Yuichiro Kumamoto

Southward spreading of the Fukushima-derived radiocesium across the Kuroshio Extension in the North Pacific

The accident of the Fukushima Dai-ichi nuclear power plant in March 2011 released a large amount of radiocesium into the North Pacific Ocean. Vertical distributions of Fukushima-derived radiocesium were measured at stations along the 149°E meridian in the western North Pacific during the winter of 2012. In the subtropical region, to the south of the Kuroshio Extension, we found a subsurface radiocesium maximum at a depth of about 300 m. It is concluded that atmospheric-deposited radiocesium south of the Kuroshio Extension just after the accident had been transported not only eastward along with surface currents but also southward due to formation/subduction of subtropical mode waters within about 10 months after the accident. The total amount of decay-corrected 134Cs in the mode water was an estimated about 6 PBq corresponding to 10–60% of the total inventory of Fukushima-derived 134Cs in the North Pacific Ocean.

Impact of Fukushima-derived radiocesium in the western North Pacific Ocean about ten months after the Fukushima Dai-ichi nuclear power plant accident.

We measured vertical distributions of radiocesium ((134)Cs and (137)Cs) at stations along the 149°E meridian in the western North Pacific during winter 2012, about ten months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident. The Fukushima-derived (134)Cs activity concentration and water-column inventory were largest in the transition region between 35 and 40°N approximately due to the directed discharge of the contaminated water from the FNPP1. The bomb-derived (137)Cs activity concentration just before the FNPP1 accident was derived from the excess (137)Cs activity concentration relative to the (134)Cs activity concentration. The water-column inventory of the bomb-derived (137)Cs was largest in the subtropical region south of 35°N, which implies that the Fukushima-derived (134)Cs will also be transported from the transition region to the subtropical region in the coming decades. Mean values of the water-column inventories decay-corrected for the Fukushima-derived (134)Cs and the bomb-derived (137)Cs were estimated to be 1020 ± 80 and 820 ± 120 Bq m(-2), respectively, suggesting that in winter 2012 the impact of the FNPP1 accident in the western North Pacific Ocean was nearly the same as that of nuclear weapons testing. Relationship between the water-column inventory and the activity concentration in surface water for the radiocesium is essential information for future evaluation of the total amount of Fukushima-derived radiocesium released into the North Pacific Ocean.

Fukushima-derived radiocesium in the northwestern Pacific Ocean in February 2012

We measured radiocesium ((134)Cs and (137)Cs) in seawaters collected at stations in the northwestern Pacific Ocean in February 2012. Activity concentration of Fukushima-derived radiocesium was highest in the transition area between the subarctic and subtropical regions, which was due to the direct discharge. The direct discharged radiocesium was transported southwardly across the Kuroshio Extension along isopycnal mixing. More than 80% of the Fukushima-derived radiocesium at stations both in the transition area and subtropical region was derived from the direct discharge.

Effects of the Anticyclonic Eddies on Water Masses, Chemical Parameters and Chlorophyll Distributions in the Oyashio Current Region

Data from the R/V Mirai cruise (May–June 2000) have been examined to discover how mesoscale processes associated with eddy dynamics direct affect the water masses, the distributions and the vertical fluxes of the dissolved oxygen, nutrients and dissolved inorganic carbon in the western subarctic Pacific. Using maps of the temperature, salinity, dissolved oxygen, nutrients, chlorophyll and sea-air pCO2 difference we show that the boundaries of the anticyclone eddies in the study region were composed of high productivity coastal Oyashio water. The coastal waters were wrapped around the anticyclone eddies (thus creating a high productivity belt) and intruded inside of them. Using SeaWifs data we demonstrate that temporal variations in the position and the strength of anticyclone eddies advected the Kuril island coastal high productivity waters to the pelagic part, resulting in temporal variations of the chlorophyll in the Oyashio region. Computed vertical fluxes of the dissolved oxygen (DO), inorganic carbon (DIC) and silicate show that the anticylonic eddies in the Kuroshio-Oyashio Zone are characterized by enhanced vertical fluxes of the DO and DIC between the upper (σθ = 26.7–27.0) and lower (σθ = 27.1–27.5) intermediate layer, probably due to the intrusions of the Oyashio waters into the eddies.

Intrusion of Fukushima-derived radiocaesium into subsurface water due to formation of mode waters in the North Pacific.

The Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 released radiocaesium (137Cs and 134Cs) into the North Pacific Ocean. Meridional transects of the vertical distribution of radiocaesium in seawater were measured along 147 °E and 155 °E in October–November 2012, 19 months after the accident. These measurements revealed subsurface peaks in radiocaesium concentrations at locations corresponding to two mode waters, Subtropical Mode Water and Central Mode Water. Mode water is a layer of almost vertically homogeneous water found over a large geographical area. Here we show that repeated formation of mode water during the two winter seasons after the Fukushima accident and subsequent outcropping into surface water transported radiocaesium downward and southward to subtropical regions of the North Pacific. The total amount of Fukushima-derived 134Cs within Subtropical Mode Water, decay-corrected to April 2011, was estimated to be 4.2 ± 1.1 PBq in October–November 2012. This amount of 134Cs corresponds to 22–28% of the total amount of 134Cs released to the Pacific Ocean.

Bomb radiocarbon invasion into the northwestern North Pacific

Invasion of the bomb radiocarbon was investigated in the northwestern North Pacific, including the subarctic (Oyashio region), the subtropical (Kuroshio region), and the subarctic/subtropical mixing (transition) areas. After the GEOSECS Pacific expedition in 1973, the bomb radiocarbon decreased in the Oyashio region and increased in the Kuroshio region. The former was caused by rapid ventilation of the Oyashio water. The latter was owing to accumulation of the bomb radiocarbon in the intermediate water of the Kuroshio region. The reverse trends in the temporal variation of radiocarbon between the Oyashio and the Kuroshio regions lead to virtually small temporal change of the bomb radiocarbon in the transition area where the two waters mix. The temporal changes of the bomb radiocarbon suggest that the North Pacific Intermediate Water is formed in the northwestern North Pacific.

Meridional distribution of Fukushima-derived radiocesium in surface seawater along a trans-Pacific line from the Arctic to Antarctic Oceans in summer 2012

In summer 2012, we measured activity concentration of radiocesium in surface seawater collected in the Arctic Ocean, Bering Sea, western Pacific Ocean, and Antarctic Ocean. The radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 was found in the Bering Sea and western North Pacific between 25°N and 63°N, which agrees with model simulation results of atmospheric deposition. A semi-synoptic view suggests that a main body of radiocesium discharged directly had been transported eastward to 170°W, northward to 50°N, and southward to 30°N by summer 2012, about one and half years after the accident.

A preliminary report on the characteristics of a CO2 gas ion source MGF-SNICS at NIES-TERRA

The AMS system at the National Institute for Environmental Studies has a cesium-sputtering source for the production of C− ions from CO2 gas which is connected with a simultaneous injection beam line. The source, MGF-SNICS, is composed of a sample-target assembly containing twelve sets of a cylinder-needle valve-transfer tube-cathode target combination, a positioning mechanism, and a cesium sputtering chamber evacuated by a turbo molecular pump. An outline of the source and future plans are described.

Decadal Changes in Bomb-Produced Radiocarbon in the Pacific Ocean from the 1990s to 2000s

In the 2000s, radiocarbon in dissolved inorganic carbon was measured during 7 revisit cruises along the lines of the World Ocean Circulation Experiment in the Pacific Ocean. Comparison of 14C data along these lines from the 1990s and 2000s revealed decadal changes of 14C concentration in the thermocline, most of which were due to temporal changes in the bomb-produced 14C. Vertical profiles and vertical-integrated inventories of the bomb 14C in the subarctic and equatorial regions did not change appreciably. In the southern subtropical region, 14C decreased in the upper thermocline from the surface to ~500 m depth. In contrast, 14C increased in the lower thermocline below ~500 m depth. The opposing directions in 14C change resulted in small temporal changes in the total inventory of bomb 14C. On the other hand, the water-column inventory significantly decreased in the northwestern subtropical region due to the 14C decrease in the upper thermocline. These decadal changes in bomb 14C indicate that the turnover time of thermocline circulation in the northwestern subtropical region is faster than that in the southern subtropical region, and imply an interbasin transport of bomb 14C from the North Pacific to other basins.

Detecting the progression of ocean acidification from the saturation state of CaCO3 in the subtropical South Pacific

Progression of ocean acidification in the subtropical South Pacific was investigated by using high-quality data from trans-Pacific zonal section at 17°S (World Ocean Circulation Experiment section P21) collected in 1994 and 2009. During this 15 year period, the CaCO3 saturation state of seawater with respect to calcite (Ωcal) and aragonite (Ωarg) in the upper water column (<400 dbar) decreased at rates of 0.037 a−1 and 0.025 a−1, respectively, east of 145°W longitude; these rates are among the fastest in the worlds oceans. In contrast, at longitudes 170°E–145°W, Ωcal and Ωarg decreased relatively slowly, at 0.008 a−1 and 0.005 a−1, respectively. The Ωarg saturation horizon occurred at a depth of about 1200 dbar at the westernmost end of the section and shoaled eastward to about 20 dbar. From 1994 to 2009, it migrated upward at a rate of 5.2 dbar a−1 west of 145°W. Decomposition of the temporal changes of Ω (ΔΩ) showed that the accumulation of anthropogenic CO2 in the ocean accounted for more than half of ΔΩ. The more rapid rate of decline of Ω in the eastern section was attributable to a relatively large contribution of organic matter remineralization, whereas the slower rate in the central section was attributed to a decrease of anthropogenic CO2 uptake caused by rising water temperatures. An important finding of this study was that acidification of the upper water column was enhanced by processes related to the oxygen minimum zone in the eastern subtropical South Pacific Ocean.