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| Titel |
Depth distributions of uranium-236 and cesium-137 in the Japan/East Sea; toward the potential use as a new oceanic circulation tracer |
| VerfasserIn |
A. Sakaguchi, A. Kadokura, P. Steier, Y. Takahashi, K. Shizuma, M. Yamamoto |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067751
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| Zusammenfassung |
| 137Cs (T1-2=30.2 y) has been spread all over the world as a fission product of atmospheric
nuclear weapons tests in the 1960s. This nuclide has been used as a powerful tool for
oceanography due to the well-defined origin and conservative behaviour in water .
However, the number of atoms has decayed already to one thirds compared with its
initial levels, and it will become more difficult to measure. In this situation, we
focus on 236U (T1-2=2.342Ã107 y) as a candidate for a new isotopic tracer for
oceanography. The detection of 236U in the environment has become possible only
recently, by the development of measuring techniques with high sensitivity based on
AMS. Our group showed that global fallout from bomb tests contains 236U, which
might be produced as nuclear reactions of 235U(n,γ) and/or 238U(n,3n). So 236U has
been therefore globally distributed in the surface environment. Thus, 236U has a
similar potential as a tracer for environmental dynamics as 137Cs, especially for
oceanography.
In this study, a comprehensive attempt was made to measure the concentration of 236U in
marine samples such as water, suspended solid and bottom sediments to clarify
the environmental behaviour of this isotope. Furthermore, the discussion of the
circulation of deep and bottom water in "Miniature Ocean", the Japan Sea, has been
attempted.
Bottom sediments (4 sites) and seawater samples (7 sites) were collected from the Japan
Sea. The sediment core was cut into 1 cm segments from the surface to 5 cm in depth within a
few hours after the sampling. About 20 L of seawater samples were collected from some
depths in each site, and immediately after the sampling, the water was filtered with
0.45 μm pore-size membrane-filters. After the appropriate pre-treatment for each
sample, uranium isotope and 137Cs were measured with AMS and Ge-detector,
respectively.
236U was successfully detected for all seawater samples, and 236U/238U atom ratios in
seawater were in the range of (0.19-1.75)Ã10-9. The dissolved 236U concentration showed a
subsurface maximum and decreased steeply with depth. The minimum value was found at a
depth of 2500 m and bottom (about 3000 m in depth) in the northern and the southern areas,
respectively. These profiles are markedly different from that of natural 238U which is nearly
constant over the depth, suggesting that 236U has not yet reached steady state. For the SS
sample, 236U could not be detected in significant levels. The total 236U inventory of the water
column was estimated at 1012-1013 atom/m2. This value is nearly the same as the global
fallout level (17.8Ã1012 atom/m2). 236U was also found in the bottom sediments,
and the inventory was about 1/40 compared with that in water column. All above
characters are comparable with 137Cs which is anthropogenic conservative nuclide in
ocean. Actually, the diffusion coefficients for both nuclides show the nearly same
value.
The detail discussion including the circulation of deep-water in the Japan Sea will be
given in our presentation. |
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