SECT. 1] 



RADIOISOTOPES AND LARGE-SCALE OCEANIC MIXING 



107 



result from changes in the production rate of 14 C, their magnitude must be 

 considered as an upper limit on any effect resulting from variations in the rate 

 of oceanic mixing. Consideration of model IIIA will serve to demonstrate the 

 sensitivity of the 14 C/ 12 C ratio in the atmosphere to changes in the rate of 

 oceanic mixing. If the ocean-atmosphere exchange rate is assumed to remain 

 constant and if the two water exchange rates are assumed to undergo similar 

 fractional changes, the 14 C concentrations in the four reservoirs can be shown 

 as a function of mixing rate as in Fig. 12. It is apparent that the atmospheric 



0.7 0.8 0.9 1.0 I.I 1.2 1.3 



FRACTIONAL CHANGE IN MIXING RATE 



Fig. 12. Dependence of the steady-state 14 C concentrations in the reservoirs of model IIIA 

 on the rate of oceanic mixing. Although the two rates J?i3 and i?23 are assumed to 

 undergo similar fractional changes, variation in i?i3 does not cause any major re- 

 distribution in 14 C, Jf?23 being the dominant rate parameter. As the atmospheric 

 1 4 C/ 12 C ratio has not varied outside 2.5%, its 19th-century value over the past 2000 

 years, the suggestion is that the rate of deep-water replacement in the ocean has not 

 varied on a long term basis by more than 25% during this span of time. 



14 C concentration is quite sensitive to variations in the rate of oceanic mixing, 

 changes of 25% in the transfer rates being of sufficient magnitude to produce 

 the variations observed in the tree-ring data. It should be emphasized, how- 

 ever, that the short-term variations in mixing rate or variations confined to a 

 local area would not produce measurable variations in the atmospheric 14 C/ 12 C 

 ratio. Thus the tree-ring data can only be used to place limits on the degree 

 of nonequilibrium in the oceans. 



10. Conclusions 



From this discussion it is clear that isotope studies offer the key to many 

 problems involving mixing rates within the sea. To date, from 14 C data, it 

 seems probable that the residence times of water in the deep ocean may exceed 

 800 years in the Pacific and 500 years in the Atlantic. 14 C results on tree rings 

 suggest that the oceans have been reasonably close to steady state over the 



