A number of hypotheses have been used to explain how free oxygenfirst accumulated in Earth's atmosphere some 2.4 billion years ago, buta full understanding has proven elusive. Now a new model offersplausible scenarios for how oxygen came to dominate the atmosphere, andwhy it took at least 300 million years after bacterial photosynthesisstarted producing oxygen in large quantities.
The big reason for the long delay was that processes such asvolcanic gas production acted as sinks to consume free oxygen before itreached levels high enough to take over the atmosphere, said MarkClaire, a University of Washington doctoral student in astronomy andastrobiology. Free oxygen would combine with gases in a volcanic plumeto form new compounds, and that process proved to be a significantoxygen sink, he said.
Another sink was iron delivered to the Earth's outer crust bybombardment from space. Free oxygen was consumed as it oxidized, orrusted, the metal.
But Claire said that just changing the model to reflectdifferent iron content in the outer crust makes a huge difference inwhen the model shows free oxygen filling the atmosphere. Increasing theactual iron content fivefold would have delayed oxygenation by morethan 1 billion years, while cutting iron to one-fifth the actual levelwould have allowed oxygenation to happen more than 1 billion yearsearlier.
"We were fairly surprised that we could push the transition abillion years in either direction, because those levels of iron in theouter crust are certainly plausible given the chaotic nature of howEarth formed," he said.
Claire and colleagues David Catling, a UW affiliate professorin atmospheric sciences, and Kevin Zahnle of the National Aeronauticsand Space Administration's Ames Research Center in California willdiscuss their model tomorrow (Aug. 9) in Calgary, Alberta, during theGeological Society of America's Earth System Processes 2 meeting.
Earth's oxygen supply originated with cyanobacteria, tinywater-dwelling organisms that survive by photosynthesis. In thatprocess, the bacteria convert carbon dioxide and water into organiccarbon and free oxygen. But Claire noted that on the early Earth, freeoxygen would quickly combine with an abundant element, hydrogen orcarbon for instance, to form other compounds, and so free oxygen didnot build up in the atmosphere very readily. Methane, a combination ofcarbon and hydrogen, became a dominant atmospheric gas.
With a sun much fainter and cooler than today, methane buildupwarmed the planet to the point that life could survive. But methane wasso abundant that it filled the upper reaches of the atmosphere, wheresuch compounds are very rare today. There, ultraviolet exposure causedthe methane to decompose and its freed hydrogen escaped into space,Claire said.
The loss of hydrogen atoms to space allowed increasinglygreater amounts of free oxygen to oxidize the crust. Over time, thatslowly diminished the amount of hydrogen released from the crust by thecombination of pressure and temperature that formed the rocks in thecrust.
"About 2.4 billion years ago, the long-term geologic sourcesof oxygen outweighed the sinks in a somewhat permanent fashion," Clairesaid. "Escaping to space is the only permanent escape that we envisionfor the hydrogen, and that drove the planet to a higher oxygen level."
The model developed by Claire, Catling and Zahnle indicatesthat as hydrogen atoms stripped from methane escaped into space,greenhouse conditions caused by the methane blanket quickly collapsed.Earth's average temperature likely cooled by about 30 degrees Celsius,or 54 degrees Fahrenheit, and oxygen was able to dominate theatmosphere because there was no longer an overabundance of hydrogen toconsume the oxygen.
The work is funded by NASA's Astrobiology Institute and theNational Science Foundation's Integrative Graduate Education andResearch Traineeship program, both of which foster research tounderstand life in the universe by examining the limits of life onEarth.
"There is interest in this work not just to know how an oxygenatmosphere came about on Earth but to look for oxygen signatures forother Earth-like planets," Claire said.
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