THE BIOSPHERE 3 



The total of those parts of the lithosphere, the hydrosphere and the 

 atmosphere, in which hfe is present, is called the biosphere. What are its 

 limits ? In the direction of interstellar space it is bounded by the ozono- 

 sphere, at about 20 km from the ground. Towards the earth's core the 

 boundar}' is reached \Q.vy soon at the surface of the continents and is not 

 generally deeper than this by more than 10 m. On the other hand, in the 

 ocean life can be found at depths down to 10,000 m. 



No matter to which theory of the earth's origin one subscribes, it seems 

 probable that there was a time when the terrestrial atmosphere consisted 

 almost entirely of nitrogen and carbon dioxide. It is probable that at the 

 beginning of the earth's history the atmosphere just described had been 

 rapidly transformed into a mixture of oxygen and nitrogen, for the study 

 of the rocks does not show the existence of any marked variations, which 

 shows that the composition of the atmosphere has been much as it is now 

 throughout a great part of geological time. For instance, the state of oxi- 

 dation of pre-Cambrian rocks is not significantly different from that of more 

 recent rocks. The change from an atmosphere of carbon dioxide and 

 nitrogen to one of oxygen and nitrogen is generally considered to be a 

 result of the process of photosynthesis. 



In the course of time, considered on a geological timescale, the atmos- 

 phere has had various chemical substances added to it: volcanic gases 

 (chiefly CO2), oxygen resulting from photosynthesis, COo as a result of 

 metabolism and the decay of organic material, helium-4 produced by the 

 radioactivity of uranium and thorium, argon-40 from the decay of potassium- 

 40. (Natural argon contains the isotopes of mass 36, 38 and 40. Only 

 argon-40 is derived from potassium-40. Argon-40 constitutes 99-63% of 

 natural argon. Similarly for helium : only helium-4 is derived from the 

 a rays emitted by several natural isotopes.) 



On the other hand, subtraction of certain chemical substances has taken 

 place : loss of oxygen by oxidation of iron, sulphur, manganese, etc., 

 loss of COo through formation of carbon, petroleum and dead organisms, 

 loss of CO2 in the formation of carbonates, loss of nitrogen by fixation, loss 

 of nitrogen by formation of oxides by electrical or photochemical action, 

 loss of hvdrogen and helium due to the weakness of the earth's 

 gravitational field, etc. 



The equilibrium between losses and gains which has been obtained is 

 witnessed by the constancy of composition of the atmosphere over extended 

 periods of geological time. In the special case of carbon dioxide the 

 regulatory role of the hydrosphere also plays a part. 



Certain geochemists consider that the hydrosphere remained shallow 

 up to the end of the Paleozoic period, so that the oceans as we know them 

 today have a relatively short histor\\ Others say that the volume of the 

 oceans has not changed much since the pre-Cambrian period. Whichever 



