RADIOCHEMICAL STUDIES ON MANGANESE NODULES OF DEEP-SEA ORIGIN

Manganese nodules from different parts of the ocean floor were analyzed for their uranium, thorium, protactinium, and radium contents. (1) The uranium : thorium ratio in nodules is quite variable, and is 3 to 6 orders of magnitude lower than that in sea water. Uranium concentrations range from 4 to 13 ppm, with most values falling between 6 and 10 ppm. Thorium concentrations have a much wider spread (3 to 150 ppm) and reflect both a geographic and depth dependence. (2) The 230Th and 231Pa in the outermost layers of a nodule greatly exceed the amounts which may be supported by the uranium present. The decrease with depth of these two isotopes allow the growth rates of the nodules to be determined. The rates for eighteen specimens vary from < 1-6 mm/106 year; most of these fall within rather narrow limits (2-4 mm/106 year). (3) The 230Th : 231Pa ratios at the nodule surface are considerably smaller than that predicted from the production of these two nuclides by the uranium dissolved in the ocean. While an activity ratio of 11 might be anticipated, values as low as 2·4 were observed. Since 230Th : 231Pa ratios considerably greater than 11 are found at the tops of deep-sea sediment cores, the two species must become separated during the sedimentary process. (4) The distribution of 230Th and 231Pa on the surface of a given nodule is not uniform. Higher concentrations occur on the upper than on the lower sides. This suggests that these nuclides are precipitated only on those portions of the nodule in contact with sea water. (5) Measurements of 226Ra reinforce the conclusion that this element migrates in sedimentary deposits. (6) For the oceans as a whole the amount of 230Th and 231Pa in manganese nodule deposits is probably no more than 5% of that generated by the uranium dissolved in the sea. © 1969.