92 



BROECKER 



[CHAP. 4 



arising from non-steady -state operation are beyond resolution, the simple 

 models developed to date all assume steady-state operation. As the interpreta- 

 tion of the isotope results is strongly dependent on this assumption, discussions 

 of the evidence supporting the steady-state hypothesis and of the consequences 

 of non-steady-state operation are given below. 



The available radiocarbon data for surface ocean water are summarized in 

 Table I as averages for various geographic localities. The uncorrected averages 

 are given in column 4 ; the averages after correction for the increase in con- 

 centration resulting from the addition of bomb 14 C in column 5. The reduction 



-30(3) 



_4_ 



-35(18) 



i_ 



-46(16) 



-121(2) 



NACW -71 (3) -92(2) -61 (6) 

 -98(8) -118(6) 



SASW 

 AAIW 



-72 (8) 



-102 (5) 



-130(3) 



-105 (15) 



104(16) -144(3) 



60°N 40° 20° 0° 



LATITUDE 



40° 60° S 



Fig. 1 . Vertical profile through the western basin of the Atlantic Ocean showing idealized 

 water-mass distribution. The numbers represent the average 14 C concentrations 

 given as per mil difference from the age-corrected concentration of 14 C in 19th- 

 century atmospheric CC>2- All the 14 C results are normalized to a common 13 C con- 

 centration in order to eliminate differences resulting from isotope fractionation. 

 Details of this so-called A 14 C scale are given by Broecker and Olson (1959). The 

 numbers in parentheses indicate the number of samples upon which the average is 

 based. 



in oceanic 14 C/ 12 C ratios resulting from CO 2 released by the combustion of coal 

 and oil is assumed negligible. The corrected results are assumed to be the steady- 

 state 14 C concentrations. The results indicate a nearly uniform 14 C/ 12 C ratio in 

 the region from 50°N to 50°S with significantly lower values in the Antarctic 

 and perhaps slightly higher values in the northern North Atlantic. 



The results on sub -surface samples in the Atlantic Ocean are summarized in 

 Table II and in Fig. 1. The majority of water originating in the northern North 

 Atlantic has a fractionation-corrected 14 C/ 12 C ratio 10% below that in the 

 steady-state atmosphere. There are three exceptions: (1) the North Atlantic 

 Central Water has a value intermediate between that of the deeper water and 

 that of the surface water, (2) a tongue of water in the western North Atlantic 



