Chapter 11 



Tracer Studies of the Sea and Atmosphere 



107 



homogeneous deep sea below the thermocline 

 marking the interface between the layers, Craig 

 (In press (a) ) derived equations relating the 

 production rate of a radioactive isotope to the 

 concentrations of the isotope in the two layers 

 of the sea and the mixing time through tlie 

 thermocline. (These functions are discussed 

 briefly in a separate paper by the writer in this 

 report, in which calculations on the disposal 

 of fission products in the sea and their ultimate 

 steady state concentrations are discussed.) The 

 applications of such calculations to the distribu- 

 tion of radiocarbon in the atmosphere and sea 

 were demonstrated; these results are discussed 

 in Section IV of this paper. 



Application of such calculations to the dis- 

 tribution of natural tritium (Craig, 1957 (b) 

 and manuscript in preparation) shows that for 

 reasonable internal mixing rates of the sea, most 

 of the world inventory of tritium must actually 

 be in the deep sea below the thermocline. Thus 

 for a deep water replacement time, or residence 

 time of a water molecule in the deep sea before 

 mixing into the surface layer, of to 1000 

 years, and with a surface concentration of 0.24 

 T.U., the tritium flux into the sea must be 

 between 7.6 and 0.3 atoms cm^/sec. For the 

 most reasonable deep sea residence time of the 

 order of a few hundred years, the flux must 

 be somewhere between 0.4 and 0.8. It is found 

 that about | of the total tritium in the sea is 

 below the thermocline, with a deep-sea tritium 

 concentration of about 0.014 tritium units. 



The tritium production rate over the North 

 American continent was recalculated (Craig, 

 op. cit.) by taking into account the removal of 

 tritium from the continent by the outgoing 

 water vapor which does not condense over the 

 land. This calculation gives a world average 

 production rate of from 0.6-0.8 after correction 

 for the latitudinal geomagnetic eflfect on the 

 incoming cosmic rays. A tritium production 

 rate of this order of magnitude indicates an 

 average deep-sea residence time of water of 

 about 250 years, for a simple two-layer ocean. 

 Calculations based on a second-order ocean 

 model in which the deep sea reservoir is exposed 

 to the atmosphere at high latitudes would give 

 a longer residence time relative to the mixed 

 layer of the sea because of direct entry of 

 tritium from the atmosphere to the deep sea. 

 (See the discussion of radiocarbon residence 

 times in Section IV of this paper.) 



However, if the bulk of the tritium is not 

 produced by cosmic radiation, but by solar 

 accretion (see below), the world average pro- 

 duction may be as high as 1.7 atoms cm^/sec 

 because the geomagnetic correction applies only 

 to tritium produced by cosmic rays in the trop- 

 osphere. 



The calculated production rate over the 

 oceans of about 0.14 is obtained by considering 

 only the transfer of tritium into the sea by 

 rainfall. Since rainfall appears to account for 

 only about one-tenth of the tritium which ac- 

 tually enters the sea, it appears that the trans- 

 fer of tritium from atmosphere to sea by direct 

 molecular exchange across the sea surface is 

 about 9 times as effective as the scrubbing action 

 of precipitation. 



A production rate of 1.4 atoms of tritium/ 

 cm^sec means that the world inventory of trit- 

 ium, before thermonuclear tests, was about 

 20 kg of tritium, or 200 megacuries, essentially 

 all of which is in the ocean. However, from 

 the experimental data obtained by the workers 

 cited above on the production of tritium by 

 the action of protons on nitrogen and oxygen, 

 it appears very doubtful that the cosmic ray 

 production rate can be much higher than about 

 0.2. In fact it is probably necessary to assume 

 that tritium is produced on the surface of the 

 sun and is directly accreted into the earth's 

 atmosphere, rather than being a secondary re- 

 sult of the action of the cosmic ray protons on 

 the atmosphere, as postulated by Feld and Craig 

 (Craig, 1957(b)). 



From a study of the fall-out rate of strontium 

 90 pushed into the stratosphere by large atomic 

 detonations, Libby (1956a, b) calculates the 

 stratospheric residence time of strontium to be 

 about 10 years (cf. Section IV of this paper). 

 Since at least half of the tritium production 

 should take place in the stratosphere even if 

 all the production is due to the action of pro- 

 tons on the earth's atmosphere, slow mixing 

 through the tropopause will pile up tritium in 

 the stratosphere in the same way that slow 

 exchange across the sea surface builds up the 

 radiocarbon concentration in the atmosphere. 

 One-sixth of the atmosphere is above the tropo- 

 pause on the average, but the water vapor con- 

 centration is so low that only about 0.3 per cent 

 of the total water vapor in the atmosphere is 

 in the stratosphere; thus the tritium concentra- 

 tion of the stratospheric water vapor will be 



