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Journal Articles

Processes affecting long-term changes in $$^{137}$$Cs concentration in surface sediments off Fukushima

Otosaka, Shigeyoshi

Journal of Oceanography, 73(5), p.559 - 570, 2017/10


 Times Cited Count:22 Percentile:84.77(Oceanography)

Based on monitoring data from 71 stations off the coast of Fukushima, Miyagi and Ibaraki Prefectures, changing tendencies of $$^{137}$$Cs concentration in surface (0-10 cm) sediment are analyzed, and primary processes affecting the temporal changes are determined. In the coastal region (water depth $$<$$ 100m), between 2011 and 2015, concentrations of $$^{137}$$Cs in the surface sediment decreased at the rate of 27% per year in average. Such a remarkable temporal change in the $$^{137}$$Cs concentration of sedimentary radiocesium was not observed in the offshore regions. By applying observed data of vertical distribution of sedimentary $$^{137}$$Cs into a pulse input sediment mixing model, it was estimated that more than a half of the decreasing effect was explained by vertical mixing of $$^{137}$$Cs-bound surface sediment with less-contaminated sediment in the deeper layers.

Journal Articles

Horizontal and vertical distributions of $$^{137}$$Cs in seabed sediments around the river mouth near Fukushima Daiichi Nuclear Power Plant

Tsuruta, Tadahiko; Harada, Hisaya*; Misono, Toshiharu; Matsuoka, Toshiyuki; Hodotsuka, Yasuyuki*

Journal of Oceanography, 73(5), p.547 - 558, 2017/10

 Times Cited Count:10 Percentile:55.23(Oceanography)

The seafloor topography was divided into flat and terrace seafloors based on their topographical features and seabed sediments were distributed in an area that was half a degree of the entire investigation area. The $$^{137}$$Cs inventory was several tens of kBq/m$$^{2}$$ and the grain sizes (the D50 values) were nearly constant (fine sand) on the flat seafloor. On the terrace seafloor, the $$^{137}$$Cs inventory was larger than that on the flat seafloor, and the grain size varied from silt to coarse sand. The grain size distributions appear to be influenced by the mean shear stress at the seafloor bottom, and a significant factor in the mean shear stress is thought to be the seafloor topography. Distributions of remarkably large $$^{137}$$Cs inventories, more than several thousands of kBq/m$$^{2}$$, are thought to be confined to a small area. Vertical changes in the $$^{137}$$Cs inventories suggested that the $$^{137}$$Cs inventories have significantly decreased in large areas of the shallow sea.

Journal Articles

Preface "Radionuclides in coastal sediments after the accident of Fukushima Daiichi Nuclear Power Plant; Distribution, dynamics and fate"

Nagao, Seiya*; Otosaka, Shigeyoshi; Kaeriyama, Hideki*

Journal of Oceanography, 73(5), P. 527, 2017/10

 Times Cited Count:1 Percentile:47.19(Oceanography)

More than five years have passed since the accident at the Fukushima Daiichi Nuclear Power Plant, and many investigations have been carried out in the marine environment. Regarding the radiocesium concentration in seabed sediment, from May 2011, monitoring survey is continuing mainly in the coastal areas of Miyagi, Fukushima, Ibaraki and Chiba prefectures. However, due to its heterogeneity, the difficulty of observation, etc., the assessment of the influence of the accident-derived radionuclides on seabed sediments and the benthic ecosystem is delayed compared to other environmental investigations. As the Guest Editors, the authors set up a special section in "Journal of Oceanography", including four papers arguing about factors affecting the temporal change of the radiocesium concentrations in estuarine, coastal and offshore sediments. This preface shows the planning intention of the special issue, and also outlines the contents.

Journal Articles

Numerical experiment for strontium-90 and cesium-137 in the Japan Sea

Kawamura, Hideyuki; Ito, Toshimichi; Kobayashi, Takuya; Otosaka, Shigeyoshi; Hirose, Naoki*; Togawa, Orihiko

Journal of Oceanography, 66(5), p.649 - 662, 2010/10

 Times Cited Count:2 Percentile:6.8(Oceanography)

A numerical experiment is performed to reproduce a distribution of concentrations of $$^{90}$$Sr and $$^{137}$$Cs and estimate their total amounts in the Japan Sea. The concentrations of $$^{90}$$Sr and $$^{137}$$Cs in the surface layer is in the range of 1.0-1.5 Bq/m$$^{3}$$ and 2.0-2.5 Bq/m$$^{3}$$. The concentrations in the intermediate and deep layer are higher than those observed in the northwestern Pacific Ocean, indicating active winter convection in the Japan Sea. The total amounts of $$^{90}$$Sr and $$^{137}$$Cs in the seawater is evaluated to be 1.34 PBq (1 PBq = 10$$^{15}$$ Bq) and 2.02 PBq, which demonstrates an estimation by observational data in the Japan Sea expeditions between 1997 and 2002 by Japan Atomic Energy Agency. The total amounts of $$^{90}$$Sr and $$^{137}$$Cs vary corresponding to deposition at the sea surface with the maximums of 4.86 PBq for $$^{90}$$Sr and 7.33 PBq for $$^{137}$$Cs in the mid-1960s.

Journal Articles

Modeling of the branches of the Tsushima Warm Current in the eastern Japan Sea

Kawamura, Hideyuki; Ito, Toshimichi; Hirose, Naoki*; Takikawa, Tetsutaro*; Yoon, J.-H.*

Journal of Oceanography, 65(4), p.439 - 454, 2009/08

 Times Cited Count:4 Percentile:13.46(Oceanography)

This study aims at reproducing branches of the Tsushima Warm Current making use of an ocean general circulation model, which is important for movement of pollutants in the Japan Sea. The model was laterally exerted by volume transports measured by ADCP through the Tsushima Straits and the Tsugaru Strait. Sea level variation measured by coastal tide-stations as well as satellite altimeters is assimilated into the numerical model. It was demonstrated that an assimilation of sea level variation at the coastal tide-stations is useful to simulate oceanic condition in the nearshore region.

Journal Articles

Deep sea circulation of particulate organic carbon in the Japan Sea

Otosaka, Shigeyoshi; Tanaka, Takayuki; Togawa, Orihiko; Amano, Hikaru; Karasev, E. V.*; Minakawa, Masayuki*; Noriki, Shinichiro*

Journal of Oceanography, 64(6), p.911 - 923, 2008/12

 Times Cited Count:20 Percentile:45.51(Oceanography)

Transport processes of particulate organic carbon (POC) were inferred from sediment trap experiments in the three regions of the Japan Sea (western and eastern Japan Basin and Yamato Basin) and radiocarbon measurement. Annual mean $$^{14}$$C/$$^{12}$$C isotopic ratio decreased with depth and the vertical changes in the isotopic signature were considered to indicate mixing of two fractions; labile POC produced in the surface and refractory POC. From seasonal variations of POC flux of the two fractions, transport processes of POC in the Japan Sea were summarized as follows: (1) In the Japan Basin, both labile and refractory POC were supplied in spring and a reservoir of refractory POC was formed, and (2) in the Yamato Basin, larger amount of refractory POC were carried the interior and accumulated rapidly.

Journal Articles

Temporal and spatial variations of radiocarbon in Japan Sea Bottom Water

Kumamoto, Yuichiro*; Aramaki, Takafumi*; Watanabe, Shuichi*; Yoneda, Minoru*; Shibata, Yasuyuki*; Togawa, Orihiko; Morita, Masatoshi*; Shitashima, Kiminori*

Journal of Oceanography, 64(3), p.429 - 441, 2008/06

 Times Cited Count:10 Percentile:25.17(Oceanography)

In 1995 and 2000, radiocarbon ratio ($$Delta$$$$^{14}$$C) of total dissolved inorganic carbon was measured in the Japan Sea, a semi-closed marginal sea in the western North Pacific, where deep and bottom waters are formed in itself. Compiling them with historical radiocarbon data in the Japan Sea, temporal and spatial variations of the radiocarbon in the bottom water below 2000 m depth were elucidated. $$Delta$$$$^{14}$$C in the bottom waters in the western Japan and Yamato Basins increased by about 20 ‰ between 1977/79 and 1995 and did not changed between 1995 and 1999/2000, suggesting penetration of surface bomb-produced radiocarbon into the bottom waters due to bottom ventilation in the earlier period and stagnation of the bottom ventilation in the following period, respectively. In the eastern Japan Basin, the bottom $$Delta$$$$^{14}$$C increased by about 10 ‰ between 1977/79 and 2002, suggesting less ventilation of the bottom water in the basin. The temporal changes of the radiocarbon, tritium, and dissolved oxygen suggest sporadic occurrences of the bottom ventilation between 1979 and 1984 and its stagnation between 1984 and 2004 in the eastern Japan and Yamato Basins. The former is probably due to spreading of a newly ventilated bottom water in the western Japan Basin in the severe winter of 1976-1977 along the abyssal circulation in the Japan Sea. The latter does not conflict with temporal changes of bomb-produced $$^{137}$$Cs and chlorofluorocarbon-11 in the bottom water.

Journal Articles

Sequential forecasting of the surface and subsurface conditions in the Japan Sea

Hirose, Naoki*; Kawamura, Hideyuki; Lee, H. J.*; Yoon, J.-H.*

Journal of Oceanography, 63(3), p.467 - 481, 2007/06

 Times Cited Count:28 Percentile:54.65(Oceanography)

This study estimates and forecasts a realistic change of the Japan Sea by assimilating satellite measurements into an eddy-resolving circulation model. Suboptimal but feasible assimilation schemes of approximate filtering and nudging play essential roles in a system. The sequential updates of error covariance significantly outperforms the asymptotic covariance due to irregular sampling patterns from multiple altimeter satellites. The best estimate shows an average rms difference of 1.2 $$^{circ}$$C only to the radiometer data, and also explain about half of sea level variances measured by the microwave observation. It is demonstrated that a forecast persistency strongly depends on variable, depth, and location.

Journal Articles

Formation rate of water masses in the Japan Sea

Kawamura, Hideyuki; Yoon, J.-H.*; Ito, Toshimichi

Journal of Oceanography, 63(2), p.243 - 253, 2007/04

 Times Cited Count:9 Percentile:20.95(Oceanography)

Water masses in the subsurface and intermediate layer are formed vigorously due to strong winter convection in the Japan Sea. Taking it into account that a part of pollutants is carried into the layer below the sea surface together with such water masses, it is significant to estimate a formation rate and turnover time of water masses to study a fate of pollutants. This study aims to estimate the formation rate and turnover time of water masses by using a three-dimensional ocean circulation model and particle chasing method. The total formation rate of water masses below the sea surface amounts to about 3.63 Sv in the Japan Sea. The formation rates of the Upper portion of the Japan Sea Proper Water (UJSPW) and Japan/East Sea Intermediate Water (JESIW) are estimated to be about 0.37 and 1.51 Sv, respectively. An estimate of turnover time shows that the UJSPW and JESIW circulate in the subsurface and intermediate layer with time scales of about 22.6 and 2.1 years, respectively.

Journal Articles

Benthic front and the Yamato Basin Bottom Water in the Japan Sea

Senju, Tomoharu*; Isoda, Yutaka*; Aramaki, Takafumi*; Otosaka, Shigeyoshi; Fujio, Shinzo*; Yanagimoto, Daigo*; Suzuki, Takashi; Kuma, Kenshi*; Mori, Kosuke*

Journal of Oceanography, 61(6), p.1047 - 1058, 2005/12

 Times Cited Count:8 Percentile:18.19(Oceanography)

The detail structure of the Bottom Water in the Japan Sea was revealed by the hydrographic observations. The Yamato Basin Bottom Water (YBBW) exhibits higher temperature, lower dissolved oxygen, and higher nutrients concentrations than those in the Japan Basin Bottom Water (JBBW). Both Bottom Waters meet around the boundary region between the Yamato and the Japan Basins, and form a clear benthic front. It is inferred from the property distributions that the JBBW flowing into the Yamato Basin is trapped by the cyclonic circulation in the basin, and modified to the YBBW in the course of the circulation through the vertical diffusion, geothermal heating and oxygen consumption. The thermal balance of in the YYBW was examined using a box-model. The results show that the effect of geothermal heating has about 70 per-cent of magnitude to the vertical thermal diffusion and both terms cancel the advection term of the cold JBBW from the Japan Basin. The box-model also estimated the averaged residence time for the YBBW was 9.1 years.

Journal Articles

Relationship between composition of settling particles and organic carbon flux in the western North Pacific and the Japan Sea

Otosaka, Shigeyoshi; Noriki, Shinichiro*

Journal of Oceanography, 61(1), p.25 - 40, 2005/01

 Times Cited Count:25 Percentile:49.37(Oceanography)

Settling particles play an important role in transport of organic carbon from the surface to the deep ocean. It is known that major components of settling particles are biogenic silicates (opal), biogenic carbonate, lithogenic clays and organic matters. In this study, we estimated organic carbon fluxes that are carried by each component in the western North Pacific Ocean (WNP) including the Japan Sea. Mean organic carbon flux at 1 km depth in the WNP (13.5 mg/m$$^{2}$$/day) was larger than that in the Eastern North Pacific (7.4), central North Pacific (1.1), Equatorial Pacific (4.2), Southern Ocean (5.8) and Eastern North Pacific (1.8). In the WNP, opal was a dominant component and 55 % of particulate organic carbon was carried by opal particles. Opal is known as a major component even in the Eastern North Pacific and Southern Ocean and opal fluxes in these areas are as much as in the WNP. However, organic carbon flux that was carried by opal particles in the WNP was significantly larger than that in the Eastern North Pacific and the Southern Ocean. It was cleared that opal particles in the WNP had great abilities not only to activate the biological pump in the surface ocean but also to transport organic carbon to the deep ocean.

Journal Articles

CFC indicating renewal of the Japan Sea deep water in winter 2000-2001

Tsunogai, Shizuo*; Kawada, Kentaro*; Watanabe, Shuichi*; Aramaki, Takafumi

Journal of Oceanography, 59(5), p.685 - 693, 2003/10

 Times Cited Count:25 Percentile:46.52(Oceanography)

The water column distributions of CFC (chlorofluorocarbon) were determined twice in 2000 and 2001 in the northwestern Japan Sea. In 2000, the CFC-11 concentration decreased almost exponentially with depth from 6 pmol/kg at a few hundred m deep to 0.3 pmol/kg or less at the bottom of about 3400 m depth at 3 stations about 300 km off Vladivostok. In 2001, the CFC-11 concentration increased extremely up to 2 pmol/kg in the bottom water, while it did not increase at a station about 450 km away to the northeast. This is due to the renewal of the bottom water replaced by the surface water flowing down along the continental slope. The increase in the CFC-11 concentration was observed even in the whole water column above 3000 m depth. The increase in inventory is almost four times larger than that in the bottom water below 3000 m depth and equals about 1/6 of the total inventory found in 2000. The increase also means that 3 % of the deep water was replaced by the recent surface water or the turnover time of the deep water to be about 30 years, if the turnover occurs every year.

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