The article can be found at: Tomkins et al. (2016) Ancient Micrometeorites Suggestive of an Oxygen-Rich Archaean Upper Atmosphere.
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Source: Monash University; Photo: Shutterstock.
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