FACTORS CONTROLLING CO, IN OCEANS AND ATMOSPHERE 49 



These things are all coupled together. Deep-sea water has a ratio of carbon to 

 phosphorus ten times higher than organisms, and that is why only a small 

 fraction of the carbon is used in the surface ocean. Because of this the difference 

 in i C to i 2 C is only 2 per mil, and the atmosphere has a C0 2 partial pressure 

 of 320 rather than 1000 ppM. Suppose the 3 13 C difference were 4 mils in an 

 ocean at some time past. Then the excess of carbon over that needed by 

 phosphorus would be reduced by a factor of 2, and the CO2 partial pressure 

 would be even further depressed from the deep-sea value by another factor of 2 

 down to roughly 160 ppM. If we went the other way and had an even greater 

 excess of carbon by a factor of 2 relative to phosphorus, the 13 C-to- 12 C 

 differences would be cut to 1 per mil and the CO2 partial pressure would rise by 

 a factor of 2 (to 640 ppM). What we find, then, is that the ocean is much more 

 sensitive in its surface CO2 content to the ratio of carbon to phosphorus than it 

 is to other environmental parameters, such as temperature and salinity. In trying 

 to measure the ' 3 C-to- 1 2 C difference between benthic and planktonic forams of 

 different ages, we find that measuring carbon in deep-sea cores is frustrating. 

 Results from planktonics in a Caribbean core suggest that there is a distinct ! 3 C 

 difference between the last glacial period and all the rest of time (including other 

 cold periods). We have data on benthics during this period indicating that there 

 was no change in the benthic composition; this would mean that the difference 

 between planktonics and benthics was enhanced to about 2.5 per mil on the 

 average, which would suggest lower CO2 partial pressures in the atmosphere 

 during the last glacial period. 



In summary, this has been an attempt to show how one geochemist looks at 

 the factors controlling carbon chemistry in the ocean and the conclusion is that 

 it is very sensitive to the interaction between the life cycles and the mixing rate 

 of the ocean and, therefore, potentially variable in time. Studies of deep-sea 

 sediments may provide valuable clues as to the nature and magnitude of these 

 changes and how they relate to the climatic history of the planet. 



DISCUSSION BY ATTENDEES 



Wetzel: Would you comment on your opinion of the interactions of 

 dissolved organic compounds adsorbed to or coprecipitated with CaC0 3 in 

 regard to inhibition of total dissolution of CaC0 3 on sedimentation? 



Broecker: The kinetics of CaC0 3 deposition are fearfully complicated in the 

 inorganic layer of the ocean and of solution are at least moderately complicated. 

 There is a fantastic range of opinion on this subject — to some people what I 

 have said in a sense is heresy, although my argument could be adapted to 

 anybody's model of what is going on. I am trying to say that, if you go deep 

 enough in the sea, there is a very strong tendency for carbonate to dissolve and 

 that, if you stay shallow enough in the sea, there is a strong tendency for the 



