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Origin of radioactivity at Niue Island

1967; Taylor & Francis; Volume: 10; Issue: 6 Linguagem: Inglês

10.1080/00288306.1967.10423221

1175-8791

J. C. Schofield,

Geological and Geochemical Analysis

Abstract A high content of radioactivity in sediments, when sea water is the source of the radioactive elements, depends on the slow rate of sedimentary deposition. The radioactive content of the Niuean soils is comparable with that found in deep-sea sediments, but the rate of sedimentation in deep-sea areas is unlikely to have been as rapid as in the now dry lagoon at Niue. Hence a sea-water origin for the abnormally high radioactivity at Niue is unlikely. A separation from uranium of the daughter products, ionium and radium-226 (the main elements at Niue), takes place in hydrothermal solutions as well as in sea water. The latter could have come from the volcanic substructure at Niue, which could have been phonolite—often one of the most radioactive igneous rocks. The anomalous distribution of radioactive soil values at Niue favours a volcanic origin rather than a sea-water origin. Evidence is also presented that shows radioactivity in the limestone has been derived from ground water and not percolating rain water. The radioactive rock values are also higher at some levels than at others, the high values being due to past water-table stillstands that can be correlated with sea-level stillstands. Further work might establish the relative lengths of time occupied by each stillstand, as well as their ages. Fieldes et al. (1960) give an age of 200,000 years for the lagoonal basin, but this need not be more correct than their maximum age of 700,000 years. Ground water will be the main source of freshwater in the near future at Niue and if hydrothermal activity is still continuing the ground water may be injurious to health and should be tested for radioactivity. A coincidence of best crop areas with greatest radioactivity, directly above the centre of the buried caldera, may mean that the radioactive elements, or other elements derived from hydrothermal solutions are favouring plant growth.