The geochemistry of fallout plutonium in the North Atlantic: I. A pore water study in shelf, slope and deep-sea sediments

The distribution of fallout plutonium between pore waters and sediments in the Northwest Atlantic is examined in order better to define the potential for Pu migration from and within marine sediments. Along a transect of seven cores collected between Woods Hole and Bermuda, the solid phase 239,240Pu, 210Pbex and 137Cs results do not suggest that any of these tracers are preferentially mobile within the cores. Pore water Pu activities appear to be controlled primarily by the distribution of Pu in the solid phase, with an operationally determined distribution coefficient, Kd, in the range of 0.2-23 x 106. There is a trend in the Kds along the transect, with higher values in the 104-105 range on the shelf and slope, to lower values < 104 in the deep-sea cores. The exact mechanism for this shift has yet to be determined. While the Pu pore water activity at the sediment-water interface is elevated over near bottom water activities, the potential for Pu remobilization out of the sediments is small. Sediment mixing models are not significantly altered by the inclusion of Pu diffusion terms since the pore water terms are so small relative to typical sediment mixing rates