Artificial radionuclides in the western Northwest Pacific (II):137Cs and239,240Pu inventories in water and sediment columns observed from 1980 to 1986

Concentrations and inventories of¹³⁷Cs and^239,240Pu were determined in sea waters and sediments columns from the western North Pacific from 1980 to 1986.The^239,240Pu/¹³⁷Cs activity ratio in the water column shows a tendency to increase from the surface (10^–3) to bottom waters (10^–1), but the ratio in sediment is within a rather narrow range (10^–2 to 10^–1), indicating more effective removal of plutonium from the water column than¹³⁷Cs. In regions south of 40N, the radionuclide inventories in the water column significantly exceed the estimated global fallout (stratospheric fallout due to the atmospheric nuclear explosion), especially in the case of^239,240Pu. These excess inventories imply that local or close-in fallout derived from nuclear explosions in the equatorial North Pacific are well-preserved and retained in the regions, despite about 20 years since the atmospheric nuclear explosion moratorium. Data suggesting lateral transport of¹³⁷Cs in surface water from north of 40N to southern regions is shown. Some data on⁹⁰Sr contents are also shown.     

Skewed Occurrence Frequency of Water Temperature and Salinity in the Subarctic Regions

In a previous paper (Oguma and Nagata, 2002), it was shown that frequency distributions of temperature and salinity in the sea off Sanriku Coast, Japan are skewed, and sometimes observed values exceed m + 5σ (m = mean, σ = standard deviation). This means that, if we apply a 3σ criterion for a range check, many real data would be lost. We have expanded our analysis to the subarctic North Pacific, the subarctic North Atlantic and their surrounding areas, by computing the distributions of skewness and kurtosis. It is found that the region of high positive skewness extends in an east-north-east direction in the Mixed Water Region from off Sanriku, and reaches to about 155°E. A high negative skewness zone is recognized along the southern margin of the Kuroshio Extension. These are thought to be generated by the breaking of the meander of the Kuroshio Extension and subsequent ejection of warm and cold eddies to the north and south, respectively. Other high positive skewness areas are found to the south of Kuril Islands and in the Japan Sea. These are generated due to very sharp vertical gradients of temperature and salinity. The situation in the North Atlantic is very similar to the North Pacific, though the detailed nature is changed due to differences of oceanographic condition. The effect of grid size on the skewed nature of the distribution is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modification of North Pacific Intermediate Water around Mixed Water Region

The mixing processes in the Mixed Water Region (MWR) that lead to changes in the properties of North Pacific Intermediate Water (NPIW) have been studied using observational data sets obtained in May–June 1998. Neutral surfaces, the equation of water mass conversion rate on neutral surfaces and the equation of vertical velocity across neutral surfaces have been used to distinguish dominant processes by assuming the horizontal scale to be the streamer scale (under 100 km). The possibility of double diffusive convection is also discussed in relation to the density ratio. These results may be summarized as follows: (1) the difference between the potential density surface and the neutral surface may rise to −0.04 kg/m³ around the source water of NPIW; (2) horizontal diffusion causes strong modifications of the source water of NPIW; (3) the density range within which strong modification of the source water of NPIW occurs becomes dense from the northern part of MWR near the Oyashio Front to the southern part near the Kuroshio Front, and to the eastern part. Our modeling of these processes shows that cabbeling has effects on the density increment of the source water of NPIW in the northern and southern part of MWR. Double diffusive convection has effects on the density increment of the source water of NPIW, mainly in the northern part of MWR. The possible density increment due to cabbeling in these areas is estimated to be 0.01≈0.03 kg/m³. The possible density increment due to double diffusive convection is 0.01≈0.03 kg/m³. The total density increment due to cabbeling and double diffusive convection amounts to 0.06 kg/m³. This revised version was published online in July 2006 with corrections to the Cover Date.     

Growth and Survival of Pacific Saury Cololabis saira in the Kuroshio-Oyashio Transitional Waters

Growth and mortality rates of larval and early juvenile Pacific saury Cololabis saira were estimated for spring and autumn spawning seasons in the Kuroshio-Oyashio transitional waters and for winter spawning season in the Kuroshio waters in 9 years from 1990–1998, based on quantitative fish sampling and otolith daily ring readings. Growth and mortality rates were more variable in the Kuroshio-Oyashio transitional waters than in the Kuroshio waters. The estimated production of 40-mm preschooling juveniles was a positive function of larval production in the hatching length class (5.9–9.9 mm) in the Kuroshio waters. In the Kuroshio-Oyashio transitional waters, rather than larval production in the hatching length class, cumulative survival through the larval and early juvenile stages determined the juvenile production. Variable growth and survival rates of saury observed in the transitional waters seem to be associated with large environmental variability in the waters, including shifts of the Kuroshio and Oyashio fronts and development of streamers and eddies between the fronts. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hydrographic Structure and Transport of Intermediate Water in the Kuroshio Region off the Boso Peninsula, Japan

Hydrographic structure and transport of intermediate water were observed in the Kuroshio region south of Japan, focusing on the 26.6–27.5σ_θ density in six cruises from May 1998 through September 2001. In the section off the Boso Peninsula where the Kuroshio exfoliates eastward, the intermediate water was clearly clustered into three groups meridionally composed of the coastal water, the Kuroshio water and the offshore water. Compared with the Kuroshio water characterized by warm, salty water transported by the Kuroshio, the coastal and offshore waters significantly degenerated due to mixing with cold, fresh waters originated from the subarctic region: the former was affected by alongshore spread of the coastal Oyashio and the latter by direct intrusion of the new North Pacific Intermediate Water (NPIW) into the southern side of the Kuroshio current axis. Particularly the offshore water showed higher apparent oxygen utilization (AOU) in layers deeper than 26.9σ_θ while it showed lower AOU in layers shallower than 26.9σ_θ, which indicated that colder, fresher and higher AOU water was distributed on the southeastern side of the Kuroshio in deeper layers. In May 1998, the Oyashio-Kuroshio mixing ratio was estimated to be typically 2:8 for the offshore water on the assumption of isopycnal mixing. Moreover, northeastward volume transport of the Kuroshio water was obtained from geostrophic velocity fields adjusted to lowered acoustic Doppler current profiler (LADCP) data to yield 6.1 Sv at 26.6–26.9σ_θ and 11.8 Sv at 26.9–27.5 σ_θ. This revised version was published online in July 2006 with corrections to the Cover Date.     

Zircon U–Pb sensitive high mass-resolution ion microprobe dating of granitoids in the Ryoke metamorphic belt, Kinki District, Southwest Japan

Abstract Zircon U–Pb sensitive high mass-resolution ion microprobe dating was carried out on three types of granitic rock (gneissose biotite granodiorite, biotite granite and two-mica granite) from the Cretaceous Ryoke belt of the Kinki district, Southwest Japan. The results give the ages of granitic magmatism in the Shigi-san area of between 87 and 78 Ma and suggest extensive melting of the Cretaceous Ryoke granitic crust to form the two-mica granite, probably at ca 80 Ma. Discrimination into older and younger granites based on development of gneissosity does not appear to represent the sequence of magma generation, although there is some scope in the interpretation of the zircon U–Pb data that would allow all three granites to form at 83 Ma. Compilation of chemical Th-U-total Pb isochron dating method ages, whole rock Rb–Sr isotope ages and U–Pb isotope ages indicates that most Ryoke plutonism occurred from ca 70 Ma to ca 100 Ma. Younger (85 Ma–70 Ma) plutonism with the formation of two-mica granite occurred only in the eastern sector of the Ryoke belt, including the Kinki District.     

Temporal and Spatial Variability of Phytoplankton Pigment Concentrations in the Japan Sea Derived from CZCS Images

Temporal and spatial variability of phytoplankton pigment concentrations in the Japan Sea are described, using monthly mean composite images of the Coastal Zone Color Scanner (CZCS). In order to describe the seasonal changes of pigment concentration from the results of the empirical orthogonal function (EOF) analysis, we selected four areas in the south Japan Sea. The pigment concentrations in these areas show remarkable seasonal variations. Two annual blooms appear in spring and fall. The spring bloom starts in the Japan Sea in February and March, when critical depth (CRD) becomes equal to mixed layer depth (MLD). The spring bloom in the southern areas (April) occurs one month in advance of that in the northern areas (May). This indicates that the pigment concentrations in the southern areas may increase rapidly in comparison with the northern areas since the water temperature increases faster in spring in the southern than in the northern areas. The fall bloom appears first in the southwest region, then in the southeast and northeast regions, finally appearing in the northwest region. Fall bloom appears in November and December when MLD becomes equal to CRD. The fall bloom can be explained by deepening of MLD in the Japan Sea. The pigment concentrations in winter are higher than those in summer. The low pigment concentrations dominate in summer.     

On Structure and Temporal Variation of the Bifurcation Current off the Kii Peninsula

The Kii Bifurcation Current is often found along the southwest coast of the Kii Peninsula, and its frequency of occurrence reaches about 70% in the period from 1988 to 1996 (Takeuchi et al., 1998a). In order to clarify the structure and short-period variability of the Kii Bifurcation Current, detailed observations were made four times on board the R/V Seisui-maru of Mie University on October 29–31, 1996, on June 24–26, 1997, October 14–16, 1997, and December 3–4, 1997. The measured horizontal structure of the Kii Bifurcation Current indicates that the eastern portion of the Current (eastward flow near Cape Shionomisaki) consists of a part of the current zone of the Kuroshio. It is shown that the current structure, including the Kii Bifurcation Current in the vicinity of Cape Shionomisaki, is stable when the Kuroshio is flowing in a stationary straight path, but that the current structure is considerably changed when small-scale eddies pass by the cape. Such short-period variation can be monitored by using the daily variation of the sea level difference between Kushimoto and Uragami. In particular, in the case of October 29–31, 1996, when an eminent small-scale eddy passed by Cape Shionomisaki, and when the Kuroshio axis tentatively moved southwards about 50 km apart from the coast, the Kii Bifurcation Current seems to have disappeared.