Radium isotopes in coastal and open ocean surface waters of the Western North Pacific

Abstract

Using manganese-impregnated fiber extraction and high-efficiency gamma counting techniques, we measured the distribution of 228Ra and 226Ra in surface waters near the coast of Japan and in the western North Pacific. There is no evidence in our data that any significant amount of 228Ra is added to open ocean surface waters from the coastal waters around Tokyo Bay. High 228Ra concentrations (> 10 dpm/10 3 kg), were observed along the Kuroshio Current as compared to < 2.5 dpm/10 3 kg between 10° and 30°N of the central gyre, and hence the major source of 228Ra in the surface water is likely to be the East Asian continental shelf zones. A simple one-dimensional eddy diffusion and advection model is used to explain the observed decrease of 228Ra from coast to the open ocean. The model results indicate two mixing regimes across the Kuroshio Current System with apparent eddy diffusion coefficients of K y = 4 × 10 5 cm 2 s −1 at distance y < 200 km from the coast, and K y = 4 × 10 7 cm 2 s −1 at y > 200 km. Along ∼40°N where an eastward flow of the ‘Kuroshio Extension’ prevails, an advective flow of > 0.1 knot is consistent with the observation of nearly constant 228Ra along the track. The geographical distribution pattern of 228Ra is clearly different from that of atmospherically derived 210Pb. Thus the 228Ra in surface water serves as a useful tracer that accompanies fluvially and coastally derived elements during their subsequent lateral transport toward the central gyre.