SECT. 1] 



RADIOISOTOPES AND LARGE -SCALE OCEANIC MIXING 



Table V 

 Strontium-90 in Atlantic Ocean a 



105 



a Bowen and Sugihara (1960). Results are given as dis/min/100 1. of water. 



upward. The rates of vertical mixing required to maintain this distribution 

 would be an order of magnitude higher than those allowed if the surface depth 

 contrast in 14 C concentration is to be maintained. No explanation for this 

 anomaly is apparent. Further data, especially on pairs of fallout isotopes (for 

 example, 137 Cs and 90 Sr) and on bomb 14 C, will allow a better evaluation of 

 vertical mixing rates in the upper portion of the sea. 



9. Non- Steady- State Conditions 



Having considered the various box models it is necessary to reconsider the 

 assumption of steady state. This assumption implies that the concentration of 

 any one of the natural radioactive isotopes should have been constant through- 

 out time at any point in the ocean-atmosphere system. As demonstrated by the 

 hypothetical case in Fig. 10, cyclic mixing in the ocean would lead to time 

 variations in the 14 C concentrations in the atmosphere and surface ocean. 

 Furthermore, the distribution of 14 C in the ocean would not be simply related 

 to radioactive decay as is the case in the steady-state models. Because of the 

 backlogging of newly produced 14 C in the atmosphere and surface oceans, large 

 differences between the 14 C concentration in the deep and the surface ocean 

 waters could be generated quite rapidly if the rate of deep-water formation 

 were severely reduced for a period of 20 to 100 years. Hence, estimates of iso- 

 tope concentrations at times in the past would allow an evaluation of the 

 steady-state assumption. Fortunately 14 C measurements on tree rings and 

 historically dated samples yield such data for the atmospheric reservoir (see 

 Fig. 11 for a summary of the available data). Although measurable variations 

 have occurred over the past 2000 years, they are restricted to a range of slightly 

 more than 2% either side of the mean. As such variations could equally well 



