Plutonium and americium in anoxic marine sediments: Evidence against remobitization |
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| Authors: | R Carpenter TM Beasley |
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| Institution: | Department of Oceanography, University of Washington, Seattle, WA 98195, U.S.A.;School of Oceanography, Oregon State University, Marine Science Center, Newport, OR 97365, U.S.A. |
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| Abstract: | 239 + 240Pu activities of 100–450dpm/kg are found down to 15–18 cm in anoxic Saanich Inlet sediments, with a subsurface maximum in undisturbed deposits. Integrated 239 + 240Pu inventories which overlap delivery estimates are present both in two cores of anoxic sediments from Saanich Inlet and in one core of oxic sediments 65 km away in Dabob Bay, Washington. ratios in Saanich Inlet sediments overlap ratios in unfractionated midnorthern latitude fallout, in oxic sediments from the Washington continental shelf, and in anoxic sediments from two basins off southern California and Mexico. The ratios in three intervals of Saanich Inlet sediments are also in agreement with ratios previously reported for oxic coastal marine sediments. The Pu inventories, the and ratios, and the Saanich Inlet Dabob Bay comparison all argue that Pu is not rapidly remobilized in anoxic sediments.The subsurface 239 + 240Pu activity maximum is not in agreement with the historical record of peak Pu fallout in 1963–1964 unless our 210Pb-derived sedimentation rates are incorrectly high. However, they are in good agreement with previous 210Pb and varve chronologies in Saanich Inlet, and also give reasonable dates for times when 239 + 240Pu and SNAP-9A supplied 238Pu first appear in the sediments. We conclude they properly date the maximum in sedimentary 239 + 240Pu activity at 1970–1973, and seek explanations for the 7–10yr time lag after peak fallout.239 + 240Pu inventories in one core from the eastern basin of the Cariaco Trench and in two cores from Golfo Dulce. an anoxic basin off the Pacific coast of Costa Rica, are also in reasonable agreement with fallout delivery to these latitudes when excess 210Pb inventories and fluxes are used to verify recovery of at least a major fraction of the most recently deposited sediments. |
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