Surface and subsurface samples of seawater collected in the Atlantic, Arctic, Indian, Pacific, and Antarctic oceans have been analyzed for 228Ra. The results agree with those obtained by Moore (1969) and show that near‐shore surface waters normally contain much higher 228Ra concentrations than surface waters collected in the open sea, that surface waters normally contain much higher 228Ra concentrations than intermediate and deep waters, but that nearbottom waters are somewhat enriched in 228Ra. Surface waters of the North Atlantic and Indian oceans contain significantly higher concentrations of 228Ra than those of the Pacific and Antarctic oceans. Vertical profiles of 228Ra activities have been used to calculate values of the constant for vertical eddy diffusion Kz of about 0.3 cm2/sec in the Arctic Ocean in the zone between the cold water mass that originates in the Bering Sea and the underlying warmer but more saline water mass that originates in the Atlantic. Values of Kz of about 0.5 to about 7 cm2/sec have been calculated for the upper 200 meters of the Atlantic and Pacific and of about 5–100 cm2/sec for the bottom water masses of the Atlantic and Pacific. The. meridional gradient of 228Ra in the surface waters south of Australia and New Zealand has been used to calculate a value of the constant for horizontal eddy diffusion perpendicular to the wind‐driven circulation KY of about 107–108 cm2/sec.

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Measured anthropogenic radionuclide profiles in sediment cores from the Hudson River estuary were compared with profiles computed by using known input histories of radionuclides to the estuary and mixing coefficients which decreased exponentially with depth in the sediment. Observed 134Cs sediment depth profiles were used in the mixing rate computation because reactor releases were the only significant source for this nuclide, whereas the inputs of 137Cs and 239,240Pu to the estuary were complicated by runoff or erosion in upstream areas, in addition to direct fallout from precipitation. Our estimates for the rates of surface sediment mixing in the low salinity reach of the estuary range from 0.25 to 1 cm2/yr, or less. In some areas of the harbor adjacent to New York City, where fine‐particle accumulation rates are generally >3 cm/yr, and often as high as 10 to 20 cm/yr, sediment mixing rates as high as 10 cm2/yr would have little effect on radionuclide peak distributions. Consequently, anthropogenic radionuclide maximum activities in subsurface sediments of the Hudson appear to be useful as time‐stratigraphic reference levels, which can be correlated with periods of maximum radionuclide inputs for estimating rates and patterns of sediment accumulation.