When did oxygen first appear in Earth’s atmosphere? The answer to this crucial question has evolved over the years. And today, once again, researchers are providing new answers. According to them, it would not have been significantly present before the episode of the Great Oxygenation, 2.3 billion years ago.
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[EN VIDÉO] Kézako: photosynthesis, or how plants produce oxygen Humans and animals would find it difficult to survive without the oxygen generated by plants. This gas is the result of a chemical reaction called photosynthesis, which takes place in the heart of these plants. Unisciel and the University of Lille 1 explain to us, during this episode of Kézako, how this reaction takes place.
Our Earth formed just over 4.5 billion years ago. And scientists have long believed that oxygen did not appear significantly in its atmosphere before the period they call the Great Oxygenation – some also speak of Great oxidation or Oxygen Catastrophe. About 2.3 billion years ago. In question, cyanobacteria who would have started consuming carbon dioxide (CO2) while producing oxygen (O2). The beginnings of photosynthesis.
But for almost 15 years already, some work suggested that the appearance of oxygen in Earth’s atmosphere may be older than that. Even go back 3 billion years at least. Transiently. What researchers call “Breaths of oxygen”.
Today, University of Michigan researchers (United States) come to undermine this hypothesis of “Breaths of oxygen” prior to the Great Oxygenation. According to them, the samples analyzed at the time could also contain minerals formed after their filing. Younger signals mixed with older signals. A story that is complex to disentangle and likely to distort interpretations.
In 2007, researchers had indeed worked, among other things, on shale samples collected on Mount McRae (Australia). They had carried out chemical analyzes on these powdered rocks and found evidence of oxidation and reduction of the molybdenum and sulfur. The new works are based on high-tech resolution. Among which, fluorescence spectroscopy of X-rays.
Ensure the correct interpretation of chemical signatures
The use of this type of method has notably enabled researchers to specify a relative timetable for the processes that led to the formation of these shales. This shows in particular that the identified molybdenum comes from volcanoes. It was indeed concentrated in the shales of Mount McRae at the time of the supposed “Breaths of oxygen”. Then changes physical and chemical transformed the sediment in rocks. In the process, pathways have opened up allowing several fluids to propagate oxidation signals dating actually hundreds of millions of years after the supposed “Breaths of oxygen”.
The work of researchers at the University of Michigan therefore provides an alternative explanation for the composition of the Mount McRae shales. They also confirm that the atmospheric oxygen level before the Great oxygenation was very low. It could even be called “Negligible” in the approximate period 150 million years before this abrupt change.
Results which thus call into question theearly existence of cyanobacteria. Rather, they support the hypothesis that oxygen-generating photosynthesis only evolved shortly before the great oxygenation event. And beyond being of interest to scientists who study the Earth, they could also attract the attention of those who probe other planets. With no doubt, a general reflection to come on how the chemical signatures of rocks billions of years old should be analyzed and interpreted.
The atmosphere enriched with oxygen sooner than expected
Over two billion years ago, the level of oxygen in the atmosphere sharply increased. Photosynthetic organisms are accused of this large-scale pollution. But it would have started tens of millions of years before, which complicates the explanation and makes play a role in geological phenomena.
Article by Jean-Luc Goudet published on 09/28/2007
” We thought these scans were going to be boring »Admits Gail Arnold, one of the scientists who took part in this work, which could indeed seem tedious. During the summer of 2004, a team was formed to study section by section a carrot 908 meters long, from a borehole in rocks in the Hamersley region, Western Australia. The researchers were particularly interested in the molybdenum content, rhenium and in uranium. The amount of these three elements in the sediment is highly dependent on the oxygen concentration in the environment.
The researchers did not expect any surprise in this routine work. Error. By attacking the carrot at levels corresponding to the Late Archean, around 2.5 billion years before the present, American scientists were surprised to find an oxygen content much higher than expected. At that time, in fact, the Earth’s atmosphere, it was assumed, was still very poor in oxygen, as it had always been since the beginning of Earth’s history 4.56 billion years ago. We know that, suddenly, between 2.4 and 2.3 billion years ago, the oxygen concentration increased rapidly, reaching the current 21%. This ” Great oxidation “Or” Oxygen disaster “, to use the expressions in use, was probably caused by cyanobacteria, the first organisms to draw their energy of the light solar through photosynthesis.
By recovering carbon from gas Carbonic, these tiny beings produced a terrible pollutant, oxygen, which caused a massacre in living beings. The cyanobacteria themselves probably struggled to endure this molecule chemically aggressive. The fact that they succeeded already suggested that they might have been able to adapt to it before.
Volcanoes and living beings, these big polluters
At the end of last year, researchers hypothesized thathydrogen (H2O2, L’oxygenated water) could have been produced in large quantities during an episode of glaciation global Earth (an event called a “snowball”). Preventing the maintenance of a ozone layer, this cooling would have left the rays ultraviolet hitting the Earth’s surface, turning water into hydrogen peroxide, released into the atmosphere. The cyanobacteria would thus have learned gradually and early to protect themselves against an oxidizing environment.
The Australian carrot tells a different story: the oxygen itself has already skyrocketed in the atmosphere – the authors speak of a breath of oxygen – 50 to 100 million years before the Great Oxidation . Why ? The question remains open. Some already imagined that photosynthetic organisms must have started producing oxygen before the Great Oxidation. Others confirm by explaining that geological phenomena (interaction with the gases emitted by the volcanism very active) destroyed this oxygen (these are of course geologists).
In the end, we realize that we still have very little understanding of this extraordinary event which breathed so much oxygen into the atmosphere and that we do not know better how to explain why this rate then stabilized at 21% with a so beautiful precision. The conclusion is that a better understanding of the interactions between geological and biological phenomena remains to be done. A little more multidisciplinarity will undoubtedly be necessary for scientists to tell the story of the Earth …
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