108 



Atomic Radiation and Oceanography and Fisheries 



much higher than that of the tropospheric va- 

 por, which averages about 1 T.U. From the 

 strontium data we assume that the mixing time 

 of water vapor through the tropopause is at 

 least 10 years. 



Assuming a tritium production rate of 1.4, 

 half of which is in the stratosphere, the trit- 

 ium concentration of stratospheric water vapor 

 is then calculated to be at least 300,000 tritium 

 units. This is an astounding concentration fac- 

 tor relative to tropospheric water vapor. Re- 

 cently the present writer and F. Begemann 

 analyzed a series of samples of atmospheric 

 molecular hydrogen for deuterium and tritium 

 content respectively. Mass spectrometric meas- 

 urements showed that all samples contained 

 about 2-10 per cent less D than ocean water, 

 falling just in the range of meteoric waters, and 

 containing far too much deuterium to represent 

 thermodynamic equilibrium with water vapor. 

 These data confirmed a few previous measure- 

 ments (cf. Harteck, 1954) which showed that 

 the molecular hydrogen in the atmosphere must 

 form by direct photodissociation of water vapor 

 in the region around 70 km altitude, rather than 

 by bacterial decomposition of organic matter 

 which has been shown to produce hydrogen in 

 isotopic equilibrium with water. We may thus 

 assume that the tritium content of stratospheric 

 molecular hydrogen is about the same as that of 

 the stratospheric water vapor. 



Assuming that the hydrogen is statistically 

 distributed in the atmosphere, so that i is 

 above, and | below, the tropopause, and taking 

 again the mixing time through the tropopause 

 as 10 years, we then calculate the tritium con- 

 tent of the molecular hydrogen in the trop- 

 osphere. This figure is found to be 100,000 

 tritium units, probably as a minimum figure 

 because of slow vertical mixing from the base 

 of the stratosphere to the 70 km level where 

 the hydrogen is made, and because of the indi- 

 cation that more than half the tritium is found 

 initially in the stratosphere. The tritium con- 

 tents measured by Begemann on a dozen sam- 

 ples of tropospheric hydrogen range from 50,- 

 000 to 100,000 tritium units, averaging about 

 80,000 T.U., in excellent agreement with the 

 calculated value when the various uncertainities 

 are considered. 



It thus appears that the high tritium content 

 of tropospheric hydrogen can be satisfactorily 

 explained by purely geophysical reasoning based 



on the stratosphere-troposphere exchange time 

 as estimated from Libby's Sr^o data, and the 

 known concentration of water vapor in the 

 stratosphere. This explanation seems more likely 

 than the intricate series of photochemical mech- 

 anisms proposed by Harteck (1954) which at 

 best may account for a tritium concentration of 

 about 1000 T.U. in the molecular hydrogen. 



Beryllium 7 



Beryllium 7 is formed in cosmic ray stars, 

 the peak production occurring at about 15 km. 

 It decays by electron capture to lithium 7 with 

 a half -life of about 53 days. The discovery, 

 and the elucidation of the geochemical history, 

 of this cosmic ray produced nuclide is due to 

 Arnold and Al-Salih (1955). 



Once formed in the atmosphere, the beryl- 

 lium burns to the nonvolatile BeO or possibly 

 Be (OH) 2, either of which diffuses until en- 

 countering a dust particle and adhering thereon. 

 It is thus a tracer for the atmospheric dust, on 

 which it is washed out of the atmosphere by 

 rain, ultimately going into the ocean. Arnold 

 and Al-Salih detected radioberyllium in 22 rain 

 and snow samples from Chicago and Indiana, 

 the average absolute assay being 6x lO*' atoms/ 

 liter. The estimated world-wide average pro- 

 duction rate is 0.04 atoms per cm^ per second, 

 based on estimated rates of transfer and mix- 

 ing in the stratosphere and troposphere. Most 

 of the mixing rates involved are of the order 

 of magnitude of the half-life, which makes 

 calculation of the production rate difficult but 

 greatly enhances the utility of the isotope for 

 studying atmospheric processes, especially when 

 used in conjunction with tritium. 



A detailed discussion of the beryllium 7 pro- 

 duction rate and atmospheric residence time has 

 recently been given by Benioff (1956). He 

 calculates the production rate to be 5.0 atoms/ 

 cm-min in the stratosphere and 1.3 atoms/cm-- 

 min in the troposphere, and he finds that a 

 stratospheric residence time of the order of 

 years is required to match these production rates. 

 Thus his stratospheric residence time agrees with 

 that found by Libby for fission products. 



Beryllium 10, a yS" emitter with a half-life 

 of 2.5 X 10*^ years, is also formed in the cosmic 

 ray stars. J. R. Arnold has recently identified 

 this isotope in deep sea sediment samples 

 (manuscript in press) ; it should be of great 



