The amount of oxygen in the atmosphere would have long stagnated at particularly low levels, impacting the development of the biosphere. New results that could lead to a revision of the search strategies for life on exoplanets.
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[EN VIDÉO] Kézako: photosynthesis, or how plants produce oxygen Humans and animals would be hard pressed 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.
Life on earth is intimately linked to the oxygen present in theatmosphere and the oceans. However, if today our Planet has the particularity of counting 21% of oxygen in its atmosphere, this has not always been the case. Far from there !
If we look at the history of the Earth, we realize that this high oxygen level is relatively recent, and that the vast majority of this history is dominated by rather anoxic conditions, that is, with very little oxygen. If life was already present during these earlier stages, this low oxygen level seems to have been a highly limiting factor in the development of the biospherefor several billion years.
When did the oxygen levels on Earth reach a sufficient level to allow the expansion and complexification of living organisms? This is one of the questions that researchers from McGill University (Canada) have tried to answer.
1 billion years ago, the oxygen level was 1% of today’s
To confirm this point, the researchers analyzed samples of sedimentary rocks very old (of age Proterozoic), rich in iron, from various locations around the globe. These rocks bear witness to the environmental conditions that prevailed in a shallow marine environment, of the coastal type, 1 to 2 billion years ago. Chemical analysis of the oxidized iron contained in these rocks has allowed scientists to estimate the amount of oxygen present in this medium when these rocks were formed. The results show that the rates were terribly low at that time and were less than 1% of today’s rates for the same type of environment.
The study also shows that the increase in oxygen levels in the atmosphere took place in stages, and that these are correlated with the different major stages in the evolution and expansion of terrestrial ecosystems. However, these low oxygen conditions would have persisted stably for almost a billion years, before the amount of O2 begins to climb rapidly, 750-900 million years ago. This transition correlates closely with the appearance of much more complex organisms. If life was indeed present before the increase in oxygen levels, these environmental conditions certainly slowed down its development and expansion over a long period.
Search for ozone rather than oxygen as a biological tracer on exoplanets
These results, to be published in the journal PNAShave important implications for the exploration of life on other planets. Searching for traces of oxygen in the atmospheres of exoplanets is currently one of the ways of considering the presence of extraterrestrial life, past or present. Oxygen is indeed considered a biosignature. Except that the results of the study show that the atmospheres of telluric planets could stabilize, like the Earth, at relatively low oxygen levels. However, it is difficult to detect such low rates. The authors therefore propose that the best chance of detecting oxygen in an exoplanet’s atmosphere is to search for its byproduct, ozone.
L’ozone is in fact more easily detectable, even in small quantities, thanks to its ability to absorb ultraviolet. A process that would perhaps increase our chances of finding signs of life on other planets, outside the Solar system.
Earth was rich in oxygen two billion years ago
At that time, researchers believed that oxygen levels on Earth were declining. But evidence found in a drill core suggests otherwise. Two billion years ago, our planet was rich in oxygen.
Article of Nathalie Mayer published on May 14, 2020
Two billion years ago, the Earth was much more rich in oxygen than scientists thought. What pave the way for the evolution of complex forms of life. This is the conclusion of a study conducted by University of Alberta researchers (Canada) on a Russian drill core.
The researchers analyzed the core containing shungite, a sedimentary rock rich in carbon deposited at a time when oxygen levels on our planet were rapidly declining. At least according to the models of carbon cycles and oxygen established. After what the paleontologists call the Lomagundi-Jatuli event which occurred between 2,220 and 2,060 million years ago.
Conditions favorable to life
“What we found contradicts the prevailing opinion”comments Kaarel Mänd, PhD student, in the communicated. the material that he has analyzed indeed presents unequaled levels — with others of the primeval land — of molybdenumofuranium and of rhenium especially. But these metals are only considered common in the oceans and sediment than when oxygen, itself, is abundant in the Earth’s oceans.
This discovery also offers insight into the evolution of complex life forms on our planet. The eukaryotes, indeed — considered the precursors of all complex life — need a high level of oxygen to thrive. And this study reinforces the idea that favorable conditions for their evolution may have appeared on Earth much earlier than researchers thought.
Two billion years ago, atmospheric oxygen collapsed!
Not only was there already oxygen in the Earth’s atmosphere 2.8 billion years ago, but the O2 fluctuated significantly before the Cambrian explosion. These surprising results come from a team of geochemists who studied isotopes chromium trapped in rocks deposited in the oceans during thearchaean.
Article of Laurent Sacco released on 09/11/2009
Reality is always more complex than the weak capacities of the brain of the’Homo Sapiens. This is a rule that the history of science never ceases to verify… We have a new example with the publication in Nature of an article dealing with the variations in the rate of oxygen in the atmosphere of the planet during the period known as the Archaean.
Until now, it was thought that this oxygen level only began to increase during a period extending between -2.45 and -2.2 billion years approximately. It is only then that photosynthetic organisms, such as those responsible for stromatolites, would have become numerous enough to release large amounts of oxygen into the oceans. We have proof of this with the famous banded iron deposits (in English, band iron trainingabbreviated as BIF) who were deposited during this period.
Indeed, the world ocean then contained a lot of iron in solution and this naturally reacted with the massively released oxygen to then precipitate. At that time, the beaches therefore had the color of the rust. Geoscientists have designated this event by the term Great Oxidation, or even of oxygen crisis because for many living organisms of the time, such a quantity of oxygen was toxic.
As the iron was soon no longer able to capture the released oxygen, it began to outgas into the atmosphere from the saturated ocean. The oxygen level in the atmosphere would then have remained relatively constant until a second event that occurred between -800 and -542 million years ago. The atmosphere was then suddenly enriched in oxygen and, strangely, it was towards the end of this period that the famous Cambrian explosion occurred.
Before the oxygen crisis, the atmosphere contained quite a bit of O2 but it came from chemical reactions linked to the photodissociation of molecules of atmospheric water under the action of solar radiation. The quantity was infinitesimal and as the stromatolites had only appeared shortly before the Great Oxidation, there were no sources to produce large amounts of oxygen sooner. At least that’s what we thought…
Amazing fluctuations
In order to follow more precisely the evolution of the oxygen level in the atmosphere, an international team of geochemists focused on the isotopes of chromium trapped in the iron-rich sediments. The amount of these isotopes in seawater depends on the rate of weathering and erosion of continental rocks by rainwater, which in turn is related to the level of oxygen in the atmosphere. L’variation study fines of the rate of trapped isotopes seems to be a good indicator of variations in the rate of atmospheric oxygen and, as the researchers explain in Nature, the results were amazing.
First surprise, the rate of O2 began to increase earlier than imagined, -2.8 billion years ago. Then, after the oxygen crisis, the rate of this gas dropped !
According to Simon Poulton from the University of Newcastle, one of the authors of the article Nature, the increase in the rate of oxygen in the atmosphere -2.8 billion years ago shows a very unstable situation with fluctuations and brief episodes of appearance of free oxygen in the atmosphere before this fall. On the other hand, the data again strongly support that the Cambrian explosion was indeed concomitant with a sudden rise in the level of oxygen in the Earth’s atmosphere.
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