8 STUIVER 



0a-m C a A = m _ a C m A + XC N 



where a _ m , m _ a = exchange rates of C0 2 (moles rrf 2 year -1 ) through the 



interface of atmosphere and surface of the mixed layer of 

 the oceans 

 C a , Cm, C = 14 C activities, 2 respectively, of atmospheric C0 2 , surface 

 ocean C0 2 , and average ocean C0 2 (moles 14 C0 2 /mole 

 C0 2 ) 

 A = area of ocean— atmosphere interface (m 2 ) 

 X = ' 4 C decay constant 

 N = total amount of C0 2 in the oceans (moles) 



The rate of loss of C0 2 to the sediments and the influx of bicarbonates from 

 rivers are negligible; thus the exchange rate from atmosphere to oceans equals the 

 rate from oceans to atmosphere for a steady state. Substituting m _ a = a -m» 

 one arrives at 



X(C /C a )N 

 0a-m " 



[l-(C m /C a )]A 



The knowledge of the ratio of the specific 1 C activities in average ocean 

 water, mixed layer of the oceans, and the atmosphere leads directly to a value 

 for an average exchange rate. Broecker 2 estimates C /C a as 0.85 ± 0.05 and 

 C m /C a as 0.95 ± 0.015. These values yield a rate of 20 ± 7 moles m~ 2 year -1 . 

 With, on the average, 100 moles C0 2 /m 2 in the atmosphere, which is equivalent 

 to 140 moles C0 2 /m 2 of oceanic surface, the mean residence time for 

 atmospheric C0 2 is 7 years. 



For the above-mentioned calculation, the C0 2 exchange between oceans and 

 atmosphere has to be uniform on a global scale. This seems unlikely because 

 wind velocity influences the rate of exchange. Kanwisher measured the rate of 

 gas exchange across the air— seawater interface and found a rate of exchange 

 approximately proportional to the square of the wind speed. Wind velocities are, 

 on the average, considerably higher in the Antarctic region than in the rest of the 

 world, and the C0 2 exchange rate in the Antarctic will be several times higher 

 than in the rest of the oceans. This would lead to an increase in residence time 

 for C0 2 in the atmosphere because the Antarctic Ocean has a 14 C to 12 C ratio 

 about 8% lower than the remaining mixed layer of the oceans. A residence time 

 of about 10 years seems to be an upper limit. 



Residence times for atmospheric C0 2 can also be obtained from the rate of 

 uptake of bomb 14 C0 2 in the oceans. For instance, between 1963 and 1965 the 

 atmospheric excess 14 C inventory decreased at a rate equivalent to a half -life of 

 3.3 years, which gives an upper limit of 4.8 years for the mean residence time of 

 a 14 C atom in the atmosphere. 4 About 18% of the 14 C0 2 leaving the 

 atmosphere goes into the biota, 1 giving an upper limit of 5.8 years for the mean 



