Chapter 11 



Tracer Studies of the Sea and Atmosphere 



117 



3 per cent is higher than the average world- 

 wide figure, and represents an increased local 

 contamination in trees growing near sites of 

 industrial activity. The latest measurements in- 

 dicate a world-wide effect of about 1 .7 per cent. 

 Revelle and Suess (1957) have discussed the 

 relationships between the exchange rate, the 

 Suess effect, the effect of an increase in the 

 atmospheric CO, content on the atmospheric 

 and oceanic reservoirs, and the buffering effect 

 of the sea water alkalinity on carbon transients. 

 They conclude that, all things considered, the 

 residence time of COg in the atmosphere, rela- 

 tive to exchange with the sea, is of the order of 

 10 years. Though the uncertainty in their esti- 

 mate is a good deal larger than in the case of 

 the steady-state considerations discussed above, 

 the close agreement of the figures obtained by 

 these different considerations is gratifying, and 

 indicates that the factors governing the natural 

 distribution of radiocarbon are now fairly well 

 understood. 



The size of the terrestrial biosphere and the 

 annual rate of photosynthesis on land have been 

 estimated by Schroeder and Noddack, and from 

 their figures it appears that the terrestrial plants 

 consume about 3 per cent of the atmospheric 

 CO2 per year, corresponding to an atmospheric 

 residence time before entrance into the bio- 

 sphere of 33 years. With a residence time of 

 7 years, prior to exchange into the sea, the total 

 residence time of a CO2 molecule in the atmos- 

 phere is 6 years, after which it goes either into 

 the sea (9 chances out of 11) or into the ter- 

 restrial biosphere (2 chances out of 11). Thus 

 the carbon dioxide flux into the sea is about 

 4.5 times larger than the flux into the biosphere, 

 and about 82 per cent of the COo leaving the 

 atmosphere goes into the sea, while only about 

 18 per cent goes into the terrestrial plants. This 

 ratio represents a considerable departure from 

 previous estimates, and indicates that the spatial 

 distribution of plants and soils is probably not 

 the dominant factor in determining the steady- 

 state CO, concentration in the atmosphere. In 

 fact it appears more likely that the spatial pat- 

 tern of absorption and release of CO„ by the 

 sea, and the seasonal variations in this pattern, 

 are the dominant factors. 



The various considerations outlined above are 

 all consistent with any deep-sea residence time 

 of carbon up to a few thousand years, and do 

 not yield any closer estimate for this figure. 



Recent unpublished data by Broecker and co- 

 workers at the Lamont Geological Observatory 

 indicate that the bicarbonate of deep ocean 

 waters probably averages about 8 per cent lower 

 in C^* content than the surface mixed layer, 

 corresponding to a radiocarbon "age" of the 

 order of 670 years. However, considerations by 

 Craig (in press), based on a second order 

 oceanic model in which the deep sea reservoir 

 is exposed to the atmosphere in high latitudes, 

 show that about half of the radiocarbon in the 

 deep sea is derived directly from the atmosphere. 

 The other half enters the deep sea from the 

 surface mixed layer of the ocean by the mixing 

 and interchange of water. 



Because of this dual source of radiocarbon, 

 the residence time calculated for carbon in the 

 deep sea is only about half of the actual resi- 

 dence time of a water molecule in the deep sea 

 relative to the mixed layer; thus the deep-sea 

 residence time of water relative to the mixed 

 layer is probably of the order of 1000 years as a 

 world-wide average. However the actual inter- 

 pretation of such residence times in the sea is 

 quite complicated, and reference is made to the 

 paper cited above for a detailed discussion of 

 carbon and water residence times. 



Deuterium and Oxygen 18 



As discussed previously, the stable isotopes 

 are of great value in the study of ocean water 

 mixing as additional parameters related to salin- 

 ity. One particular case in which information 

 can be gained from such studies is the problem 

 of meltwater dilution of the oceans in the polar 

 regions. A salinity decrease can be caused by 

 addition of fresh water from river runoff, or 

 from the melting of sea ice, and from salinity 

 data alone these sources cannot be differentiated. 

 However, the isotopic composition of the two 

 sources is quite different; the sea ice should 

 have a composition quite similar to that of the 

 ocean water, while, as shown above, the runoff 

 of rivers in polar areas is greatly depleted in 

 deuterium and oxygen 18 relative to ocean 

 water. Thus from consideration of salinity and 

 isotopic data taken together, a quantitative eval- 

 uation of the mixing conditions can be made. 

 Friedman of the U. S. Geological Survey is 

 currently studying such problems with deute- 

 rium analyses of Atlantic waters. The isotopic 

 data should also be useful in material balance 



