The proportion of oxygen in the Earth's atmosphere remained extremely low after the appearance of the first photosynthetic microbes, due to the fact that the rocks of ancient continents actively absorbed its molecules, preventing them from accumulating in the ocean and air, according to an article published in the journal Nature Geoscience.
“Saturation of the Earth's atmosphere with oxygen could occur at any moment. All that was needed for this was the “correct” chemical composition of the continents. We found that the chemistry of the continents changed dramatically just as oxygen began to accumulate in the planet's primary ocean,”says Matthijs Smit of the University of British Columbia in Vancouver, Canada.
As scientists today believe, the Earth in the distant past did not resemble itself in anything today - there was no oxygen in its atmosphere and there was a lot of carbon dioxide and methane. Its waters, reminiscent of boiling thick soup in temperature and consistency, were inhabited by bizarre extremophile bacteria, traces of which, in the form of deposits of peculiar “blankets” of microbial colonies, are often found by scientists in the oldest rocks of the Earth.
Nobody knows exactly when life was born - there is conflicting evidence that it already existed 3.3-3.7 billion years ago or even 4 billion years ago, in fact, immediately after the completion of the formation of the Earth and the Moon and the end of their "bombardment" large asteroids and comets that brought the "bricks of life" to Earth.
This life, Smith says, lasted until what geologists call the "great oxygen catastrophe." About 2.4-2.32 billion years ago, the oxygen concentration in the atmosphere rose sharply, rising from 0.0001% to 21% today. The reason for its occurrence today is considered to be the first photosynthetic organisms, cyanobacteria, which cleared the atmosphere of CO2 and filled it with oxygen.
On the other hand, as scientists note, it remains unclear what exactly constrained the growth of oxygen concentration in water and in the Earth's atmosphere in those hundreds of millions of years when cyanobacteria already existed in the primary ocean of the planet.
Some scientists suggest that the "extra" oxygen was absorbed by the primary continental rocks of the Earth, formed at a time when there was virtually no oxygen in its atmosphere, while others believe that the role of the "absorber" of oxygen was taken by the remains of living organisms that accumulated at the bottom Earth's oceans are hundreds of millions of years old.
Smith and his colleague Klaus Mezger from the University of Bern (Switzerland) found new evidence in favor of the first hypothesis by analyzing the chemical composition of tens of thousands of crustal samples formed long before the start of the "oxygen catastrophe" and at a time when the proportion of oxygen in the atmosphere grew at the fastest pace.
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For this analysis, the scientists used an ingenious technique - they measured the proportions of chromium and uranium in these rocks, which react differently to the process of destruction of rocks by oxygen and water. Accordingly, the greater these differences, the longer and stronger the elements acted on these rocks, which makes it possible to understand what role the continents of the Earth played in the emergence of its oxygen reserves.
As these measurements showed, the proportions of chromium and oxygen in the continental rocks began to change about three billion years ago, which coincides with the appearance of the first photosynthetic organisms. Approximately 300 million years before the "oxygen catastrophe", their proportion sharply changes, which indicates an equally sharp change from one type of rock to another, which almost did not absorb oxygen. This, according to scientists, was the reason for the onset of the "oxygen catastrophe", which dramatically changed the appearance of the Earth and its first inhabitants and made it suitable for the existence of humans and other modern living beings.