The atmosphere of the Earth has lost oxygen

A group of researchers from Denmark, Great Britain and Uruguay, led by Professor Robert Frei from the University of Copenhagens (Kebenhavns Universitet, www.ku.dk ), for the first time managed to restore the detailed picture of the appearance of oxygen -free oxygen atmospheres in the Earth’s Earth. back. In total, the new analysis covers a period of 3.8 billion years (publication in the journal Nature, www.nature.com/doifinder/10.1038/nature08266 ).

According to the previous geochemical studies, oxygen in the initially non-acidic earthly atmosphere began to accumulate 2.45-2.2 billion years ago (at the beginning of the prophene). Geologists respectfully call this event a great oxygen disaster or global oxidation (Great Oxidation Event - GOE). The next noticeable (almost “explosive”, according to geological concepts) the growth of oxygen content in the atmosphere was observed about 750 million years ago. It was not possible to find out exactly how exactly the level 02 in the interval between these events was not possible. Attempts to use the molybdenum isotopes as “markers”, Renia and other metals gave very conflicting results. It remained incomprehensible when oxygen first began to accumulate in the atmosphere, what were the pace of this accumulation and whether there were periods when the level of oxygen, on the contrary, was reduced. Meanwhile, the question of the accumulation of oxygen is unusually important, since it is closely related to the process of appearance and further evolution of life on our planet.

The Frey group conducted its analysis by studying the content of different chromium isotopes in the ancient oceanic sedimentary layers rich in iron. In particular, they managed to show that an increase in the level of atmospheric oxygen 580 million years ago coincided with the most important evolutionary changes in the earth's fauna - the appearance of the first large and quite difficult living organisms that have reached land over time.

According to Frey and his colleagues, oxygen appeared in the surface waters of the ocean already 2.8-2.6 billion years ago-this is at least 200 million years earlier than the results of other isotopic methods. However, the most amazing conclusion can be considered the statement according to which about 1.9 billion years ago, the level of oxygen content suddenly decreased almost to the insignificant amount that the Earth’s atmosphere has cost the great oxygen catastrophe (less than 1% of today's level). That is, the flow of oxygen into the atmosphere in the early stages of the existence of our planet was very unstable (and in the future the content of oxygen was never reduced in such a noticeable way).

The method used in this work to indicate the amount of oxygen in the Earth’s atmosphere is based on the observed dependence of the degree of mobility of the chromium and its compounds on the concentration of atmospheric oxygen. If the amount of oxygen in the atmosphere decreases, then the chromium in rocks is binding, during which each chromium atom loses three electrons and oxidation occurs (in the most stable chromium compounds the degree of its oxidation is +3 (SG3 +)).

On the contrary, when the level of oxygen grows, the metal manganese contained in the same breeds can turn into a manganese oxide, which in turn takes electrons from the associated chromium atoms, transferring it into a 6-band state (SG6+). As a result, the maximum oxidized forms of chromium are most likely to be washed with rainwater and get into the ocean. Once there, chrome reacts with iron and is included in its compounds in its most stable form +3.

It is important to note that the heavy chromium isotope (CR-53) is more washed from breeds to the ocean than lighter CR-52. This means that by comparing the relative amounts of severe and mild chromium isotopes in glandular quartzites, you can find out how much oxygen was in the atmosphere at a time when these isotopes were contained in a bound state in rock.

Of course, the general picture of the interaction of chromium with oxides of manganese, iron, etc. can be more complicated, and the processes of elimination of chromium should be additionally investigated so that all conclusions contained in this work have become indisputable. However, it is already important that scientists have now been able to not only determine the overall trend associated with the accumulation of oxygen, but also track the fluctuations in the level of its content in the atmosphere.

The next natural question: how closely are these fluctuations with the appearance and development of the first living organisms capable of photosynthesis? (Apparently, they initially were the so -called anaerobic microorganisms that did not need oxygen for photosynthesis and, conversely, releasing it from compounds with carbon.) Unfortunately, the final answer to this question has not yet been received.

Maxim Borisov

1. Chromium Isotopes Track Oxygen's Rise (Nature News)
   www.nature.com/news/2009/090909/full/news.2009.901.html
2. Ancient Oceans Offer New Insight Into The Origins of Animal Life (Newcastle University)
   www.physorg.com/news171722551.html