Plutonium isotopes in the North Atlantic
Buesseler, Ken O.
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KeywordRadioactive pollution of the sea; Nuclear fuels; Radioactivity; Diagenesis; Oceanus (Ship : 1975-) Cruise OC152; Oceanus (Ship : 1975-) Cruise OC173
The artificial radionuclide Plutonium (Pu) has been introduced into the environment primarily as fallout from atmospheric nuclear weapons testing during the 1950's and 1960's. Earlier studies of Pu geochemistry are generally based upon the measurement of the combined activities of 239Pu and 249Pu (detected by alpha-counting and written as 239,240Pu) and assume an identical geochemical behavior for Pu from any of its fallout sources. A major focus of this thesis is the development of a mass spectrometric (m.s.) technique for the analysis of Pu in marine sediments, pore waters, sediment trap material and sea water from the North Atlantic. With the m.s. technique, not only is the detection limit for 239,240Pu increased by over an order-of-magnitude, but the 240Pu and 239Pu isotopes can be separated as well. The increased sensitivity for Pu provided by m.s. allowed me to measure Pu in deep-sea pore waters for the first time. Pore water studies are sensitive indicators of early diagenetic reactions, and can be used to examine the unresolved question of the extent of Pu remobilization out of marine sediments. Along a transect of cores ranging from highly reducing muddy sediments on the shelf to more oxic and carbonate-rich sediments in the deep-sea, I have found that the solubility of Pu is predominantly controlled by the distribution of Pu in the solid phase. The calculated 239,240Pu distribution coefficients (Kd = dpm per kg on solids/dpm per kg in solution) range from 0.2-23 x l04, with some suggestion of a trend towards lower values in the deeper cores (Kd's <104 in cores from water depths >2500m). Diffusive flux calculations based upon the observed Pu pore water gradients suggest that since its introduction, negligible Pu has been remobilized out of the sediments at all of the sites. On a time scale of 102 -103 years however, Pu remobilization may be significant. A large suite of sedimentary Pu and 210Pbex inventory data are also examined from the Northwest Atlantic shelf, slope and deep-sea sediments. Comparisons between Pu and 210Pb are of interest since both isotopes are predominantly supplied by atmospheric delivery to coastal waters, and since both isotopes are used to study recent accumulation and mixing processes in marine sediments. Inventories of these tracers will reflect their source function, removal efficiencies, and lateral transport in water and particles. A major conclusion is that the sediment inventories decrease with increasing water depth, reflecting a decrease in the net scavenging of these elements off-shore. Pu sediment inventories drop-off with increasing water depth much more rapidly than 210Pbex inventories, due to either the shorter residence time of 210Pb compared to Pu with respect to water column removal processes, or due to comparisons between the naturally occurring 210Pb steady-state scenario and the more recently introduced fallout Pu. When Pu and 210Pbex inventories are summed over water depths out to 4000 m in the Northwest Atlantic, the sediments can account for roughly 24 ± 8% of the expected Pu and 83 ± 15% of the expected 210Pbex inputs. The 240Pu/239Pu ratio data provided by the m.s. provide a unique insight into the relationship between the specific sources of fallout Pu and its geochemical behavior in the oceans. I find a systematic decrease in the 240Pu/239Pu ratio in sediments from 0.18 on the shelf to 0.10 in deep-sea (4500- 5000 m) sediments from the Northwest Atlantic. This trend is consistent with a model whereby Pu from surface based testing at the Nevada Test Site (240Pu/239Pu = 0.035) is carried by tropospheric fallout particles of a distinct physical/chemical form which are rapidly removed from the water column at all depths, in contrast to global stratospheric fallout (240Pu/239Pu = 0.18) which is only efficiently deposited to the sediments in the shallower cores where scavenging is more intense. This two source model was chosen since there is no evidence for the present day fractionation of 239Pu from 240Pu in the water column and sediment trap data. This two source model is supported by the analysis of 240Pu/239Pu ratios in marine sediments from the 1950's and 1960's which show lower or equivalent 240Pu/239Pu ratios than present day samples from the same locations (Nevada fallout was confined to 1951-1958 while global fallout inputs peaked in 1961/62). Also, while all of the North Atlantic deep-sea sediments show some evidence of the Nevada inputs (i.e. 240Pu/239P < 0.18), the net inventory of Pu from the Nevada source can be shown to decrease with increasing distance away from the Nevada source. Using the observed sediment 240Pu/239Pu ratios and a two end-member mixing model, the Pu supplied by the Nevada source in deep Northwest Atlantic sediments (>4500 m) is shown to account for roughly 40% of the total sediment 239,240Pu inventory. The very low inventories of 239,240Pu in the deep-sea sediments in general serves to accentuate the Nevada fallout signal at these sites. A pronounced dis-equilibrium is observed between the solid phase 240Pu/239Pu ratios (which range from 0.10-0.18) and the pore water 240Pu/23pPu ratios (which are constant at ≈ 0.18 ) at all sites. The low ratio Nevada fallout Pu is apparently more tightly bound by its solid phase carrier than Pu from global fallout sources and is therefore not participating in the general solid/solution exchange reactions. Within an individual sediment profile, the 240Pu/239Pu ratios are relatively constant from core top to core bottom. Using a sediment mixing model which combines the Pu activity data and the resulting 240Pu/239Pu ratios given the two Pu sources, I have been able to constrain the input function of Pu to the slope and deep ocean sediments. The data are consistent with a model which suggests that the bulk of the Pu deposited to the deep ocean sediments arrived early-on in the fallout record.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1986
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