Abstract
A systematic decrease in the 240Pu 239Pu ratio in marine sediments is found with increasing water depth along a transect of cores between Woods Hole and Bermuda. The 240Pu 239Pu atom ratios range from ≅O.18 on the shelf to ≅O.10 at 5000 m but do not change with depth in individual cores. A model is presented which can account for the range of 240Pu 239Pu ratios found in this and other similar studies ( Noshkin and Gatrousis, 1974; Scott et al., 1983). We propose that there have been at least two distinct sources of fallout Pu to this region. The major source of Pu is global stratospheric fallout, characterized by a 240Pu 239Pu ratio of 0.18 and a relatively long residence time in seawater. The second source is characterized by a much lower 240Pu 239Pu ratio, and relative to global fallout it must have been much more efficiently removed from the water column to deep-sea sediments. We suggest that surface-based low yield testing at the Nevada Test Site is the only source of low ratio fallout Pu which could account for the timing, inventories, and refractory characteristics of this second component of fallout Pu inputs to the North Atlantic.
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