50 ao undoubtedly has cuused un inerensed reduction in levels over and above rudioactive decay. The action of rain with subsequent runoff would carry some » amounts into the ocean. Question 9D | | ‘ Apparently some nuclides--like uranium and thorium--sink to the ocean floor, where they concentrate. What other fission products do that? . , Answer 9D Uranium, Thorium, and Actinium comprise the three major scries of naturally occurring radionuclides. All three series end up, following radio- ‘active decay through a number of daughter products over many thousands of years, as stable isotopes of lead. A review of the behavior of these elements | in sea water and occurrence in marine sediments can be found in reference 1. Although fission products comprise more than 200 nuclides of elements” ranging from zine to dysprosium, the major ones of interest in oceanography are listed below.