Carbon dioxide h Pa ms

crust or, latterly, by way of volcanoes. Volcanic gases may consist of 80 per cent water vapour, 12 per cent carbon dioxide, 6 per cent sulphur dioxide, for instance, with small amounts of other gases but no free oxygen. The original lack of the oxygen which now comprises just over a fifth of today's suface air, and makes our present atmosphere so distinctive amongst planets, is demonstrated by rocks with grey-green unoxidised iron sulphide, until 1.9 GaBP. Significant amounts of oxygen subsequently are indicated by abundant rocks coloured brown or red by oxidized iron. Thereafter the amount of oxygen increased, as indicated in Figure 1.1.

It remains uncertain how free oxygen first came about. There were probably several mechanisms. One involves photolysis, the splitting of molecules of the volcanic water vapour by solar radiation, thus forming separate hydrogen and oxygen atoms. Hydrogen is light and relatively mobile, and so escaped from the Earth's gravity into space, whilst the oxygen remained. That oxygen would absorb precisely the radiation responsible for its formation by photolysis, so that less and less could form. Such an automatic limitation of further oxygen creation is sometimes called the Urey effect. It would result in an oxygen concentration less than 0.3 per cent of present levels, unless there were other processes creating oxygen.

Further creation depended on photosynthesis, which occurs in some bacteria and in the leaves of plants (Note 1.B). It is the basic process of plant life and involves the combination of carbon dioxide, water and sunlight energy, to form oxygen and kinds of carbohydrate, the buildingblock of plant tissue. Photosynthesis within blue-green algae may have occurred from 3 GaBP, and green-plant photosynthesis from about 2 GaBP. The outcome was an oxygen concentration of about 1 per cent of that now,

Figure 1.1 Estimates of the maximum and minimum possible concentrations of oxygen at various times, as a fraction of the PAL. PAL stands for 'present atmospheric level'. The bold line is a suggested best estimate.

by 1.6 GaBP. Until that time, most of the oxygen released by photosynthesis was captured by unoxidised iron and dissolved in the sea.

The creation of oxygen accelerated with the evolution of plant respiration (Note 1.B), allowing increasingly complicated vegetation, suited to a wider range of environments. The increased area of plants raised oxygen concentrations rapidly, to about 10 per cent of present levels by 1 GaBP (Figure 1.1).

Initially, the lack of oxygen meant that the Sun's sterilising ultra-violet radiation (see Chapter 2) could reach to ground level, preventing the development of any life in exposed situations. As a consequence, life forms could exist only under at least 10 metres of water. Subsequently, life in ever shallower depths became possible as the concentration of atmospheric oxygen rose and protected the living tissue. The shallowness allowed better access to the air's carbon dioxide, needed for photosynthesis, and so yet more oxygen formed. In fact, the atmosphere appears to have had enough oxygen to allow life on land by 0.4 GaBP. As a result, another rapid acceleration of the evolutionary process took place, with an increase of oxygen to present levels by maybe 0.3 GaBP, i.e. 300 million years ago.

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