The concentrations of
228Ra and
226Ra in the surface and subsurface sea water samples collected from the East Indian and the West Pacific Ocean were determined. Radium isotopes were analyzed by γ-ray spectrometry or
228Th milking method. Surface waters of the northern Indian Ocean to the north of latitude 30°S contained higher concentrations of
228Ra (63-32dpm/1, 000l) than those of southern area and the West Pacific Ocean (less than 8dpm/1, 000l). The samples from near shore water of Indonesia and the South China Sea showed high
228Ra contents, 629-164dpm/1, 000l. The concentrations of
228Ra in subsurface waters were very low except for the South China Sea. A major source of
228Ra in the Indian Ocean may be coastal sediments of Indonesia. The mixing rate of surface water masses across the subtropical convergence was calculated by using a simple box model to be about 41 km/y.
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