. The Composition of sea-water : comparative and descriptive oceanography. Seawater -- Composition. SECT. 1] RADIOISOTOPES AND LARGE-SCALE OCEANIC MIXING 107 result from changes in the production rate of 14C, their magnitude must be considered as an upper limit on any effect resulting from variations in the rate of oceanic mixing. Consideration of model IIIA will serve to demonstrate the sensitivity of the 14C/12C ratio in the atmosphere to changes in the rate of oceanic mixing. If the ocean-atmosphere exchange rate is assumed to remain constant and if the two water exchange rates are assumed
. The Composition of sea-water : comparative and descriptive oceanography. Seawater -- Composition. SECT. 1] RADIOISOTOPES AND LARGE-SCALE OCEANIC MIXING 107 result from changes in the production rate of 14C, their magnitude must be considered as an upper limit on any effect resulting from variations in the rate of oceanic mixing. Consideration of model IIIA will serve to demonstrate the sensitivity of the 14C/12C ratio in the atmosphere to changes in the rate of oceanic mixing. If the ocean-atmosphere exchange rate is assumed to remain constant and if the two water exchange rates are assumed to undergo similar fractional changes, the 14C concentrations in the four reservoirs can be shown as a function of mixing rate as in Fig. 12. It is apparent that the atmospheric. FRACTIONAL CHANGE IN MIXING RATE Fig. 12. Dependence of the steady-state 14C concentrations in the reservoirs of model IIIA on the rate of oceanic mixing. Although the two rates J?i3 and i?23 are assumed to undergo similar fractional changes, variation in i?i3 does not cause any major re- distribution in 14C, Jf?23 being the dominant rate parameter. As the atmospheric 14C/12C ratio has not varied outside , its 19th-century value over the past 2000 years, the suggestion is that the rate of deep-water replacement in the ocean has not varied on a long term basis by more than 25% during this span of time. 14C concentration is quite sensitive to variations in the rate of oceanic mixing, changes of 25% in the transfer rates being of sufficient magnitude to produce the variations observed in the tree-ring data. It should be emphasized, how- ever, that the short-term variations in mixing rate or variations confined to a local area would not produce measurable variations in the atmospheric 14C/12C ratio. Thus the tree-ring data can only be used to place limits on the degree of nonequilibrium in the oceans. 10. Conclusions From this discussion it is clear that isotope studies offe
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