Chapter 11 



Tracer Studies of the Sea and Atmosphere 



111 



reservoir. There is some doubt as to whether 

 the data of Lane and Dole can actually yield 

 a material balance without invoking some spe- 

 cial mechanisms relating the productivities of 

 the oceans and the land, and a good deal of 

 further study on this question is needed. The 

 intent here is to point out that the isotopic 

 transfer rates involved in this problem of the 

 isotopic composition of atmospheric oxygen, 

 and the variations in the isotopic composition 

 and amounts of oxygen dissolved in ocean 

 waters, may well be important parameters for 

 the study of transfer phenomena in the oceans 

 and the atmosphere and the interaction between 

 them. 



Carbon 13 



About one per cent of natural carbon con- 

 sists of the stable isotope O^; the ratio C^^/C^-, 

 and thus effectively the C^^ concentration, in 

 natural material shows a range of variation of 

 about 6 per cent. The details of the natural 

 variation have been described (Craig, 1953, 

 1954), and reference is made to these papers 

 for extended discussion. The delta values for 

 carbon are referred to a standard which has 

 the composition of average limestone; on this 

 scale the characteristic compositions of natural 

 materials are shown below: 



Material d C^ (%c) 



Limestones and shell 



Ocean bicarbonate — 1.5 



Atmospheric CO2 — 7 



Marine biosphere — 13 



Terrestrial biosphere — 25 



Coal —25 



Petroleum — 28 



Shales —28 



The difference between the compositions of 

 atmospheric carbon dioxide and ocean bicar- 

 bonate probably reflects the isotopic equilibrium 

 constant for the exchange of carbon isotopes 

 between these compounds; the other variations 

 shown in the table are due to kinetic factors 

 which cause a selection of the isotopes in the 

 various processes involved in the biogeochemi- 

 cal cycle of carbon. The carbon 14 variations 

 caused by such processes should be almost 

 exactly twice the C^^ values shown above, and, 

 as noted previously, the knowledge of the C^^ 

 variations has been of great value in under- 

 standing the transfer rates and mixing phenom- 

 ena involved in the distribution of radiocarbon. 



A particularly fertile field for study is the 

 marine biosphere and the phenomena involved 

 in the isotopic partition of carbon between 

 carbonate and organic matter. One critical 

 parameter in the kinetic processes involved is 

 the rate of uptake of CO, by photosynthesis 

 versus the relative rates of CO., replenishment 

 by mixing and by reassociation of bicarbonate 

 ions, and such studies may well lead to an 

 improved knowledge of the carbon flux through 

 local ecological systems and the interaction of 

 the local system with the general marine reser- 

 voir. Keeling (manuscript in preparation) has 

 studied the isotopic variations in carbon dioxide 

 over the land, and has found that the isotopic 

 parameters are critical indicators of the atmos- 

 pheric transfer phenomena through local bio- 

 topes, as a result of the large difference in 

 isotopic composition between normal atmos- 

 pheric carbon dioxide and carbon dioxide pro- 

 duced in respiration during the night. 



III. Contribution of radioisotopes to the geo- 

 sphere by nuclear fission and detonations 



The steady state isotopic distributions dis- 

 cussed in the preceding section have, in the 

 case of radioactive elements, been altered to 

 some extent by contribution to the geosphere 

 of radioisotopes produced in nuclear fission in 

 both reactors and nuclear detonations. Such 

 contributions, rather than being detrimental 

 to the study of natural transfer phenomena, 

 have, on the whole, provided extra parameters 

 of great value for such studies. It is of course 

 obvious that addition of such elements under 

 carefully controlled conditions in selected loca- 

 tions and at planned times would have con- 

 tributed a great deal more to our knowledge of 

 geophysical phenomena than the actual dis- 

 persal of the material has resulted in; never- 

 theless it is possible, even though working in 

 almost total ignorance of the amounts of ma- 

 terial added, to deduce a great deal of valuable 

 information about mixing rates and even to 

 make detailed studies of certain specific prob- 

 lems. 



The fission of uranium in reactors and nu- 

 clear weapons results in a great variety of 

 elements distributed mass-wise into a spectrum 

 known as the fission yield curve; the propor- 

 tions of the various masses produced are a 

 unique function of the atomic mass and vary 



