Non-destructive alpha-spectrometry and radiochemical studies of deep-sea manganese nodules |
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| Authors: | Yoshiyuki Nozaki Han-Soeb Yang |
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| Institution: | Ocean Research Institute, University of Tokyo, Nakano-ku, Tokyo 164, Japan |
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| Abstract: | A deep-sea Mn nodule handpicked from top of a 20 cm diameter North Pacific tripod core was directly counted by an α-spectrometer with a silicon surface barrier detector. A distinct 210Po peak was recorded for the nodule upper side in contact with bottom seawater. Based on the spectrum, the 210Po (supported by 210Pb with 22 year half life) is thought to be confined in the top few microns of the nodule. Since the 210Po peak diminishes considerably for the bottom side resting on sediment, nondestructive α-spectrometry is useful for determining the sea floor orientation of nodules. This method was tested for a western North Pacific dredged Mn nodule, and was successful in identifying the orientation (and also surface loss of the nodule by dredging).Radiochemical analysis was also carried out for carefully scraped samples from the top and bottom of the two nodules. The growth rates of the hand-picked nodule based on excess 230Th and 231Pa profiles were 1 to 4 mm/106 y and apparently varied with time. The dredged sample has grown at 2 to 6 mm/ 106 y and the growth has likely been episodic. The radionuclide composition was significantly different between the top and bottom. Evidence was found that the radionuclide fluxes of 230Th and 231Pa into the nodules considerably changed during the growing histories. The 230Th/232Th activity ratios in the nodule tops are comparable with those in bottom seawater suggesting that the top is growing with metals of seawater origin. However it remains unresolved whether the nodule bottoms are growing by utilizing metals in the sediment pore water. |
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