SECT. 1] RADIOISOTOPES AND LARGE-SCALE OCEANIC MIXING 95 



with a 14 C/ 12 C ratio about 3% higher than the surrounding water, and (3) 

 the deepest water in the eastern basin of the Atlantic is about 2.5% lower in 

 14 C than both the overlying N.A.D.W. and adjacent N.A.D.W. on the opposite 

 side of the mid-Atlantic ridge. The A.A.I.W. and A.A.B.W., which originate in 

 the southern South Atlantic, are significantly lower in 14 C than the deep water 

 originating in the north. Residence time estimates given in the table are based 

 on the assumption that the difference in activity between the water at depth 

 and that in the corresponding source area is the result of radioactive decay 

 ( 14 C decays 1% each 80 years). 



Results from the sub -surface Pacific are given in Table III. The deep water 

 ( > 2000 m) ranges from 0.687 to 0.850 the atmospheric value. As the Antarctic 

 surface is deficient in 14 C, residence times are similar to those for the Atlantic. 

 As shown by Bien et al. (1960) there appears to be a south-to-north decrease in 

 the 14 C/ 12 C ratio in the deep Pacific. Again the central waters have 14 C con- 

 centrations intermediate between those of the deep and of the surface waters. 



4. Ocean- Atmosphere CO 2 Exchange Rates 



As all of the water masses of the ocean simultaneously interact with one 

 another it is necessary to consider the entire ocean as a unit before a complete 

 understanding of the C0 2 cycle can be obtained. Before discussing such models, 

 it is necessary to obtain estimates of the rate of exchange of CO 2 between the 

 oceans and the atmosphere. As mentioned above, since this exchange is im- 

 portant to the CO 2 cycle and not that of dissolved solids, it must be quantita- 

 tively understood before 14 C data can yield estimates of water mixing rates. 

 Fortunately evaluation of this exchange can be carried out without reference 

 to oceanic mixing processes. 



If it is assumed that the exchange rate is geographically uniform the following 

 equation holds (since at steady-state the 14 C inventory in the ocean must 

 remain constant): 



/C A ^ A -s = ECsA A - S + AC N , 



where / and E are the invasion and evasion rates of CO 2 through the ocean- 

 atmosphere interface (moles/m 2 /yr), C A , Cs and Co are the 14 C concentrations 

 in average atmospheric CO2, average surface ocean CO2 and average oceanic 

 CO 2 respectively (moles 14 C 2 /mole CO 2), -4a-s is the area of the ocean- 

 atmosphere interface (m 2 ), 2V is the quantity of C0 2 in the ocean (moles) and 

 A is the decay constant of 14 C (yr _1 ). Since the rate of loss of C0 2 to the sedi- 

 ments as CaC03 and the influx of bicarbonates from rivers are negligible, E 

 and / should be equal at steady state. Therefore, 



I = A(C /C A )iVo 



[1-(Cs/C a )]^a-s 



Thus, from a knowledge of the ratio of the average 14 C concentration in the 

 dissolved CO2 in oceans as a whole to that in atmospheric CO2 (Co[C A ) and of a 



