Our planet is a complex, dynamically developing system for more than 4.5 billion years. All the components of this system (the Earth’s solid body, hydrosphere, atmosphere, biosphere), interacting with each other, continuously changed in a difficult, sometimes not obvious relationship. Modern Earth is an intermediate result of this long evolution.
One of the most important components of the system that the Earth represents is the atmosphere, which is in direct contact with the lithosphere, and with the water shell, and with the biosphere, and with solar radiation. At some stages of the development of our planet, the atmosphere was undergoing very significant changes with far-reaching consequences. One of these global changes is called the oxygen catastrophe. The significance of this event in the history of the Earth is extremely great. After all, it was with him that the further paths of the development of life on the planet were connected.
What is an oxygen disaster
The term arose at the beginning of the second half of the 20th century, when, on the basis of the study of Precambrian sedimentation processes, it was concluded that the oxygen content jumps up to 1% of its current amount (Pasteur point). As a result, the atmosphere assumed a stable oxidative character. This, in turn, led to the development of life forms that use significantly more effective oxygen respiration instead of enzymatic fermentation (glycolysis).
Modern studies have made significant refinements to a previously existing theory, showing that the oxygen content on the Earth before and after the Archean and Proterozoic boundaries fluctuated significantly, and in general the history of the atmosphere is much more complicated than previously imagined.
The oldest atmosphere and activity of primitive life
The primary composition of the atmosphere cannot be established with absolute accuracy, and it was unlikely that it was constant in that era, but it is clear that it was based on volcanic gases and the products of their interaction with the rocks of the earth's surface. The fact that among them there could not be oxygen is significant - it is not a volcanic product. The early atmosphere was thus restorative. Almost all atmospheric oxygen has a biogenic origin.
The geochemical and insolation conditions probably contributed to the formation of mats - layered communities of prokaryotic organisms, some of which could already carry out photosynthesis (first anoxigenic, for example, based on hydrogen sulfide). Pretty soon, apparently, already in the first half of the Archean, cyanobacteria mastered high-energy oxygen photosynthesis, which became the culprit of the process, called the oxygen catastrophe on Earth.
Water, atmosphere and oxygen in Archean
It must be remembered that the primitive landscape was distinguished primarily by the fact that it is hardly legitimate to speak of a stable land-sea boundary for that era due to the intensive erosion of land due to the absence of plants. It would be more correct to imagine the often flooded vast areas with an extremely inconsistent coastline, such were the conditions for the existence of cyanobacterial mats.
The oxygen secreted by them — waste products — entered the ocean and the lower, and then the upper layers of the Earth’s atmosphere. In water, he oxidized dissolved metals, primarily iron, in the atmosphere - gases that were part of it. In addition, it was spent on the oxidation of organics. No oxygen accumulation occurred; only local increases in its concentration took place.
Long formation of an oxidizing atmosphere
At present, the oxygen jump in the end of the Archean is associated with changes in the tectonic regime of the Earth (the formation of a real continental crust and the formation of plate tectonics) and the change in the nature of volcanic activity caused by them. Its consequence was a decrease in the greenhouse effect and prolonged Huron glaciation, stretching from 2.1 to 2.4 billion years. It is also known that after the jump (about 2 billion years ago), a decrease in the oxygen content followed, the reasons for which are still unclear.
During almost the entire Proterozoic, up to 800 million years ago, the oxygen concentration in the atmosphere fluctuated, remaining, however, on average very low, although already higher than in the Archean. It is assumed that such an unstable composition of the atmosphere is associated not only with biological activity, but also to a large extent with tectonic phenomena and the regime of volcanism. We can say that the oxygen catastrophe in the history of the Earth stretched almost 2 billion years - it was not so much an event as a long complex process.
Life and oxygen
The appearance of free oxygen in the ocean and atmosphere as a by-product of photosynthesis has led to the development of aerobic organisms capable of assimilating and using this toxic gas in life. This partly explains the fact that for such a long period, oxygen did not accumulate: life forms appeared quite quickly that utilized it.
The oxygen burst at the Archaean – Proterozoic boundary correlates with the so-called Lomagundi-Yatuli event, an isotopic anomaly of carbon that has passed through the organic cycle. Perhaps this surge led to the heyday of early aerobic life, an example of which is the Frenchville biota dating to about 2.1 billion years ago, which includes the supposedly first primitive multicellular organisms on Earth.
Soon, as already noted, the oxygen content fell and further fluctuated near fairly low values. Perhaps the outbreak of life, which caused an increased oxygen consumption, which was still still very small, played a role in this fall? In the future, however, a kind of “oxygen pockets” should inevitably arise, where aerobic life existed quite comfortably and made repeated attempts to “reach the multicellular level”.
The consequences and significance of oxygen catastrophe
So, global changes in the composition of the atmosphere were not, as it turned out, catastrophic in nature. However, the consequences of them really radically changed our planet.
Life forms have arisen that build their life activity on highly effective oxygen respiration, which created the prerequisites for the subsequent qualitative complication of the biosphere. In turn, it would have been impossible without the formation of the ozone layer of the Earth’s atmosphere — another consequence of the appearance of free oxygen in it.
In addition, many anaerobic organisms were unable to adapt to the presence of this aggressive gas in their environment and became extinct, while others were forced to limit their existence to oxygen-free “pockets”. According to the figurative expression of the Soviet and Russian scientist, microbiologist G. A. Zavarzin, the biosphere “turned inside out” as a result of the oxygen catastrophe. The consequence was the second great oxygen event at the end of the Proterozoic, resulting in the final formation of multicellular life.