Increase of the oxygen level during geological times

5 Increase of the oxygen level during geological times 

There is considerable documentation in the literature to show that the major part of the increase in the oxygen level during geological time was due to the photosynthetic activity of the biosphere, which can be summarized as follows  

A part of the biosphere, specifically plants, uses water and carbon dioxide according to this equation to synthesize organic materials under the influence of solar radiation of appropriate wavelengths. An important peculiarity of this reaction is that the 02 on the right-hand side of [2.4] comes from the oxygen of the water. 

It has already been mentioned that life came into being in a reducing environment. The constituents of the reducing atmosphere (e.g. methane, ammonia) were absorbed in waters to produce a solution in which organic compounds were formed under the effect of electric discharges and short-wave radiations (Miller, 1953). The first anaerobic organisms originated from these compounds (e.g. Bernal, 1967). 

In the Cambrian period life began to develop very quickly. For this reason the oxygen concentration increased rather rapidly. Thus, in the late Silurian (420 millions years ago) the oxygen level was as high as 0.1 PAL (Fig. 3) which is termed the second critical level. With the increase of the oxygen concentration the quantity of ozone in the atmosphere increased, together with an increase in the altitude of maximum ozone production. This latter, in the late Silurian period reached 20 km level, which made the spread of life onto dry land possible. At the same time the thermal structure of the atmosphere was drastically changed, which resulted in the appearance of the stratosphere. It was shown previously that our atmosphere has an 

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