Our planet is a complex system that has been dynamically developing for more than 4.5 billion years. All components of this system (the solid body of the Earth, hydrosphere, atmosphere, biosphere), interacting with each other, continuously changed in a complex, sometimes non-obvious relationship. The modern Earth is an intermediate result of this long evolution.
One of the most important components of the system that the Earth 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 has undergone very significant changes with far-reaching consequences. One such global change is called the oxygen catastrophe. The significance of this event in the history of the Earth is exceptionally great. After all, it was with him that the further development of life on the planet was connected.
What is an oxygen catastrophe
The term arose at the beginning of the second half of the 20th century, when, based on the study of the processes of Precambrian sedimentation,conclusion about the abrupt increase in the oxygen content up to 1% of its current amount (Pasteur points). As a result, the atmosphere assumed a steadily oxidizing character. This, in turn, led to the development of life forms that use much more efficient oxygen respiration instead of enzymatic fermentation (glycolysis).
Modern research has made significant refinements to the previously existing theory, showing that the oxygen content on Earth both before and after the Archean-Proterozoic boundary fluctuated significantly, and in general the history of the atmosphere is much more complicated than previously thought.
Ancient atmosphere and activities 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. It is significant that among them there could not be oxygen - it is not a volcanic product. The early atmosphere was thus restorative. Almost all atmospheric oxygen is of biogenic origin.
Geochemical and insolation conditions probably contributed to the formation of mats - layered communities of prokaryotic organisms, and some of them could already carry out photosynthesis (first anoxygenic, for example, based on hydrogen sulfide). Quite soon, apparently already in the first half of the Archean, cyanobacteria mastered high-energy oxygen photosynthesis,which became the culprit of the process, which received the name of the oxygen catastrophe on Earth.
Water, atmosphere and oxygen in the 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 the land due to the lack of plants. It would be more correct to imagine vast areas often flooded with a highly unstable coastline, such were the conditions for the existence of cyanobacterial mats.
The oxygen released by them - waste products - entered the ocean and into the lower, and then into the upper layers of the Earth's atmosphere. In water, he oxidized dissolved metals, primarily iron, in the atmosphere - the gases that were part of it. In addition, it was spent on the oxidation of organic matter. No accumulation of oxygen occurred, only local increases in its concentration took place.
Long establishment of an oxidizing atmosphere
At present, the oxygen surge of the end of the Archean is associated with changes in the Earth's tectonic regime (formation of the real continental crust and formation of plate tectonics) and the change in the nature of volcanic activity caused by them. It resulted in a decrease in the greenhouse effect and a long Huron glaciation, which lasted from 2.1 to 2.4 billion years. It is also known that the jump (about 2 billion years ago) was followed by a drop in the oxygen content, the reasons for which are still unclear.
During almost the entire Proterozoic, up to 800 million years ago, the concentration of oxygen 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 for 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 oxygen did not accumulate for such a long period: life forms appeared rather quickly to utilize it.
The oxygen burst at the Archean-Proterozoic boundary correlates with the so-called Lomagundi-Yatulian event, an isotopic anomaly of carbon that has passed through the organic cycle. It is possible that this surge led to the rise of early aerobic life, as exemplified by the Francville biota dated to about 2.1 billion years ago, which includes supposedly the first primitive multicellular organisms on Earth.
Soon, as already noted, the oxygen content dropped and then fluctuated around fairly low values. Maybe a flash of life that caused an increased consumption of oxygen,which was still very small, played some role in this fall? In the future, however, some kind of “oxygen pockets” were bound to arise, where aerobic life existed quite comfortably and made repeated attempts to “reach the multicellular level.”
Consequences and significance of the oxygen catastrophe
So, the global changes in the composition of the atmosphere were not, as it turned out, catastrophic. However, the consequences of them really radically changed our planet.
Life forms emerged that build their life activity on highly efficient oxygen respiration, which created the prerequisites for the subsequent qualitative complication of the biosphere. In turn, it would not have been possible 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 could not adapt to the presence of this aggressive gas in their habitat and died out, while others were forced to limit themselves to existence in 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 of this was the second great oxygen event at the end of the Proterozoic, which resulted in the final formation of multicellular life.
