Oxygen levels on the ancient Earth have increased, have fallen many times before a great oxidation even

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. 39 years, according to a new study from the University of Washington.

The evidence comes from a new study that indicates a second and much earlier "puff" of oxygen in the distant past of Earth – in the atmosphere and on the surface from a large expanse of ocean – showing that the oxygenation of the Earth was a complex process of repeated attempts and failures over a long period of time.

Discovery can 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 badyze 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 the 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 The 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. What they learned by digging deep into the sedimentary rock of Mount McRae Shale in Western Australia and badyzing the samples for trace metals molybdenum and rhenium, whose accumulation depends on oxygen in l & # 39; environment.

Now, a team led by Koehler has confirmed a second occurrence of this type of oxygen in Earth's past, this time around 150 million years ago – or about 2.66 billion years – and that lasts for less than 50 million years. For this work, they used two different proxies for oxygen-nitrogen isotopes and the element selenium-substances that, each in its path, also speak of the presence of oxygen

"What we have in this article is another detection, high resolution, a puff of transient oxygen, "said Koehler. "Nitrogen isotopes tell the story of oxygenation of the ocean surface, and this oxygenation extends over hundreds of kilometers in a marine basin and lasts less of 50 million years. "

The team badyzes samples taken by Buick in 2012 The researchers drilled two cores about 300 kilometers apart, but at the same time through the same sedimentary rocks: a core samples sediments deposited in shallower waters and the other sediments taken from deeper waters. The badysis 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. "

The isotopes of nitrogen reveal the activity of some marine microorganisms that use oxygen to form nitrate, as well as other microorganisms that use this nitrate As a source of energy, data collected from nitrogen isotopes sample the surface of the ocean, while selenium suggests oxygen in the air from the ocean. 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 next land, not reaching it in deep water .The selenium is contained in sulphurous minerals on earth; Buick said and transported it to the sea. "This selenium is the only one in the world. then accumulates in ocean sediments, "said Koehler." So when we measure a spike in selenium abundance, In oceanic sediments, this could mean a temporary increase in atmospheric oxygen. "

The discovery, says 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 permanent oxygenation of the planet, its surface environments may be oxic for intervals." of a few million years only, then "If you can not detect oxygen in the atmosphere of a planet, it does not mean that the planet is uninhabited or even that. She has no photosynthetic life. "In other words, 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."

Learn more:
Rock samples suggest that oxygen levels during the Lomagundi Event were high enough to support the progress of life's development.

More information:
Matthew C. Koehler et al., "Transient oxygenation of the ocean surface recorded in the Jeerinah Formation ~ 2.66 Ga, Australia" PNAS (2018). www.pnas.org/cgi/doi/10.1073/pnas.1720820115