FACTORS CONTROLLING C0 2 IN OCEANS AND ATMOSPHERE 47 



TABLE 6 



CONTROL ON OCEANIC l 3 C DISTRIBUTION 



Carbon to phosphorus, 



organic debris 



3 t'Warm ~^ C Pacific = A Warm surface ~ ; ; ; 



- . Carbon to phosphorus, 



surface deep water, , r r 



r : , , . deep sea 



water water organic debris r 



/oo 1000 



= ? / 



'o 



9 ^Plankton ~ a (-Benthic- 9 C Warm surface - <* ^Pacific 



forams forams water deep water 



13 C content of shells formed by planktonic and benthic organisms. It is likely 

 that, if there is a temperature effect for carbon, it would be insignificant. 

 The absolute fractionation for oxygen isotopes between water and forams 

 is 40 per mil; for carbon, it is about 1 per mil. Hence temperature changes will 

 not alter the planktonic— benthic carbon isotope difference. The thing that sets 

 the magnitude of 2 per mil is the ratio of the carbon-to-phosphorus ratio in the 

 organic debris to that of carbon-to-phosphorus ratio in the deep sea. This ratio 

 fixes the fraction of the upwelling carbon that is fixed into organic debris and 

 falls back into the deep sea as opposed to the fraction that rides back down with 

 the water (or with the falling CaC0 3 ). Today about 10% of the carbon that 

 comes up is needed by the organisms for their soft tissue; the other 90% is not 

 needed because there are no nitrogen and phosphorus to match it. Therefore 

 about 90% goes down with the water (or CaC0 3 ), and 10% goes down with the 

 particles. That 10% that goes down with the soft-tissue particles has an average 

 fractionation of 20 per mil. The resulting 13 C enrichment in the remaining 

 carbon is 0.10 X 20, or 2%. If there was a time when the ratio of carbon to 

 phosphorus was different than today, then it ought to be recorded by the C 

 difference between benthics and planktonics grown at that time. We will assume 

 that over a period of a million years this carbon-to-phosphorus ratio demanded 

 by marine organisms has not changed, and that if we do see C differences, 

 they are due to changes in carbon-to-phosphorus ratio in the sea. This has even 

 broader implications because the major factor controlling the CO2 content of 

 the atmosphere is not the temperature changes or the salinity changes in the 

 ocean due to glaciation. They are hardly worth mentioning when compared 

 to the effects of changing the ratios of these nutrients. These nutrients all have 

 time constants for replacement in the ocean of about a hundred thousand years, 

 which is approximately the length of a glacial period. It is conceivable that the 

 ratios could change considerably over the course of a single cycle. Suppose we 

 bring up a parcel of deep water with a certain alkalinity and a certain total C0 2 



