14 C CYCLE AND ITS IMPLICATIONS FOR MIXING RATES 9 



residence time of atmospheric C0 2 before uptake by the oceaTis. This estimate is 

 an upper limit only, because during the 1963 to 1965 period much of the excess 

 C was stored in the stratosphere and unavailable for transfer to the oceans. By 

 taking into account the dependence of the CO2 exchange on wind velocity, 

 Young and Fairhall also developed a model giving mean tropospheric residence 

 times as a function of latitude. They conclude that the bulk of excess 14 C 

 produced by past nuclear explosions will eventually be taken up by the oceans 

 of the Southern Hemisphere. 



As discussed earlier, exchange rates between troposphere and the mixed 

 layer of the oceans can be derived from (a) the transfer of excess bomb 14 C to 

 the oceans and from (b) steady-state equilibrium between natural 14 C exchange 

 over the ocean— air interface and l C decay in the oceans. In addition, 

 exchange-rate constants can be calculated from (c) the change in atmospheric 

 14 C activity caused by dilution of the atmospheric reservoir through addition of 

 industrial CO2 and (d) the natural l C balance in the atmosphere where natural 

 production of 14 C balances the decay in the atmosphere and the net transfer to 

 the oceans. The results obtained from atomic-bomb radiocarbon transfer 

 between atmosphere and oceans include atmospheric C0 2 residence times of 4, 

 5.4, 25, 4, and 5 to 10 years reported, respectively, by Lai and Rama, 5 Miinnich 

 and Roether, 6 Bien and Suess, 7 Young and Fairhall, 4 and Nydall. 8 From 

 radiocarbon decay in the oceans (method b) Broecker 2 gives a residence time of 

 7 to 10 years, whereas Craig 1 arrives at a 4- to 10-year value from 

 C-production-rate considerations (d, above). Fergusson 9 and Revelle and 

 Suess calculate, from the change in specific 14 C activity caused by industrial 

 C0 2 , residence times of, respectively, 2 to 7 years and about 10 years. 



The 25-year value was calculated on the basis of major uptake of excess 

 bomb C by the biosphere. The other workers suggest a smaller biospheric 

 contribution, and a 5- to 6-year atmospheric residence time for C0 2 transfer 

 between troposphere and the oceans seems a reasonable meeting ground. 



The complexity of ocean-water circulation makes the application of box 

 models to the investigation of mixing rates and patterns in the oceans more 

 difficult. It would be possible to explain the isotope distributions with a large 

 variety of models. However, only when the isotope results are tied to other 

 oceanographic data is it possible to obtain suitable models. 



Oceanic mixing itself involves complicated contributions of both advection 

 (or convection) and diffusion. Advective processes involve regular patterns of 

 water movement, whereas in diffusive processes irregular movements of water 

 called turbulence, together with molecular diffusion, provide exchange without 

 net transport of water. Advective processes are normally large-scale events, 

 whereas diffusive processes are restricted to much smaller dimensions. 



For the box-model studies, the properties of water in each box are averaged, 

 and one attempts to see changes only as between boxes. Both horizontal and 

 vertical exchange have been studied. Some of these models are given in Fig. 1. In 

 the terminology of Broecker, 2 the models are classified as: (1) the two-layer 



