Today, 21% of the air we breathe consists of molecular oxygen. But this gas was not always in such quantity, could not always support life - moreover, it was practically absent in the atmosphere for the first 2 billion years of Earth’s history. When did oxygen first start collecting on Earth? Scientists at MIT have found the answer. In a paper published recently in Science Advances, a group of scientists reported that the Earth’s atmosphere experienced the first tangible and irreversible injection of oxygen about 2.33 billion years ago. This period marked the beginning of the Great Oxygenation - an event after which oxygen began its victorious offensive on Earth.
Scientists also determined that this initial increase in atmospheric oxygen, albeit small, occurred in just 1-10 million years and caused a series of events that subsequently led to the proliferation of multicellular life.
“This is the beginning of a very long period that has turned into a difficult life,” says Roger Sammons, senior author and professor at the Department of Earth, Atmospheric and Planetary Sciences at MIT. - It took about 1.7 billion years for animals to evolve, similar to what we have today. But the presence of molecular oxygen in the ocean and the atmosphere means that organisms that breathe oxygen could thrive. ”
The air smelled of oxygen
By and large, scientists agree that oxygen, despite a shortage in the atmosphere, most likely was boiled in the ocean as a byproduct of cyanobacteria photosynthesis already 3 billion years ago. But, as Sammons notes, oxygen in the ancient ocean was “instantly absorbed” by hungry microbes, bivalent iron, and other volunteers, preventing it from escaping into the atmosphere.
“There could have been oxygen leaks in the air before, but their duration and content cannot currently be measured,” says Sammons.
Everything changed with the period of Great oxygenation, which marked the beginning of the constant presence of oxygen in the atmosphere. Previous estimates placed the beginning of the HE at about 2.3 billion years ago with uncertainties of tens or hundreds of millions of years.
"The dating of this event has remained rather inaccurate until now," says Sammons.
Forced transition
In order to accurately determine the course of HE, the colleagues of Sammons first analyzed the rocks of that period in search of a specific sulfur isotope pattern. When volcanoes erupt, they emit sulfur gases, which can be chemically and isotopically separated by ultraviolet radiation. The structure of the isotopes produced in this process depends on whether oxygen is present above a certain threshold or not.
Scientists tried to isolate a major transition in a particular isotope pattern series - the mass-independent fraction of sulfur isotopes (S-MIF), in order to determine when oxygen first appeared in the Earth's atmosphere. To do this, they studied the sedimentary core collected during the expedition of scientists to South Africa.
"Zhenmin Luo is a very diligent guy," says Sammons, about another scientist who participated in the writing of this work. “He found traces of S-MIF in deep rocks, the absence of these marks in shallow rocks, but nothing between them.” So he went back to South Africa. ”
There, he took samples from the rest of the sediment core and from two nearby and found out that the S-MIF transition - meaning the permanent oxygen-crossing of the above threshold - occurred 2.33 billion years ago, plus or minus 7 million years. Uncertainty is much lower when compared with previous estimates.
Also, scientists have discovered a large fractionation of sulfur isotopes-34, which indicates an increase in the level of marine sulphates at the same time. Such sulphate should have appeared as a result of the reaction between atmospheric oxygen with sulphide minerals on land, and also with sulfur dioxide from volcanoes. This sulphate was then used by ocean dwellers, sulphate-breathing bacteria, to produce a specific pattern of sulfur-34 in the underlying layers of sedimentary rocks that were dated between 1 and 10 million years after the S-MIF transition.
These results suggest that the initial accumulation of oxygen in the atmosphere was relatively fast. Since its first appearance 2.33 billion years ago, oxygen has accumulated in sufficiently high concentrations to exert a weathering effect on the rocks already after 10 million years. This process of weathering leached more sulphate and some metals into the water and, therefore, into the oceans. Sammons points out that some time has passed before the earth system reached a stable state by dumping organic carbon and exceeded the oxygen threshold needed to further stimulate biological evolution.
“Complicated life could not establish itself on the planet as long as oxygen remained the lot of the ocean depths,” says Sammons. - And it took a lot, a lot of time. But this is the first step in a whole series of processes. ”
Now that scientists have limited the timing of VO, Sammons hopes to discover other clues that will lead to the cause or mechanism of this event. One of the hypotheses that scientists want to explore is the relationship between the sudden and rapid emergence of oxygen and the “Earth-Snowball”, a period when the continents and oceans of the Earth were mostly covered with ice.
In addition, you need to understand why our 21% of our oxygen in the atmosphere has remained so stable for quite a long time.
The article is based on materials .
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