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Journal Articles

$$delta^{13}$$C and $$delta^{15}$$N values of sediment-trap particles in the Japan and Yamato Basins and comparison with the core-top values in the East/Japan Sea

Khim, B.-K.*; Otosaka, Shigeyoshi; Park, K.-A.*; Noriki, Shinichiro*

Ocean Science Journal, 53(1), p.17 - 29, 2018/03

 Times Cited Count:1 Percentile:79(Marine & Freshwater Biology)

Distribution of carbon and nitrogen stable isotope ratios ($$delta$$$$^{13}$$C, $$delta$$$$^{15}$$N) in sinking particles collected in four stations in the Japan Sea was summarized. The $$delta$$$$^{13}$$C and $$delta$$$$^{15}$$N values in the sinking particles showed a clear seasonal variation, which agreed well with the variation in the chlorophyll a concentration in the surface seawater and in the sinking flux of biogenic particles. In particular, during the highly productive season of phytoplankton, a remarkable isotopic fractionation of $$delta$$$$^{13}$$N was found in the sinking particles, and it was inferred that the fractionation indicates the degree of nitrogen uptake by diatoms in the surface layer. These results are helpful information for linking the phenomenon in the ocean surface and the isotopic information recorded in the seafloor.

Journal Articles

Resuspension and lateral transport of seafloor sediment contaminated with artificial radionuclides derived from Fukushima Daiichi Nuclear Power Plant accident

Honda, Makio*; Otosaka, Shigeyoshi

Nippon Genshiryoku Gakkai-Shi, 58(4), p.225 - 228, 2016/04

Transport processes of particulate radiocesium obtained from a three year time-series sediment trap experiments at a site 115 km southeast of Fukushima Daiichi Nuclear Power Plant off Fukushima are expounded. Results show a high fraction of lithogenic material in sinking particles, suggesting a lateral source of sediments. In particular, we found that accident-derived radiocesium associate with slope and shelf sediments were resuspended and transported laterally to the offshore regions in typhoon seasons in 2013. Continuous observation of such transport processes of particulate radiocesium near the seabed would be significant for better understanding of the fate of accident-derived radiocesium.

Journal Articles

Tracking the fate of particle associated Fukushima Daiichi cesium in the ocean off Japan

Buesseler, K. O.*; German, C. R.*; Honda, Makio*; Otosaka, Shigeyoshi; Black, E. E.*; Kawakami, Hajime*; Manganini, S. M.*; Pike, S.*

Environmental Science & Technology, 49(16), p.9807 - 9816, 2015/08

 Times Cited Count:11 Percentile:47.29(Engineering, Environmental)

A three year time-series of particle fluxes is presented from sediment traps deployed at 500 and 1000 m at a site 115 km southeast of Fukushima Daiichi Nuclear Power Plant (FDNPP). Results show a high fraction of lithogenic material, suggesting a lateral source of sediments. The accident-derived radiocesium were enhanced in flux peaks that, given variations in trap $$^{137}$$Cs/$$^{210}$$Pb ratios, are characteristic of material derived from shelf and slope sediments found from $$<$$ 120 to $$>$$ 500 m. The fluxes of radiocesium are an order of magnitude higher than a previously reported for the trap located 100 km due east of FDNPP. We attribute the large difference due to the position of our trap under the southeasterly currents that carry contaminated waters and resuspended sediments in to the Pacific. These higher sedimentary fluxes of radiocesium to the offshore are still small relative to the inventory of radiocesium currently buried nearshore.

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:22 Percentile:50.69(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

Fluxes and balance of $$^{210}$$Pb in the northwestern Japan Sea

Otosaka, Shigeyoshi; Baba, Masami*; Togawa, Orihiko; Karasev, E. V.*

Pacific Oceanography, 1(2), p.149 - 157, 2003/12

A sediment trap experiment and sediment coring were carried out in the northwestern Japan Sea (Sta MS), and lead-210 ($$^{210}$$Pb) and major components in settling particles were measured. By drawing up a balance of $$^{210}$$Pb in the water column, it was estimated that the cycle of $$^{210}$$Pb in the Sta MS was controlled by 3 processes, (1) removal and vertical transport of $$^{210}$$Pb from the surface layer by settling particles, (2) decomposition of particles in the deep layer, and (3) export by the deep current. Flux of $$^{210}$$Pb at 3 km depth was large in winter and spring. The large $$^{210}$$Pb flux in this season would be caused by the horizontal import of $$^{210}$$Pb -rich seawater from the coastal region to the bottom layer of Sta MS. It was suggested that seasonal variation of particulate $$^{210}$$Pb flux at the deep layer in the northwestern Japan Basin indicated the renewal of the deep water in this area.

Oral presentation

Lateral transport of Fukushima Daiichi Nuclear Power Plant accident-derived particulate radiocesium to semi-pelagic ocean off Fukushima

Buesseler, K. O.*; German, C. R.*; Honda, Makio*; Otosaka, Shigeyoshi; Black, E. E.*; Kawakami, Hajime*; Manganini, S. M.*; Pike, S.*

no journal, , 

A three year time-series of particle fluxes is presented from sediment traps deployed at 500 and 1000 m at a site 115 km southeast of Fukushima Daiichi Nuclear Power Plant. Results show a high fraction of lithogenic material in sinking particles, suggesting a lateral source of sediments. From $$^{137}$$Cs/$$^{210}$$Pb ratio in sinking particles, it was inferred that accident-derived radiocesium associate with slope and shelf sediments were resuspended and transported laterally to the offshore regions in winter and typhoon seasons. Continuous observation of such transport processes of particulate radiocesium near the seabed would be significant for better understanding of the fate of accident-derived radiocesium.

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