Atmosphere speculative history

FIGURE 15.7 This speculative history of the atmosphere shows a change from a predominantly reducing to a predominantly oxidizing atmosphere about 2 billion years ago when photosynthetic plants began to appear. Hydrogen has been lost to compound formation and to outer space. Earth is the only planet in the solar system with an oxidizing atmosphere. 

The equilibrium model allows certain inferences to be drawn concerning the geochemical environment which might have existed when sediments were formed, from knowledge of the thermodynamic properties of these sediments today. Thus, one can speculate about the evolution of the lithosphere, hydrosphere, and atmosphere, by assuming that equilibrium states were approached at various stages in geological history. 

At this time in the Earth s history the carbon dioxide abundance was higher than its present value since this gas accumulated in the absence of a biosphere. Rutten (1966) speculated that the atmospheric C02 level was 10 times PAL (present atmospheric level) about 3 x 109 years ago. On the other hand the presence of H20 in the atmosphere led, by photochemical dissociation, to the formation of free radicals and molecular oxygen. An estimate of the importance of these reactions is necessary to give some idea of the oxygen level in pre-biospheric times. The photodissociation of water vapour can be represented as follows (Suess, 1966) ... 

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