I4 C CYCLE AND ITS IMPLICATIONS FOR MIXING RATES 11 



Atlantic deep water are, respectively, about 1000 and 600 years. These numbers 

 are approximate only as the exact time is strongly dependent on the C0 2 

 exchange rate through the polar seas. 



It should be noted that not all information in the models is derived from 

 14 C data. In the simplified mixing cross model, we can evaluate the rate of 

 exchange between the three major water types in the ocean (surface water, 

 North Atlantic deep water, and Pacific and Indian deep water) by determining 

 the eight unknown fluxes from material-balance considerations for H 2 0, 14 C, 

 and total CO2 and from the requirement that the 18 0— salinity relations 

 observed in the ocean are not to be violated. This model also takes into 

 account the particulate transport of carbon to the deep-ocean reservoirs. The 

 organic material and calcium carbonate produced in surface waters by organisms 

 sink under the influence of gravity and introduce a carbon transport that is not 

 exclusively associated with water movement. Thus the unknowns in the 

 simplified mixing cross model involve six water-related fluxes and two fluxes of 

 carbon-bearing particles from the surface ocean into the deep reservoirs. 



Most of the particles transported downward are redissolved or oxidized in 

 the deep ocean. Of the particulate carbon, 1 5 about 80% is in the form of organic 

 debris and 20% is in the form of CaC0 3 . Whereas the organic debris is depleted 

 in l C by about 2% relative to the dissolved carbon in surface water, 16 the 

 CaCC>3 shows negligible isotope separation. Because the C fractionation 

 amounts to twice the ! C fractionation, the composite particulate carbon flux is 

 about 3% depleted in l C relative to surface waters. 



The mixing-cross-model calculations give a best estimate of the flux of 

 CaC0 3 necessary to provide the carbon to the deep waters of 3.2 g cm -2 1000 

 years -1 in the Atlantic and 1.4 g cm -2 1000 years -1 in the Pacific. Actual 

 measured rates in limited oceanic areas indicate calcite accumulation rates of 

 about one-half of these values. 1 4 Although agreement between predicted and 

 actual values is only within a factor of 2, it still provides support for the 

 magnitude of oceanic overturn time predicted by the mixing cross model. 



Since most models are concerned with steady-state conditions, it is essential 

 to use J C data that were obtained before appreciable quantities of bomb J C 

 were added to the surface waters. Prebomb measurements available are those of 

 Broecker 2 for the Atlantic and of Bien, Rakestraw, and Suess 1 7 for the Pacific 

 and Indian oceans. These data are given in Figs. 2 and 3. 



The C deficiency of the oldest ocean waters is about 10% for the Atlantic, 

 10% for the Antarctic, and 25% for the Pacific Ocean (after correction for 

 isotope fractionation). This would lead to apparent J C ages of 800 years for 

 the Atlantic and Antarctic oceans and 2000 years for the Pacific Ocean. The 

 apparent-age values are only crudely related to the residence time in those 

 reservoirs. For example, one of the reasons for the old age of the deep-seated 

 Pacific waters is the addition of Antarctic waters with an apparent age of about 

 800 years. Only a proper evaluation of box models, using specific 1 C activity 

 data, leads to useful residence times for the various reservoirs. 



