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.
The increase of the atmospheric oxygen level will be discussed in the following on the basis of the coherent model of Berkner and Marshall (1965 and 1967) in spite of the fact that some conclusions of these authors are not generally accepted. Thus, according to Fisher (1972) the increase of the oxygen level began earlier than in the model mentioned. This idea is supported by the calculations of Schidlowski (1978) who determined the photosynthetic oxygen quantity on the basis of the abundance of organic carbon formed by reaction [2.4].
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).
According to Berkner and Marshall (1965 and 1967) the oxygen content of the Earth's atmosphere about 3 x 109 years ago was very small: 10 ~ 3 PAL (see Section 2.3). It followed from this low oxygen level that the ozone concentration also had to be negligible. Furthermore, the altitude of the maximum ozone formation depends upon the oxygen concentration: it decreases with decreasing oxygen partial pressure. For this reason the small amount of 03 became concentrated in these early times in layers near the Earth's surface. Life was formed at such depths in the liquid water that, on the one hand, the lethal U V radiation was absorbed, but, on the other hand, the radiation with wavelengths greater than 0.29 /im still penetrated. Berkner and Marshall (1965 and 1967) estimate this depth to be about 10 m. The convection which occurred in the water transported the organic materials, produced as a result of UV radiation near the water surface, to this level. However, this convection was not so intensive as to carry the primitive organisms to the surface.
The first organisms were primitive algae and bacteria. The lack of fossils from this early age suggests that, until the Paleozoic era, organisms had no hard parts which would have been preserved in solid crust. As a result of the development of photosynthetic life around the beginning of the Cambrian (600 millions years ago) the atmospheric oxygen level reached 0.01 PAL, which made the conversion from fermentation to respiration possible for some organisms. At the same time, life could be spread even in surface waters. The oxygen concentration of 0.01 PAL is called the first critical level.
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 anomalous composition. This anomalous composition involves a rather strange temperature profile, which is caused by the presence of free oxygen and ozone. Berkner and Marshall speculated that the stratosphere was built up several hundred million years ago. However, according to the more recent study of Ratner and i 10
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