114 



Atomic Radiation and Oceanography and Fisheries 



The absolute values are probably not better 

 than ±150 years, but the relative values are 

 more precise. The age of the surface bicar- 

 bonate is somewhat older than the 400-year 

 average age mentioned in Part II as the result 

 of slow transfer of atmospheric carbon into the 

 sea, perhaps as a result of local conditions; 

 however, the important figure is the age dif- 

 ference between surface and deeper waters and 

 it is unfortunate that still deeper waters were 

 not sampled. The importance of a great many 

 vertical profiles of this sort from both oceans, 

 and their fundamental import for knowledge of 

 the mixing rates in the ocean, is obvious. 



Because of the requirements of steady state 

 balancing, the amounts of water transferred, 

 per unit time, downward and upward through 

 the thermocline in the sea must be equal, but 

 because the mixed layer contains only about 

 2 per cent of the sea, this balance requires that 

 a water molecule remain, on the average, some 

 50 times longer below the thermocline than 

 above. As a consequence of this relationship, 

 an uncertainty of 10 years in the residence time 

 of material in the mixed layer results in an 

 uncertainty of 500 years in the residence time 

 of the material below the thermocline, consid- 

 ering the world average rate of general cross- 

 thermocline mixing of the substance. As we 

 shall see below, the half-life of radiocarbon 

 happens to be so long, that considerations of 

 the extensive data on C^* distribution in the 

 atmosphere, biosphere, and mixed layer of the 

 sea, do not yield important information on the 

 internal mixing rate of the ocean itself. In fact, 

 the distribution of tritium above the thermocline 

 of the sea furnishes a much more precise esti- 

 mate of the general turnover time of water in 

 the deep sea. 



Thus the application of radiocarbon analysis 

 to mixing problems within the sea itself can 

 be made only by actually getting below the 

 mixed layer and studying the deep-sea distribu- 

 tion of C^* directly; such studies, coupled with 

 chemical analyses and physical data serving as 

 parameters for the identification of continuous 

 water masses, will probably prove to be the 

 most fruitful method for the delineation of 

 large scale mixing phenomena in the sea. 



On the other hand, the distribution of radio- 

 carbon in the atmosphere and mixed layer of 

 the sea is strongly dependent on the rate of 

 exchange of carbon dioxide between the atmos- 



phere and sea, and from a study of the relation- 

 ship between the exchange rates and the radio- 

 active decay rate, it is possible to derive rather 

 precise values for the flux of carbon into the 

 sea and downward through the thermocline. 

 For such calculations it is necessary to assume 

 a model of the atmosphere-sea system based on 

 simplifying assumptions as to the nature of the 

 sea below the thermocline. Calculations of this 

 type, outlining the factors affecting the natural 

 distribution of radiocarbon, have recently been 

 made by Suess (1953), Arnold and Anderson 

 (1957), Craig (1957 (a)), and Revelle and 

 Suess (1957) . The conclusions of these papers, 

 though reached by various means, are quite 

 similar, and we shall briefly summarize the gen- 

 eral results. 



There are two empirically observed effects, 

 of different origin, by which factors aflfecting 

 the natural distribution of radiocarbon may be 

 evaluated. The first of these is the observation 

 that the carbon in the surface layers of the sea 

 (bicarbonate, shell, and organic matter) has an 

 apparent age of about 400 years relative to the 

 terrestrial wood used as standards for radio- 

 carbon dating. The second is the observation 

 that contemporaneous wood has a radiocarbon 

 activity some 2 per cent lower than the activity 

 of 19th century wood, corrected for age to the 

 present date. This decrease in activity, reflect- 

 ing the contribution of C^* free CO2 to the 

 atmosphere by the combustion of fossil fuel, 

 was first found by Suess (1953) and we shall 

 refer to it as the Suess effect. 



The "apparent age" of carbon in the mixed 

 layer of the sea has been measured on Atlantic 

 ocean samples (and one Pacific sample) by 

 Suess (1954), and on Pacific ocean samples 

 around New Zealand by Rafter (1955). The 

 average age determined by Suess is 430 years, 

 while that of the Pacific samples was reported 

 by Rafter as only 290 years. However, the 

 Suess measurements are relative to the 19th 

 century wood standard, corrected for decay to 

 the present, while the Rafter measurements 

 were made relative to a contemporaneous stand- 

 ard which has suffered a decrease in activity 

 due to the Suess effect. Measurement of the 

 effect in the New Zealand standard shows that 

 110 years must be added to the ages reported 

 by Rafter (1955) in order to correct for this 

 effect and make the ages comparable to those 

 reported by Suess (Rafter, manuscript in press) . 



