SECT. 1] 



RADIOISOTOPES AND LARGE-SCALE OCEANIC MIXING 



101 



the surface of the polar seas. If, as in cases IIA, IIIC and HID, the magnitude 

 of this transfer is sufficiently large, the rate of transfer between the mixed layer 

 and the deep ocean is greatly reduced leading to deep-sea residence times of 

 several thousand years. Model IIB cannot reproduce the 14 C distribution in 

 the oceans because the rate of supply of 14 C through the surface of the polar 

 ocean would be too rapid to allow the low 14 C/ 12 C ratios observed in the deep 

 Pacific. 



WORLD-OCEAN MODEL (Coses 12 E and ECF) 



Ei =I|=E3 = l3^4=L, = E 6 =I 6 =22M/M' : /YR 

 I|4, = 4.8xlO l5 M/YR l^A^X.0* I0 I5 M/YR 

 I 4 /»4=l.8x I0 I5 M/YR I 6 >3 6 =0.3xlO l5 M/YR 



•MUST BE TAKEN AS LESS THAN 0.960 

 OBSERVED VALUE = 0.970 



ff 3l = ff l3 = 2.2xl0 15 M/YR 

 fi32=ff 2 3 = 2.0xl0 15 M/YR 

 «34 = «43=0.6xl0 l5 M/YR 

 R4 6 =ff 6 4 = l.8xl0 15 M/YR 

 ff 65 =ff 56 =l.8xl0 l5 M/YR 



Fig. 8. Cases E and F differ only in the thickness assigned to the mixed layer. The cases 

 in this figure differ from those in Fig. 6 in the manner in which the Atlantic Ocean 

 circulates. Return to the surface of Atlantic deep water is through the Arctic instead 

 of Antarctic yielding a noncyclic rather than cyclic pattern of circulation in the 

 Atlantic. In order to obtain non-negative mixing rates the 14 C concentration assigned 

 to the Arctic reservoir must be intermediate between that in the surface and deep 

 Atlantic. Contrary to this requirement, existing 14 C measurements on northern 

 North Atlantic samples suggest that 14 C/ 12 C ratios for the Arctic reservoir may 

 exceed those for the Atlantic surface reservoir. 



(2) Models such as HIE and IIIF requiring a return of a large fraction of 

 the deep waters directly to the mixed layer rather than via the polar reservoir 

 as is the case in the other models contradict the observed 14 C distribution. If 

 such a flow pattern existed, the 14 C concentration in polar waters should be 

 equal to or greater than that in the low latitude surface waters. This definitely 

 is not the case for the Antarctic. 



(3) The thickness chosen for the surface layer has no significant effect on the 

 deep-water residence times. The residence time of dissolved solids in the mixed 

 layer is, however, directly proportional to the thickness assumed for this layer. 



