Sea water samples of 130 to 190 liters were collected to measure the concentration of thorium and protactinium in the ocean. After concentration at sea and chemical isolation of the elements in the laboratory, the various isotopes were resolved with an α-particle spectrometer. The precipitation of Th230 and Pa231 as they are formed from uranium was found to be essentially quantitative, less than 0.05% of the Th230 and less than 0.2% of the Pa231 remaining in solution. All thorium isotopes seem to be more concentrated in the one sample of North Atlantic Bottom Water than in several samples of surface or intermediate waters. An activity ratio of 15 was observed for Th228/Th232 at all depths. This ratio is probably due to a partial solution of Ra228 as it is formed from Th232 in the sediment and/or river contribution of Ra228. The presence of excess Ra228 in coastal waters was confirmed by measurements made on samples of shell material.
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