Chapter 11 



Tracer Studies of the Sea and Atmosphere 



109 



importance, because of the long half-hfe, for 

 the dating of such sediments. 



Deuterium and Oxygen 18 



Deuterium and oxygen 18 are stable isotopes 

 of hydrogen and oxygen respectively, and it is 

 now well known that the isotopes of these 

 elements, as well as of other light elements 

 such as carbon, nitrogen, and sulphur, are 

 fractionated, or separated, by chemical and 

 physical processes in natural systems. Since 

 the fractionation factors for stable isotopes are 

 measurable and/or calculable for many separa- 

 tion processes, and since the magnitude of these 

 factors is mainly a function of temperature and 

 process, the stable isotopes are extremely well 

 adapted for the study of natural transfer rates 

 in the geochemical cycles of their elements. 



The concentrations of these isotopes show 

 rather wide variations in different natural ma- 

 terials, these variations generally ranging from 

 a few tenths of a per cent to a few per cent. 

 In this report we shall mainly be concerned 

 with the distribution of these isotopes in marine 

 and fresh waters and in the atmosphere. Craig 

 and Boato (1955) have recently reviewed the 

 present status of natural isotopic studies, and 

 reference is made to that paper for a more 

 extended discussion. 



Vapor Pressures and Relatfve Abundances of 

 THE Isotopic Water Molecules 



Relative p (mm Hg) 



abundance ,, ^ ^ 



Species (ocean water) Mass 30° C 100° C 



H2O 1 18 31.5 760 



HDO 1/3230 19 29.4 741 



H^O^" 1/500 20 31.3 756 



The above table shows the three most prom- 

 inent members of the family of isotopic water 

 molecules, their masses, relative abundances in 

 average ocean water, and their vapor pressures 

 at two temperatures. Other members of the 

 family are much less abundant and can be 

 neglected. One sees from the table that the 

 vapor pressures are not a direct function of 

 the molecular weight ; the vapor pressure differ- 

 ence between HDO and HJD is 10 times larger 

 than the vapor pressure difference between 

 H,0i8 and HoO, at 30 °C. The isotopic separa- 

 tion in an evaporation or condensation process 

 is directly proportional to these vapor pressure 

 differences, so that in water vapor in equi- 



librium with water at 30°, the percentage de- 

 pletion in deuterium, relative to the water, is 

 ten times larger than the percentage depletion 

 in oxygen 18. 



The natural isotopic variations are customarily 

 given in terms of per mil enrichment or deple- 

 tion relative to a standard, similar to the way 

 the density parameter is given in an oceano- 

 graphic temperature-salinity diagram. The data 

 are presented in terms of a function 8, defined 

 as follows: 



8 ( % ) = [ (Rsample/Rstd ) - 1 ] x 1 000 



where R is the isotopic ratio O^YO^^ or D/H. 

 In the case of deuterium, however, the quantity 

 in the brackets is multiplied by 100 and the 8 

 values are given in per cent, because of the ten 

 times higher isotopic separations encountered. 

 Rstd here refers to the isotopic ratio in average 

 ocean water. 



Since HoO^*' is the most volatile isotopic 

 species, the water vapor over the oceans is 

 depleted in the heavy isotopes relative to the 

 surface ocean water. As this vapor moves over 

 the continents, the first rain to fall out is en- 

 riched in the heavy isotopes relative to the 

 vapor, again because of the higher volatility of 

 the lightest species. Removal of the heavy iso- 

 topes, in the form of rain, then causes the vapor 

 to become continually depleted in deuterium 

 and oxygen 18. In general enough rain falls 

 out of an air mass over the oceans so that by 

 the time the mass reaches the continents the 

 rain is already "lighter" in isotopic content 

 than ocean water, and as the air mass moves 

 inland and poleward the rain which falls out 

 becomes more and more depleted in deuterium 

 and oxygen 18. 



In a recent study by Craig (ms. in prepara- 

 tion) several hundred fresh water samples from 

 all over the world were analyzed for deuterium 

 and oxygen 18 concentration. The deuterium 

 concentration varies by about 30% relative to 

 mean ocean water, 8D ranging from -f3 to 

 — 27%, while the oxygen 18 concentration 

 varies by only 4%, 80^^ ranging from -\-6%o 

 to — 34%o. The delta values for the trwo iso- 

 topes show a linear correlation such that 8D = 

 980^^, corresponding to the vapor pressure 

 difference ratio at about 25 °C. The reason for 

 the high value of the average temperature at 

 which liquid and vapor equilibrate in the at- 

 mosphere is as yet unknown; the uncertainty 



