ATMOSPHERIC CARBON DIOXIDE AND RADIOCARBON: I 61 



SIGNIFICANCE OF RADIOCARBON VARIATIONS 

 IN THE CARBON CYCLE 



Since the oceans and the land biota both rapidly exchange C0 2 with the 

 atmosphere and represent large reservoirs of carbon, knowledge of the fraction 

 of industrial CO2 remaining airborne is not sufficient information to determine 

 the partitioning of C0 2 between the geochemical reservoirs. As several studies 

 have shown, _1 ' we can learn more about that partitioning if we take into 

 account the fractional dilution of atmospheric radiocarbon, CO2 , by inactive 

 industrial C0 2 (the "Suess effect"). The distribution of ' C in the deep-ocean 

 water and estimates of atmospheric ' CO2 before 1850 offer additional data to 

 determine the quasi-steady-state preindustrial exchange rates of the carbon 

 cycle. 



Variations in ' C from the beginning of the industrial era to 1954, when 

 nuclear weapons testing added a new source of ' C to the air, are known 

 approximately from measurements of the radiocarbon/inactive carbon ratio, 

 1 C/C, in wood samples dated by counting tree rings (Fig. 7). Before we attempt 

 in the next paper 1 to deduce the Suess effect from this record, however, we will 

 consider the question of whether or not the production of ! C has varied 

 significantly during the industrial era. 



Previous studies of natural C have reached conflicting conclusions as to 

 whether the known variations in cosmic-ray flux can lead to significant 

 fluctuations in 14 C abundance outside the stratosphere where ' C is formed. 

 The slow exchange of air between that reservoir and the lower atmosphere 

 attenuates any such fluctuations, and further damping occurs when atmospheric 

 14 C0 2 exchanges with the oceans and land biota. Although the magnitude of 

 the stratospheric variation is uncertain, the degree of attenuation is the principal 

 focus of controversy. This disagreement clearly results from the variety of 

 geochemical models invented to predict the attenuation. We will now discuss this 

 problem in some detail. 



Stuiver 6 noted an inverse correlation between solar activity, as determined 

 from sunspot records, and the C concentration in modern wood. As shown in 

 Fig. 8, the principal correlative feature is a * C peak and sunspot dip in the early 

 19th century. If the relation found by Stuiver holds for the period near 1950, 

 the observed decrease in l C is partially due to a decrease in C production. 

 Taking the decrease solely as a measure of dilution of ' C by industrial C0 2 will 

 result in an overestimate of the industrial effect. 



The physical explanation for Stuiver's discover)' is believed to be that 

 galactic primary cosmic rays (principally high-energy protons) which generate 

 1 4 C-producing neutrons in the stratosphere are more likely to be scattered out 

 of the neighborhood of the earth's orbit during periods of increased solar 

 activity. This enhanced scattering is probably due to increased turbulence in the 

 solar wind along with alterations of the terrestrial magnetopause. 



