2 LIVINGSTONE 



No pools in the carbon cycle seem to be known with satisfactory accuracy. 

 Possible exceptions are the amount of carbon in the atmosphere and the amount 

 dissolved in seawater, but even these are not well enough known for some 

 purposes. For example, it would be highly desirable to have a much wider net of 

 stations measuring the atmospheric carbon dioxide and many more data for the 

 equilibrium carbon dioxide of surface ocean water. 



The other major pools are known even less well than the dissolved carbon in 

 atmosphere and ocean. There is no general agreement on the total mass of plants 

 and animals, living or dead. Leafing through a few standard texts and 

 encyclopedias, we find estimates for the size of the biosphere in a non-Val- 

 lentynian sense that range from 0.58 X 10 18 (Takahashi 4 ) to 2.8 X 10 18 g 

 (Borchert 5 ). Rankama and Sahama 6 give the mass of plants and animals as 

 20.4 X 10 18 g, but that is presumably on a wet-weight basis. To my knowledge, 

 no estimate of the total carbon in groundwater has ever been made. Even data 

 on carbonate equilibrium conditions in groundwater are very scarce (White, 

 Hem, and Waring, 1963 7 ). These are large and reactive carbon pools, and it is 

 unsettling that they should be so poorly known. 



Technical and geophysical developments of the past decade have drawn 

 attention to interactions between the earth and outer space and to plate 

 tectonics of the sea floor. Both these matters have geochemical implications but 

 are seldom treated explicitly in geochemical budgets. It is comforting to report, 

 on the basis of very crude computations, that the gain of carbon in the form of 

 meteorites seems negligibly small and tnat loss of carbon from the reactive 

 surface of the earth by sea-floor spreading, although not negligible, is not 

 ruinously large. 



The rate of sea- floor spreading seems to be between 1 and 10 cm/year, and 

 the thickness of the sediment on older parts of the sea floor seems to be some 

 300 m. The total length of the subduction zones is not known. In fact, even the 

 location of subduction zones is established with much less certainty than the 

 spreading that seems to require them. It might not be unreasonable, however, to 

 assume a length of subduction zone for the world of no more than 44,000 km. 

 This is about the length of the Pacific ring of fire which consists in large part of 

 deep-sea trenches which are probably subduction zones and of young mountain 

 belts which may be subduction zones. There is enough length left over to be 

 roughly equal to the combined length of the shorter convergences in other parts 

 of the world. If we assume a mean density of 2.4 for the down-welling rocks and 

 the carbon content of 1% given by Garrels and Mackenzie 8 for modern 

 sediments, we reach an estimate of some 0.003 to 0.03 X 10 9 tons of carbon 

 being carried down in the zones of crustal convergence. Any substantial 

 entrainment of calcareous sediments would increase the rate considerably but 

 possibly not to so much as 0.3 X 10 9 tons. This is not a small figure, being about 

 one-tenth as large as the annual combustion of fossil fuels, but it is not so large as 

 to suggest that all classical geochemical computations must be abandoned in the 

 face of sea-floor spreading. 



