106 



Atotnk Radiation and Oceanography and Fisheries 



and predicted radiocarbon inventories is that 

 they agree within present hmits of error. 



Tritium 



Tritium (H^) is made in the upper atmos- 

 phere, primarily in the "stars" or nuclear ex- 

 plosions produced by the collisions of primary 

 cosmic ray particles with the atmospheric 

 molecules; it is naturally radioactive, decaying 

 by ^' emission to helium 3 with a half-life of 

 about 12.5 years (Kaufman and Libby, 1954). 

 The T atoms "burn" very quickly to HTO and 

 enter the precipitation — evaporation cycle of 

 water. A very small amount of tritium is pro- 

 duced in rocks by the nuclear reaction of 

 lithium with neutrons produced by spontaneous 

 fission of uranium and from (a,n) reactions 

 (Morrison and Pine, 1955) ; the production of 

 tritium by this process is insignificant relative 

 to the atmospheric production. 



Detailed studies of the distribution of tritium 

 in natural waters have been made by Libby and 

 his co-workers at Chicago. The natural concen- 

 tration of tritium (before thermonuclear tests) 

 in continental waters averages about 5 X 10"^® 

 atoms of tritium per atom of hydrogen. (Fol- 

 lowing Libby's usage, such a concentration will 

 hereafter be referred to as 5 tritium units, ab- 

 breviated as T.U.) The concentration in oceanic 

 rains is about 1 T.U., while in the surface 

 waters of the ocean itself the concentration 

 appears to be as low as 0.2 T.U. The sea is, 

 of course, the ultimate resting place of the 

 tritium formed in the atmosphere, and the low 

 concentration in the oceanic rains relative to 

 continental rains is principally due to tritium 

 removal by direct molecular exchange with the 

 sea surface (see below) . 



Kaufman and Libby (1954) calculated the 

 tritium production rate in the atmosphere by 

 equating it with the rate at which tritium dis- 

 appears from the atmosphere into the ocean, 

 taken as the sum of the tritium entering the 

 ocean by run-off from continental rains and the 

 tritium entering directly via oceanic rains. For 

 this calculation only the average run-off and 

 ocean precipitation figures, and measured av- 

 erage tritium content of such waters, are needed. 

 They obtained a net production rate, averaged 

 over the earth's surface, of .12 T atoms per 

 cm2 per second. Von Buttlar and Libby (1955) 

 measured many more rain samples, and also 



analyzed 5 samples of ocean water, from which 

 they could estimate the tritium content of the 

 water vapor which evaporates from the sea 

 surface. Using this latter figure they calculated 

 the production rate over the oceans, assuming 

 that tritium is lost from the atmosphere only 

 by oceanic rain, and gained by production and 

 oceanic evaporation, and obtained a figure of 

 0.11 to 0.12 T atoms per cm- per sec. A similar 

 calculation was made for the production rate 

 over land, assuming tritium is lost from the 

 continental atmosphere only by continental rains 

 running off into the ocean, and gained by pro- 

 duction, and by transport of ocean vapor onto 

 the continents. Using the tritium data for av- 

 erage Mississippi Valley rains, they obtained a 

 figure of 0.16. Their estimated world average 

 production rate is 0.14 with a probable un- 

 certainty of less than 20 per cent. This value 

 agrees precisely with the expected world pro- 

 duction rate calculated by Currie, Libby, and 

 Wolfgang (1956) from their experimental 

 measurements on tritium production in nitrogen 

 and oxygen by bombarding protons of 450-Mev 

 and 2-Bev energies. Previous experiments and 

 calculations by Fireman and Rowland (1955) 

 gave an expected production rate of 0.2 T 

 atoms/cm- sec, also in good agreement with 

 the rate apparently observed. 



However, the tritium production rate must 

 be a good deal higher than the figures given 

 above. Von Buttlar and Libby calculated that, 

 with such a production rate, and with the ob- 

 served surface sea concentration of about 0.24 

 T.U., then the mixed layer of the sea is about 

 100 meters deep if one assumes that all the 

 tritium of the sea is in the mixed layer. Though 

 this depth is consistent with observational data 

 on the sea, such a calculation assumes that the 

 mixed layer is sealed off from the deep sea so 

 that no tritium mixes below the thermocline, 

 and the question then arises as to just how much 

 mixing across the thermocline does, in fact, 

 occur. 



As discussed by Wooster and Ketchum in a 

 separate paper in this report, various observa- 

 tions on ocean currents and on the heat flux 

 through the ocean floor, indicate that the deep 

 ocean water turns over, or mixes with surface 

 water, in times of the order of a few hundred 

 years. Assuming a generalized two-layer model 

 of the sea, consisting of a shallow mixed layer 

 about 75 meters deep on the average, and a 



