118 



Atomic Radiation and Oceanography and Fisheries 



studies over various sections of the oceans, 

 because of the latitudinal decrease in deuterium 

 and oxygen 18 concentration of oceanic water 

 vapor, and the known temperature dependence 

 of the isotopic selection in evaporation. 



Craig, Boato, and White (1956) have shown 

 how deuterium and oxygen 18 measurements 

 can be usesd to determine the proportions of 

 juvenile or magnetic water to reheated ground 

 water in thermal springs, and volcanic steam. 

 These isotopes, together with tritium, have im- 

 portant applications to practically all hydrologic 

 problems, and the exploitation of such tech- 

 niques has barely begun. 



Tritium and Strontium 90 



As described in Part II, the tritium measure- 

 ments made by Libby and his co-workers fur- 

 nish an independent value for the mixing rate 

 in the sea; more detailed studies will surely 

 provide important information on the oceanic 

 mixing phenomena. The production of tritium 

 in thermonuclear explosions provides an iso- 

 topic tracer for determination of atmospheric 

 mixing times across the face of the earth and 

 storage times in the atmosphere. 



The measurement of the world-wide distribu- 

 tion of strontium 90 produced by nuclear deto- 

 nations has been done by W. F. Libby and 

 E. A. Martell at the University of Chicago. 

 The results of their work have recently been 

 described by Libby (1956 a, b). The radio- 

 nuclides produced by low-yield kiloton weapons, 

 and part of the activity produced by the higher- 

 yield megaton weapons, are distributed within 

 the troposphere in a belt corresponding to the 

 latitude of the test site. This material has a 

 tropospheric life which is a function of particle 

 size; some of the activity may circle the earth 

 two or three times within the hemisphere in 

 which it was produced before being washed out 

 of the atmosphere. However, the mean life of 

 this tropospheric material is only a few weeks. 



More interesting is the fact that Libby and 

 Martell find that half or more of the radio- 

 strontium produced by the megaton weapons 

 is distributed over both hemispheres and falls 

 out much more slowly, the mean storage time 

 in the atmosphere being of the order of ten 

 years. They conclude that this material is car- 

 ried up into the stratosphere, above the tropo- 

 pause, where it is mixed horizontally in a time 

 comparable to the storage time at this level. 



The contrast between the distribution of 

 megaton weapon produced radiostrontium and 

 tritium is extremely significant. As noted in 

 Part III, Begemann and Libby find that the 

 artificially produced tritium is confined to a 

 single hemisphere and is rapidly washed out 

 of the atmosphere; this material thus follows 

 the pattern of the activities which remain in 

 the troposphere. The tritium and fission prod- 

 uct data thus show that over a period of months 

 there is virtually no cross-hemispheric mixing in 

 the troposphere, but that over a period of years 

 the stratosphere is well-mixed horizontally. The 

 failure to detect tritium carried up into the 

 stratosphere with the megaton weapon produced 

 radiostrontium may be due to the instantaneous 

 combustion of tritium to HTO by the catalytic 

 action of the oxides of nitrogen produced in 

 the blast (Harteck, personal communication). 

 As water, the tritium may be frozen out at the 

 lower cold trap, in the tropopause, where the 

 temperature is about — 70°C, and thus pre- 

 vented from entering the stratosphere. 



On the other hand, Martell points out (per- 

 sonal communication), that the thermal energy 

 of the fireball is still quite large by the time a 

 fireball produced by a megaton weapon has 

 risen to the height of the tropopause. In order 

 for HTO to condense and thus be trapped be- 

 low the tropopause, it is necessary to assume 

 that the lighter constituents of the fireball have 

 diffused into the cooler outer layers. Martell 

 suggests that if such is the case, then the actual 

 explanation may be that the portion of the cloud 

 containing the HTO may not have sufficient 

 thermal energy to penetrate the tropopause, and 

 as a result, this portion of the cloud merely 

 expands horizontally below the tropopause. 



V. Conclusions 



From the discussion in the preceding parts of 

 this report, it is apparent that the advent of 

 manmade nuclear reactions introduced a series 

 of geophysical and geochemical experiments on 

 a vast scale. It is fortunate that the introduction 

 of such experiments came at a time when geo- 

 chemists were well underway towards the under- 

 standing of natural transfer phenomena by 

 means of studies based on naturally CKCurring 

 isotopes in their steady state biogeochemical 

 cycles. It should be clear that the need for this 

 knowledge is such that every effort should be 



