Chapter 11 



Tracer Studies of the Sea and Atmosphere 



113 



Thermonuclear weapons may also be expected 

 to produce some carbon 14 because of the 

 neutrons released into the atmosphere in the 

 explosion. A contemporary sample of grass, 

 collected in the summer of 1955 in S.W. Kansas 

 by the writer, was analyzed for C^* content by 

 M. Rubin at the U.S. Geological Survey labora- 

 tory. This grass was found to be about 2.5 

 per cent higher in O* content than the 19th 

 century wood, corrected for age, used as the 

 U.S.G.S. radiocarbon standard (Suess, 1955). 

 The samples and standard were analyzed for 

 C^^ content by the writer and the results cor- 

 rected for isotopic fractionation, and the sample 

 was counted twice. Thus the measurement is 

 quite precise, and probably indicates that the 

 atmospheric radiocarbon content has risen about 

 2 per cent above normal at the present time, 

 due to thermonuclear neutron production. For 

 future geochemical studies with natural radio- 

 carbon it will be important to monitor continu- 

 ously the activity of contemporaneous plants 

 and atmospheric carbon dioxide, though the 

 effect will be insignificant for radiocarbon dat- 

 ing studies for some time yet. 



The situation with tritium is different. The 

 earliest rain ever analyzed for tritium content 

 fell in Chicago in May of 1951; since October 

 1952 Libby and his co-workers at Chicago have 

 produced an essentially continuous record of 

 the tritium content of Chicago rain, and have 

 analyzed a great many other samples from many 

 parts of the world. Their data show that there 

 was no significant production of tritium in the 

 November 1952 Ivy test (Kaufman and Libby, 

 1954). However, the March 1954 Castle ther- 

 monuclear tests produced an increase in tritium 

 concentration of Chicago rain from an average 

 value of 9 to a maximum value of 450 atoms 

 T/lQi^ atoms H; i.e., a factor of 50 (von 

 Buttlar and Libby, 1955). Even more striking 

 was the discovery that the tritium content of 

 southern hemisphere waters showed no signifi- 

 cant increase in tritium concentration, and snow 

 samples collected from the Antarctic as late as 

 February of 1955 showed that during this in- 

 terval no significant amounts of artifically pro- 

 duced tritium had crossed the equator (Bege- 

 mann, 1956). 



Begemann's recent data show that the tritium 

 rained out of the northern hemispheric atmos- 

 phere with a mean-life of about 40 days for 



the decrease in tritium concentration; as late 

 as the end of 1955 the tritium concentration of 

 Chicago rain was still about three times normal. 

 In Section II above it was concluded that the 

 world inventor}' of tritium was about 20 kg, 

 with about 5 kg in the mixed layer of the sea, 

 and about 15 kg in the deep sea. The Chicago 

 data show that the tritium content of the sur- 

 face ocean waters has increased by at least a 

 factor of four, indicating that the order of 

 magnitude of 20 kilograms of man-made trit- 

 ium has so far rained out into the ocean. Thus 

 the amount of tritium produced by man is now 

 about equal to the natural steady-state inventory. 



IV. Applications of tracer techniques to the 

 study of physical processes in the sea and 

 atmosphere 



In this section we describe a few of the more 

 obvious applications of the tracer techniques 

 and isotopes described in the previous sections 

 to specific problems of transfer phenomena in 

 the oceans and atmosphere. The topics are sub- 

 divided in terms of the isotopes discussed, in 

 order to facilitate reference to points in pre- 

 ceding sections and parts of this section. 



Carbon 14 



Carbon 14 is perhaps the most useful of the 

 isotopic tools available for geophysical and 

 geochemical studies, especially when used in 

 conjunction with oxygen 18 data; the 5700 

 year half-life and the universal distribution of 

 carbon in organic and inorganic reservoirs make 

 it ideal for such purposes. The most obvious 

 application of immediate interest is the dating 

 of the bicarbonate of the deep-sea waters, in 

 order to determine the mixing rate of the 

 oceans. Unfortunately, only one definitive set 

 of measurements of this type has been made, 

 namely the U. S. Geological Survey laboratory 

 measurements of waters east of the Lesser An- 

 tilles at approximately 57° W. and 16° N., 

 made by M. Rubin (personal communication) . 

 These data are shown below: 



Carbon 14 

 Depth (meters) age (years) 



Surface 652 



640 634 



1640 628 



1750 841 



