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| Titel |
Ocean dynamic processes causing spatially heterogeneous distribution of sedimentary caesium-137 massively released from the Fukushima Daiichi Nuclear Power Plant |
| VerfasserIn |
H. Higashi, Y. Morino, N. Furuichi, T. Ohara |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 23 ; Nr. 12, no. 23 (2015-12-08), S.7107-7128 |
| Datensatznummer |
250118203
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| Publikation (Nr.) |
 copernicus.org/bg-12-7107-2015.pdf |
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| Zusammenfassung |
| Massive amounts of anthropogenic radiocaesium 137Cs that were released
into the environment by the Fukushima Daiichi Nuclear Power Plant accident
in March 2011 are widely known to
have extensively migrated to Pacific Ocean sediment off of eastern Japan.
Several recent reports have stated that the sedimentary 137Cs is now
stable with a remarkably heterogeneous distribution. The present study
elucidates ocean dynamic processes causing this heterogeneous sedimentary
137Cs distribution in and around the shelf off Fukushima and adjacent
prefectures. We performed a numerical simulation of oceanic 137Cs
behaviour for about 10 months after the accident, using a comprehensive
dynamic model involving advection–diffusion transport in seawater,
adsorption and desorption to and from particulate matter, sedimentation and
suspension on and from the bottom, and vertical diffusion transport in the
sediment. A notable simulated result was that the sedimentary 137Cs
significantly accumulated in a swath just offshore of the shelf break (along
the 50–100 m isobath) as in recent observations, although the seabed in the
entire simulation domain was assumed to have ideal properties such as
identical bulk density, uniform porosity, and aggregation of particles with a
single grain diameter. This result indicated that the heterogeneous
sedimentary 137Cs distribution was not necessarily a result of the
spatial distribution of 137Cs sediment adsorptivity. The present
simulation suggests that the shape of the swath is mainly associated with
spatiotemporal variation between bottom shear stress in the shallow shelf
(< 50 m depths) and that offshore of the shelf break. In a large
part of the shallow shelf, the simulation indicated that strong bottom
friction suspending particulate matter from the seabed frequently occurred
via a periodic spring tide about every 2 weeks and via occasional strong
wind. The sedimentary 137Cs thereby could hardly stay on the surface of
the seabed with the result that the simulated sediment-surface 137Cs
activity tended to decrease steadily for a long term after the initial
137Cs migration. By contrast, in the offshore region, neither the spring
tide nor the strong wind caused bottom disturbance. Hence, the particulate
matter incorporated with 137Cs, which was horizontally transported from
the adjacent shallow shelf, readily settled and remained on the surface of
the sediment just offshore of the shelf break. |
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