ATMOSPHERIC CARBON DIOXIDE AND RADIOCARBON: II 95 



proportion to their own masses, as assumed in the preceding paper (Ekdahl and 

 Keeling 7 ). The effect of this difference in assumption can be seen by comparing 

 their Fig. 22 with our Fig. 6. 



ISOTOPIC STANDARD FOR RADIOCARBON 



We wish to compare 14 C variations of atmospheric C0 2 calculated by our 

 model with ' C variations of wood given in the literature. The latter are 

 conventionally expressed relative to an age-corrected oxalic acid standard 

 normalized to the ] 3 C/ 1 2 C ratio of average wood. 



Su w = |*-1 (7) 



whereas variations evaluated by the model are computed relative to prein- 

 dustrial times 



Su! = i-1 (8) 



Rio 



In Eqs. 7 and 8, R denotes a l C/C ratio, observed or predicted, and the 

 subscript 1 refers to the lower atmosphere, w to a wood sample, • to the 

 standard, and o to preindustrial times. To eliminate variations in 14 C content of 

 wood owing to variations in isotopic fractionation during photosynthesis, and 

 possibly during the preparation of samples, the values of R w are normalized to 

 the same ' 3 C/ 1 2 C ratio as that of the oxalic acid standard. 



The two variations Su w and Su), to a very good approximation, are related 

 by an additive constant, 5 #0 . Data listed for the base-line period in Table 1 are 

 used to evaluate 5 #0 for a particular model. 



Let Su w82 , defined equal to (R W 82/R«) — 1> denote the average of the 14 C 

 variations of wood relative to the oxalic acid standard for the years of the 

 base-line period of Table 1 (the weighted average date is 1882). Also, let Sui 82 , 

 defined equal to (R] 82 /Ri ) — 1, denote the average of the 14 C variations of 

 atmospheric C0 2 calculated by the model relative to preindustrial times for the 

 years of the base-line period of Table 1. To establish a value for Ri , we assume 

 that the model predictions are in exact agreement with the observations for the 

 base-line period 



Rl82 =a; waRw82 (9) 



where a wa , with a value 5 of about 1.039, is the 14 C fractionation factor 

 between average wood and atmospheric C0 2 . Then 



Rio _ Q\vaRw8 2/R > o; wa (l + Su w82 )_ 



K « K l82/Rlo 1 + ^ u l8 2 



Since Su #82 and Sui 82 are both of the order of 1%, 



S.o =* Su w82 - Su l82 (11) 



