ATMOSPHERIC CARBON DIOXIDE AND RADIOCARBON: II 91 



RADIOCARBON OBSERVATIONS AND 

 THE SUESS EFFECT 



Radiocarbon, ! C, is formed in the upper atmosphere by cosmic-ray 

 collisions. It then mixes with inactive carbon in the atmosphere, the oceans, and 

 the land biota. Its mean life 16 of 8267 years makes it a convenient tracer for 

 studying the short-term natural carbon cycle. Fossil fuel, owing to long 

 underground storage, contains practically no 14 C. Nevertheless, its combustion 

 dilutes and displaces the l C in all the earth's reservoirs where l C occurs 

 naturally. The relative depletion for the atmospheric reservoir, often called the 

 Suess effect, has been determined from the radiocarbon/inactive carbon ratio 

 ( 14 C/C) in independently dated wood. This effect has been measured by Suess 

 and his coworkers, 1 7 '' Fergusson, 1 and Lerman, Mook, and Vogel, among 

 others. After 1954, nuclear bomb tests complicated matters by also introducing 

 1 4 C into the atmosphere. We have considered 14 C data only up to 1954. 



During this year the Suess effect was approximately —2.3% ± 0.4 if we follow 

 the practice of counting only the variation in 1 C since a late 19th century 

 base-line period. A relatively high uncertainty is implied by the different average 

 values obtainable from the data of individual investigators. If we extrapolate to 

 1954 tree-ring data for the period 1945 to 1954, we find that seven analyses 

 reported by Houtermans et al. 18 of one tree at 45°N yield a value of about 

 —2.7%, single analyses of seven trees in both hemispheres by Lerman et al. 

 yield about —2.3%, and six analyses of four trees in both hemispheres by 

 Fergusson 19 (extrapolated by that author) give —2.0%. As we discuss later, 

 about —0.3% is to be added to the above figures if we are to include the 

 industrial dilution from the beginning of industrialization to the late 19th 

 century. This leads to a corrected Suess effect in 1954 of —2.6% ± 0.4. These 

 extrapolations do not take into account a probably negligible decrease in 

 radiocarbon between the late 1940s and 1954 which may have occurred as a 

 result of heliomagnetic perturbations of the cosmic-ray flux, as discussed later. 



The average l C/C ratios of surface and deep-ocean water for 1954 can be 

 estimated from fractionation-corrected data compiled by Broecker. These 

 data, as published, were normalized for departures of the l 3 C/ 1 2 C ratio of each 

 sample from a standard value of that ratio. It is not possible from the normalized 

 data to deduce exactly the radioisotopic ratios of the original samples, but the 

 correction for fractionation can be approximately cancelled using independent 

 observations of 13 C/ 12 C in seawater, such as reported by Craig. : We deduce 

 that the actual 14 C/C ratios relative to the ' 4 C standard (as discussed in a later 

 section) were 0.999 ± 0.019 and 0.86 ± 0.04 in 1954 for surface and deep-ocean 

 water, respectively. If the 14 C/C ratio of the standard were normalized to the 

 13 C/ 12 C ratio of atmospheric C0 2 , these ratios would be 0.963 ± 0.020 and 

 0.83 ± 0.04, respectively. The quoted uncertainties are as stated by Broecker 

 etal. 8 



