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The Earth 's oxygen levels have increased and decreased more than a hundred million years before the global success of the Grand Oxidation Event there are about 2.4 billion. 39 years, according to a new study from the University of Washington. study that indicates a second "puff" of oxygen in the distant past of the Earth – in the atmosphere and on the surface of much of the ocean – showing that the Earth's oxygenation was a complex process of repeated attempts and failures
Discovery may also have implications for the search for life beyond the Earth. The coming years will bring new powerful telescopes, based on the ground and the space, able to analyze the atmospheres of distant planets. This work could help astronomers to unduly exclude "false negatives" or inhabited planets that might not seem to be due to undetectable oxygen levels.
"The production and destruction of oxygen in the ocean and atmosphere over time," said Matt Koehler, UW PhD student in Earth Science and Technology. Space and leading author of an article published the week of July 9 in the Proceedings. of the National Academy of Sciences .
"These transient oxygenation events were battles in the war, when the scales leaned more in favor of oxygenation."
In 2007, co-author Roger Buick, UW Earth and Space Sciences were part of an international team of scientists who found evidence of an episode – a " "puff" – oxygen between 50 and 100 million years before the big event of oxidation. This is what they learned by drilling deep into the sedimentary rocks of Mt. McRae Shale in Western Australia and analyzing trace metals samples of molybdenum and rhenium, whose accumulation is dependent on the 39, oxygen in the environment.
Now, a team led by Koehler has confirmed a second appearance of this type of oxygen in Earth's past, this time about 150 million years ago – about 2.66 billion years – and that lasts for less than 50 million years. For this work, they used two different proxies for isotopes of oxygen and nitrogen and the element selenium – substances that, each in its path, also speak of the presence of 39, oxygen
"We have in this article another high resolution detection, a transient oxygen burst," Koehler said. "Nitrogen isotopes tell the story of ocean surface oxygenation, and this oxygenation extends over hundreds of kilometers in a marine basin and lasts less than 50 million years." # 39; years. "
The team analyzed samples taken by Buick in 2012 The researchers drilled two cores about 300 kilometers apart, but through the same sedimentary rocks – a core sampled sediments deposited in shallower waters and the other sediments taken from deeper waters. . The analysis of successive layers in the rocky years shows, says Buick, a "gradual" change in isotopes of nitrogen, then returns to zero, which can only be interpreted as meaning that there is oxygen in the environment.
Nitrogen isotopes reveal the activity of certain marine microorganisms that use oxygen to form nitrate and other microorganisms that use this nitrate as a source of nitrogen. ;energy. The data collected from the isotopes of nitrogen sample the surface of the ocean, while selenium suggests oxygen in the air of the ancient Earth. Koehler said that the deep ocean was probably anoxic, or without oxygen, at the time.
The team found abundant selenium in the shallow hole only, meaning that it was coming from the nearby land, not reaching it in deep water. Selenium is contained in sulphide minerals on Earth; Buick said – and transportation to the sea. "This selenium then accumulates in ocean sediments," said Koehler. "Thus, when we measure a spike in selenium abundance in ocean sediments, this could mean a temporary increase in atmospheric oxygen."
The discovery, Buick and Koehler, is also relevant for detecting life on exoplanets "One of the strongest atmospheric biosignatures is oxygen, but this study confirms that" during the transition to a permanent oxygenation of the planet, its surface environments can be oxic for intervals of a few million years only, then "If you fail to detect oxygen in the atmosphere of a planet, that does not mean that the planet is uninhabited or even that it has no photosynthetic life, "she said." In other words , the lack of oxygen can easily be a "false negative" for life. "
Koehler added," You could look at a planet and do not see oxygen – but it could be teeming with microbial life. "
