36 



BROECKER 



Supply to oceans 



Phosphorus = 0.1 mole m 2 1CT 3 years 

 Carbon 1 10 moles m~- 10~ 3 years 



Organics 



Phosphorus = 0.1 mole m -10 years 

 Carbon = 10 moles m~ 10" years 



Loss from oceans 



Phosphorus = 0.0 mole irf 2 10 years 

 Carbon = 100 moles m" 2 10" 3 years 



Fig. 3 The fate of carbon added to the sea. Fraction of carbon lost as organic 

 tissue controlled by the availability of phosphorus. 



different group of carbon atoms there. Therefore the system must be matching 

 loss to input on that time scale. This is not to say that there cannot be 

 fluctuations. There probably have been, but each hundred thousand years' input 

 must have roughly equalled output if the system is to be considered anywhere 

 near steady state. Later 1 will present data which show that over the past ten 

 million years or so the system has been rather close to steady state. Consider 

 now the input of carbon and phosphorus. Phosphorus is really god oi the plant 

 world because it is the only nutrient that is totally limited. There is controversy 

 in this regard in biology. There are people who say that it is nitrogen that is 

 limiting. 1 choose phosphorus because nitrogen is available in large quantities 

 from atmospheric N 2 . 



The ratio of carbon to phosphorus coming into the ocean from erosion is 

 about 10 times higher than it is for organic matter. This means that, if carbon is 

 being removed in the form of organic tissue of about the same composition as 

 that in plants and animals, then only about 10% of the carbon can be carried 

 away in this form. The other 90% must be going out as calcium carbonate. The 

 best averages of the carbon composition for sediments of the last 300 to 

 400 million years show that roughly 85% of the carbon is stored in the form of 

 calcium carbonate and 15% in the form of kerogen. How does the ocean manage 

 now to get rid of the 90 of 100 units of carbon supplied by rivers that cannot go 

 out in the form of organic material? 



Organisms produce calcium carbonate at a rate of about five times the rate 

 that carbon is being supplied for this purpose by rivers (Fig. 4). In other words, 

 if all the calcium carbonate produced every year by organisms were to fall and 

 be preserved in the sediments, then the ocean would be running a very large 



