Oxygen levels of the ancient Earth have increased, have fallen several times, says a new study

The Earth 's oxygen levels have increased and decreased more than a hundred million years before the global success of the Great Oxidation Event, there are about 2.4 billion Years, according to a study. The discovery, published in the journal Proceedings of the National Academy of Sciences, may have implications in the search for life beyond the Earth. According to the researchers, the coming years will bring powerful new terrestrial and space telescopes capable of badyzing the atmospheres of distant planets.

The work could help astronomers to unduly exclude "false negatives", or inhabited planets that may not initially appear to be due to undetectable oxygen levels, they said. "The production and destruction of oxygen in the ocean and in the atmosphere over time was a war without evidence of a winner, up to the Great Oxidation," Matt said. Koehler, PhD student at the University of Washington in the United States. The major event of oxidation was the biologically induced occurrence of oxygen in the Earth's atmosphere about 2.45 billion years ago. "These transient oxygenation events were battles in the war, when the scales leaned more in favor of oxygenation," Koehler said. Researchers have confirmed the appearance of oxygen in the Earth's past, about 150 million years ago – about 2.66 billion years ago – and that lasted less than 50 million years old.

They use two different proxies for oxygen-nitrogen isotopes and the element selenium – substances that, each in its path, also speak of the presence of oxygen. "Nitrogen isotopes tell the story of oxygenation of the ocean surface, and this oxygenation extends over hundreds of kilometers across a marine basin and lasts since less than 50 million years, "said Koehler. The team badyzed drill samples taken by Roger Buick, a professor at the University of Washington, in 2012 in another site in the northwestern part of Western Australia called the Jeerinah Formation

. same sedimentary rocks – one core samples sediments deposited in shallower waters and the other sediments taken from deeper waters. The badysis of successive layers in the rocks shows, says Buick, a "gradual" change in isotopes of nitrogen "then returns to zero". This 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 energy. . While selenium suggests oxygen in the air of the ancient Earth, Koehler says that the deep ocean was probably anoxic, or without oxygen, at the time, and that the team found abundant selenium in the shallow hole.

Selenium is retained in sulphide minerals on land, a higher oxygen content of the atmosphere would lead to the leaching of selenium through oxidation – "rock rust" and transport to the sea , says Buick.