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

Hydrographic observations have revealed detailed structure of the Bottom Water in the Japan Sea. The Yamato Basin Bottom Water (YBBW) exhibits higher temperatures and lower dissolved oxygen concentrations than those found in the Japan Basin Bottom Water (JBBW). Both Bottom Waters meet around the boundary region between the Yamato and the Japan Basins, forming a clear benthic front. The structure of the benthic front suggests an estuary-like water exchange between both Basins, with the inflow from the Japan Basin passing under the outflow from the Yamato Basin. It is inferred from the property distributions that the JBBW flowing into the Yamato Basin is entrained by the cyclonic circulation in the basin, and modified to become the YBBW. Vertical diffusion and thermal balance in the YBBW are examined using a box model. The results show that the effect of geothermal heating has about 70% of the magnitude of the vertical thermal diffusion and both terms cancel the advection term of the cold JBBW from the Japan Basin. The box model also estimates the turnover time and vertical diffusivity for the YBBW as 9.1 years and 3.4 × 10⁻³ m²s^{− 1}, respectively.     

Latitudinal effect on the growth dynamics of harvested stands of Typha: A modeling approach

A model was developed for Typha, to examine the effects of latitudinal changes in temperature and radiation on the partitioning of total biomass during the growing season into rhizomes, roots, flowering and vegetative shoots, and inflorescences.Regardless of initial rhizome biomass, both above and belowground biomasses converge on a equilibrium value, with the balance between total production and metabolic loss being latitude-specific. If aboveground biomass is harvested just once, then both above and belowground biomasses return to equilibrium values after several years. If the aboveground biomass is harvested annually, then both above and belowground biomasses converge on smaller equilibrium values, which are determined by the balance between the sum of production prior to harvesting and after harvesting, and the sum of annual metabolic losses and a loss due to harvesting.The model could be used in wetland management activities to predict the potential growth of Typha in given conditions as well as the responses of Typha stands to harvesting over a wide range of latitudes for times ranging from a season to several years.     

Formation, Distribution and Volume Transport of the North Pacific Intermediate Water Studied by Repeat Hydrographic Observations

In order to examine the formation, distribution and transport of North Pacific Intermediate Water (NPIW), repeated hydrographic observations along several lines in the western North Pacific were carried out in the period from 1996 to 2001. NPIW formation can be described as follows: (1) Oyashio water extends south of the Subarctic Boundary and meets Kuroshio water in intermediate layers; (2) active mixing between Oyashio and Kuroshio waters occurs in intermediate layers; (3) the mixing of Oyashio and Kuroshio waters and salinity minimum formation around the potential density of 26.8σ_θ proceed to the east. It is found that Kuroshio water flows eastward even in the region north of 40°N across the 165°E line, showing that Kuroshio water extends north of the Subarctic Boundary. Volume transports of Oyashio and Kuroshio components (relative to 2000 dbar) integrated in the potential density range of 26.6–27.4σθ along the Kuroshio Extension across 152°E–165°E are estimated to be 7–8 Sv (10⁶ m³s⁻¹) and 9–10 Sv, respectively, which is consistent with recent work. This revised version was published online in July 2006 with corrections to the Cover Date.     

OCTOPUS,an Octo parameter underwater sensor,for use in biological oceanography studies

A realtime monitoring system for observing physical, biological, and optical oceanographic parameters in shallow waters was developed. It consists of a CTD with oxygen sensor, anin situ fluorometer, anin situ transmissometer, and anin situ quanta meter together with an on-deck quanta meter. The system is also equipped with a water sampling device. Water sampling can be done during the upward cast on the basis of depth profiles of temperature, salinity, oxygen,in situ fluorescence, transmittance, and underwater irradiance taken during the downward cast.     

228Ra in surface water of the Seto Inland Sea

The concentrations of²²⁸Ra in surface waters of the Seto Inland Sea were determined. Surface waters from the central region of the Seto Inland Sea, Hiuchi Nada and Bingo Nada, contained concentrations of²²⁸Ra of 655–811 dpm/1000 l which were 100 times higher than those obtained in the Pacific Ocean. These high concentrations of²²⁸Ra must be supported by a²²⁸Ra flux from the bottom sediment. The lower limit of this flux was estimated to be more than 0.16 dpm cm^–2 y^–1. The²²⁸Ra concentrations decreased markedly from central regions of the Seto Inland Sea to about 18 dpm/1000 l in the Kii and the Bungo Channels as salinity increased. Using a box model and the²²⁸Ra data, the mean residence time of sea water in the Seto Inland Sea with respect to the exchange with the open ocean water was estimated to be less than 10 y, and the most probable value is the order of several years.     

Physical control of sediment carbon content in an estuarine tidal flat system (Nanakita River, Japan): A mechanistic case study

Temporal variations in sediment carbon content were tracked by sampling every 2 weeks for 6 to 33 months at 6 tidal flat stations with different carbon content levels (0.05–1.64%) in a single estuary. Three temporal series of current velocity at 5 cm above the sediment were also obtained. Non-cohesive and cohesive sediment stations differed in patterns of temporal variation in sediment carbon content, suggesting the difference in processes controlling sediment carbon content. In the stations of non-cohesive sandy sediment with relatively low carbon content (0.05–0.15%), sediment carbon content fluctuated within ranges specific to each station. In these stations, current velocity data suggested that frequency of sand resuspension washing out sediment carbon controls sediment carbon content level. In the stations of cohesive sediment with relatively high carbon content (0.77–1.64%), sediment carbon contents showed some unusual upward and downward peaks, recovered to usual levels specific to the stations, and was always kept higher than that in the non-cohesive sediment stations. We speculate that in the cohesive sediment stations, sand inputs may significantly control processes establishing the consolidated sediments with various carbon content levels. Spatial transitions from non-cohesive to cohesive sediments and from low to high sediment carbon contents likely occur as sand resuspension at spring tides become more irregular and less frequent. Based on these results, conceptual models describing physical processes controlling sediment carbon content at the studied stations were proposed.     

Seasonal and mesoscale variation in the surface temperature and salinity at the coastal and shelf region off Shikoku,Japan

A three-year-long time series of water temperature and salinity observed on a ferryboat in the shelf region off Shikoku Japan was analyzed, focusing on the phenomena with a time scale of more than one month. We found two remarkable fronts in the seasonal variations. One is the well-known Kii Channel Front. This front remains as a haline front in summer while a thermohaline front in winter. The other, which is formed near Cape Ashizuri-misaki, is newly found. Density gradient across the front in winter is in the opposite direction to that in summer. Next, focusing on phenomena with a shorter time scale, we found the simultaneous variation in water temperature over the observational region, the time scale of which is about three months. It has a good coherence with the variation in air temperature observed at the coast, which implies that this variation has something to do with a phenomenon including the atmospheric system. Warm water intrusion from the Kuroshio is also correlated with this variation. Short-period variations such as the eastward progression of warm water mass tend to be active when the simultaneous variation in water temperature is in the warming phase, i.e., water temperature is increasing.     

Redeposition of volcaniclastic sediments by a tsunami 4600 years ago at Kushima City,south-eastern Kyushu,Japan

The Hyuga-nada Sea, south-eastern Kyushu, Japan, is located between a strong (Nankai Trough) and a weak interplate coupling zone (Ryukyu Trench). Over the past 400 years this area has only experienced Magnitude 7·5 earthquakes or smaller and associated small-scale tsunamis. However, this short historical record most likely does not include the full range of high magnitude, low frequency giant earthquakes that might have occurred in the region. Thus, it is still unclear whether giant earthquakes and their associated tsunamis have occurred in this region. This paper reports on a prehistoric tsunami deposit discovered in a coastal lowland in south-eastern Kyushu facing the Hyuga-nada Sea. There is a reddish-brown pumiceous layer preserved in a non-marine, organic-rich mud sequence obtained from onshore sediment cores. This layer is recognized as the ca 4600 year old Kirishima-Miike tephra (that is now placed around 4500 years ago) sourced from Mount Kirishima, southern Kyushu. Another whitish pumiceous layer is evident below the Kirishima-Miike tephra in almost all of the sediment cores. A relatively high percentage of marine and brackish diatoms is recorded within this lower pumiceous layer (but not in the surrounding muds or in the overlying Kirishima-Miike tephra), indicating a marine or beach sediment source. Plant material obtained from organic-rich mud immediately below the event layer was dated to ca 4430 to 4710 cal yr bp , providing a limiting-maximum age for this marine incursion event. The presence of marine diatoms below the event layer is probably explained by pre-seismic subsidence. An absence of the resting spore of the planktonic brackish diatom Cheatoceros and the appearance of the freshwater diatom Eunotia serra immediately above the event layer probably represents a marked change to a relatively low-salinity environment. Assuming that there were no significant local geomorphological changes, such as drainage obstruction caused by formation of a new barrier spit, it is considered that co-seismic or immediate post-seismic uplift are the most likely explanations for this notable environmental change. Based on the crustal movements noted before and after the marine incursion, this event is interpreted here as an earthquake-generated tsunami. Moreover, because of these notable seismic crustal movements the tsunamigenic earthquake probably occurred immediately offshore of the study site.     

Source characteristics of main and post-burst-increase phases of solar bursts at 17 GHz

Twenty four solar bursts of peak fluxes above 50 sfu are analyzed which were observed with the 17 GHz interferometer at Nobeyama during the period from 1978 September to 1979 December. Source characteristics and their temporal evolutions are investigated on a statistical basis with high time resolutions up to 0.8 s. Use of a model-fitting technique recently developed by Kosugi (1982) is made to derive both the position of centroid and size (～ FWHM) of burst source with an uncertainty of a few arc sec. The results of this study are the following:

1. Two different phases in the burst, that is to say, the main phase and the post-burst-increase (PBI) phase, are distinguished clearly not only by the morphological difference of flux time profile, but also by the differences of brightness temperature (10⁷-?10⁹ K vs 10⁵–10⁷ K), circular polarization degree (0–50% vs 0–10%), and size (?5–25″ vs 10–70″). There is no definite correlation between the peak fluxes in the two phases.
2. The majority of the selected bursts (21 of 24) show in the main phase source characteristics of the impulsive burst. The total flux varies rapidly (characteristic time scale defined by FWHM ? 100 s), often associated with the rapid shift of position and the rapid change of polarization degree. The source height of the impulsive source is lower than that of the PBI source. On the other hand, the type IV_μ source, seen in three events, shows a gradual variation and the source ascends to a height of ～ 40 000 km above the photosphere.
3. In the PBI phase, the expansion and ascension of the source occur in general (21 of 23 for the former and 12 of 15 for the latter). The velocities of both the movements are of the order of 5 km s^?1.