Publication Date:
2022-05-25
Description:
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
Description:
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.
Description:
Financial support was provided primarily by the Education Office
of the Massachusetts Institute of Technology/Woods Hole Oceanographic
Institution Joint Program in Oceanography, by Department of Energy contract DE-FG02-85ER60358, and by the Oak Ridge Associated Universities
program for travel support to the Savannah River Laboratory.
Keywords:
Radioactive pollution of the sea
;
Nuclear fuels
;
Radioactivity
;
Diagenesis
;
Oceanus (Ship : 1975-) Cruise OC152
;
Oceanus (Ship : 1975-) Cruise OC173
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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