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Geoscientists at Newcastle University in the United Kingdom have recently confirmed that the Earth’s atmosphere switched back and forth from being hydrocarbon-rich and hydrocarbon-free- causing the atmosphere to become hazy and clear- repeatedly more than 2.5 billion years ago, during the mid-Precambrian supereon.
The alternation of haziness, resulting from microbial activity (intense enough to effect the Earth’s climate) that produced the hydrocarbons, occurred before the event in which the atmosphere underwent oxygenation- when oxygen became the principal element in the atmosphere- and sometime after the time when methane comprised much of the air.
The geoscientists analyzed marine sediments deposited 2.65 to 2.5 billion years ago in South Africa. They discovered the fossils of local microbes that produced oxygen. But if oxygen was present at the time these microbes existed, where could they have gone? The geoscientists also found carbon and sulphur isotopes, signifying that hardly any oxygen went into the atmosphere and instead stayed in the ground. It was then they ascertained that the atmosphere had not one state, but two, during which a transition took place.
“Instead of evidence for a continuously ‘hazy’ period we found the signal flipped on and off, in response to microbial activity,” Dr. Aubrey Zerkle states in the Newcastle University press release. Dr. Zerkle led the study and is a part of the School of Civil Engineering and Geosciences at Newcastle University.
“This provides us with insight into Earth’s surface environment prior to oxygenation of the planet, and confirms the importance of methane gas in regulating the early atmosphere.”
The geoscientists believe that it took so long for oxygen to dominate the atmosphere because the hydrocarbon haze blocked sunlight, affecting the evolution of microbes that depended on light to photosynthesize. Furthermore, when the amount of hydrocarbons decreased, the amount of methane (which dominated the atmosphere before the haze and haze-free switching) increased. Hence, oxygenation was delayed no matter the concentration of hydrocarbons in the atmosphere.
Evidence for the switch was further supported by models that demonstrated the transitions in action, which came from the NASA Astrobiology Institute, led by Dr. Mark Claire and Dr. Shawn Domagal-Goldman, with whom the geochemists collaborated. These models specifically showed “how the transitions could be caused by changes in the rate of methane production by microbes.”
“[These] models have [also] previously suggested that the Earth’s early atmosphere could have been warmed by a layer of organic haze,” Dr. Zerkle continues.
This discovery has changed aspects of how we come to understand the evolution of Earth and of early life, mainly of how the Earth’s atmosphere went from consisting of methane to oxygen- a concept that made scientists stumble for years.