SECT. 1] RADIOISOTOPES AND LARGE-SCALE OCEANIC MIXING 89 



present. The exact amount added, the times of addition, and the pre-bomb 

 background levels are subject to much uncertainty. Whether the numerous 

 problems involved in unraveling the pre-bomb levels of natural tritium can be 

 treated with sufficient certainty to allow valid conclusions to be drawn is 

 unlikely. If tritium is of value in studies of ocean circulation, it will be through 

 the use of the man-made tritium tracer. 



The application of 32 Si (half-life ~ 700 years) recently discovered in nature 

 by Lai et al. (1960) is also extremely difficult. The low activity levels ( ~ 15 dpm/ 

 kg Si) make direct measurements in sea-water prohibitively time-consuming 

 and expensive. Beyond this, as dissolved silica is greatly depleted in surface 

 ocean water, 32 Si must move to a large extent independently of the water 

 descending from surface to depth as diatom tests. Whether the biologic cycle 

 can be sufficiently well understood to make 32 Si a valuable water tracer is 

 somewhat doubtful. 



Koczy's (1958) work on 226 Ra has already demonstrated the value of this 

 isotope as a natural tracer. As discussed in Volume 3 release from deep-sea 

 sediments is the dominant mode of addition of radium to the oceans. Although 

 little detailed knowledge exists regarding the rates of release, there is some 

 basis for assuming that the release should be relatively constant with time and 

 with geographic location (if sufficiently large areas are considered). Once in the 

 ocean the possibility of incorporation in particulate matter and consequent 

 settling must be considered. Since experimental measurements suggest a 

 universal discrimination against radium relative to calcium during the forma- 

 tion of carbonate materials, the absence of vertical gradients in the calcium 

 concentration in the ocean water provides evidence that the transport of 226 Ra 

 as calcium carbonate is negligible compared to transport in solution. The 

 evaluation of the importance of transport in some other solid phase with a high 

 selectivity for radium awaits a more detailed study of the vertical distribution 

 of chemically similar elements such as strontium and barium. Of great im- 

 portance is the fact that radium is not released to and transported through the 

 atmosphere as is the case for radiocarbon. 



Natural radiocarbon has received the most attention as an oceanic tracer. It 

 is added to the oceans from the atmosphere, which has been shown to be a nearly 

 uniform source both in space and time. Transport in solid phases can be 

 evaluated through dissolved O2 and the total CO 2 data, and from sedimentation 

 rates of CaC03 and the degree of preservation of CaC03 in bottom deposits. 

 These considerations suggest that more than 90% of the transport of radio- 

 carbon within the ocean occurs with the water itself and that interaction with 

 the sedimentary reservoir is negligible. One rather serious complication enters 

 into the interpretation of radiocarbon results ; whereas the transport of dis- 

 solved solids or even of water itself through the atmosphere is of little impor- 

 tance, that of CO2 has great significance. The quantitative evaluation of this 

 mode of transport is therefore necessary if 14 C data is to be used for water 

 circulation rate estimates. 



Of the numerous isotopes produced during nuclear tests, 14 C, 3 H, 9C \Sr and 



