Primary productivity and solar radiation off Sanriku in May 1997

We measured the in situ primary production at four stations from the surface to 80 m off Sanriku in late May 1997. The depth-integrated daily primary production in the upper 80 m was estimated to be 391, 468, and 855 mgC m⁻²d⁻¹ in water from the Oyashio, and 336 mgC m⁻²d⁻¹ in the warm-core ring. The variation in the primary production was primarily due to the variation in phytoplankton activity (chlorophyll α-specific primary production). A combination of previous and present studies in water from the Oyashio and the warm-core ring suggested that phytoplankton activity is proportional to light intensity between 12 and 50 Ein m⁻²d⁻¹ which is close to the usual light condition (61–75 Ein m⁻²d⁻¹) off Sanriku in May and June. Light may be a limiting factor for phytoplankton off Sanriku in late spring and early summer.     

Subduction of the Woodlark Basin at New Britain Trench, Solomon Islands region

The Woodlark Basin, located south of the Solomon Islands arc region, is a young (5 Ma) oceanic basin that subducts beneath the New Britain Trench. This region is one of only a few subduction zones in the world where it is possible to study a young plate subduction of several Ma. To obtain the image of the subducting slab at the western side of the Woodlark Basin, a 40-day Ocean Bottom Seismometer (OBS) survey was conducted in 1998 to detect the micro-seismic activity. It was the first time such a survey had been performed in this location and over 600 hypocenters were located. The seismic activity is concentrated at the 10–60 km depth range along the plate boundary. The upper limit just about coincides with the leading edge of the accretionary wedge. The upper limit boundary was identified as the up-dip limit of the seismogenic zone, whereas the down-dip limit of the seismogenic zone was difficult to define. The dip angle of the plate at the high seismicity zone was found to average about 30°. Using the Cascadia subduction zone for comparison, which is a typical example of a young plate subduction, suggests that the subduction of the Woodlark Basin was differentiated by a high dip angle and rather landward location of the seismic front from the trench axis (30 km landward from the trench axis). Furthermore, as pointed out by previous researchers, the convergent margin of the Solomon Islands region is imposed with a high stress state, probably due to the collision of the Ontong Java Plateau and a rather rapid convergence rate (10 cm/year). The results of the high angle plate subduction and inner crust earthquakes beneath the Shortland Basin strongly support the high stress state. The collision of the Ontong Java Plateau, the relatively rapid convergence rate, and moderately cold slab as evidenced by low heat flow, rather than the plate age, may be dominantly responsible for the geometry of the seismogenic zone in the western part of the Woodlark Basin subduction zone.     

西南日本秋吉带石炭系高山(Ko-yama)灰岩群的褶皱构造

The study area is located in Kawakami-cho, Okayama Prefecture, which is occupied by Lower Carboniferous to Middle Permian Ko-yama Limestone Group (Yokoyama et al., 1979), Permian Yoshii Group (Sano et al., 1987) and Triassic Nariwa Group (Tera-oka, 1959). The Nariwa Group unconformably covers the Paleozoic successions (Otoh, 1985). These succes-sions belong to the Akiyoshi Belt. The Ko-yama Limestone Group is composed mainly of massive limestone, with basic volcanic rocks, acidic tuff and chert. The group was dated by foraminifer and fusu-linid as Lower Carboniferous to Middle Permian (Yo-koyama et al., 1979).     

西南日本秋吉带石炭系高山(Ko-yama)灰岩群的石炭纪(Visean – Moscovian)牙形石序列

The Carboniferous conodont faunal succession of the lower part of Ko-yama Limestone Group was studied. The confirmed upper Visean–lower Moscovian lithostratigraphy of the group is characterized by the clastic carbonates with common association of the basaltic pyroclastics and some intercalation of spicular chert beds. The faunal succession of Gnathodus semiglaber (upper Visean), Gnathodus praebilineatus – Lochriea multinodosa (upper Visean), Lochriea ziegleri – Gnathodus girtyi girtyi s.l. (lower Serpukhovian), Neoganthodus symmetricus – Idiognathodus primulus (middle – upper Bashkirian), and Idiognathoides convexus – Gondolella clarki (lower Moscovian) faunas, appears in concordance with the lithostratigraphic order. The faunas are correlative with those from the conodont zones of the Hina, Atetsu, Akiyoshi and Omi limestone groups in the Akiyoshi Belt. The Visean/Serpukhovian boundary of the section was recognized by the FAD of Lochriea ziegleri.     

Natural radionuclide analysis in chattarpur area of southeastern coastal area of Odisha, India

The energy released in a spontaneous decay process of natural radionuclides is the main source of the total radiation dose to human beings. Natural radionuclides are widely distributed in soil, rocks, air, and groundwater. In present investigation, the analysis of terrestrial radionuclides such as ²²⁶Ra, ²³²Th, and ⁴⁰K in soil and sand of Chattarpur area of southeastern coast of Odisha has been carried out using NaI(Tl) gamma ray detector. The higher activity concentrations of naturally occurring radionuclides have been reported from the study area. The gamma radiationdose originating from the terrestrial radionuclides was found to vary from 95 to 1813 nGy/h with an average of 700 nGy/h. This study is important to generate a baseline data of radiation exposure in the area. Health hazard effects due to natural radiation exposure are discussed in details.     

Multiple-indicator study of the response of groundwater recharge sources to highly turbid river water after a landslide in the Tedori River alluvial fan,Japan

The large volume of groundwater stored in the Tedori River alluvial fan, Ishikawa Prefecture, Japan, is an important source of local drinking and industrial water. The Tedori River was observed to be highly turbid from the beginning of May 2015 to at least November 2017 due to a landslide in the upper reach of the river. After the landslide, the groundwater level was drawn down by several to 10 m near the middle river section during paddy irrigation periods in 2015 and 2016. This study addresses the impacts of the highly turbid water on groundwater recharge from the river and paddy fields. In 2016, we sampled groundwater, river water, paddy irrigation water, paddy ponding water, and precipitation five times at 2-month intervals. We analysed the H, O, and Sr stable isotopic compositions and major dissolved ion (and Sr) concentrations and compared our data to previous data obtained in June 2011. Ca, Sr, Cl, SO₄, and TN concentrations and δ¹⁸O values were higher in June 2016 than in June 2011; these increases were more extreme along the left bank of the Tedori River than along the right bank. We explored the mixing of Tedori River water with groundwater using a two-endmember mixing model based on their Sr concentrations and isotopic compositions. Compared to June 2011, mixing ratios were decreased near the Tedori River in 2016, and larger decreases were observed along the left bank and in the middle stream area. These results confirm that the contribution to groundwater recharge from the river decreased during the turbidity event, particularly along the left bank.     

Analyses of age and population genetic structure of the broadbanded thornyhead Sebastolobus macrochir in North Japan suggest its broad dispersion and migration before settlement

Body size, age, and population genetic structure of the broadbanded thornyhead Sebastolobus macrochir inhabiting areas off the Pacific coast of Tohoku and off Abashiri, the Sea of Okhotsk, Japan, were analyzed. On the basis of significant differences in body size (growth) between certain year classes at Tohoku and Abashiri, it appears that S. macrochir does not migrate extensively after settlement and subsists on food within the settled environment. Meanwhile, no genetic isolation was observed between the populations at these two sites. Thus, it is highly likely that its pelagic egg balloons, larvae, and juveniles widely disperse and migrate before settlement.     

Decline in heavy metal contamination in marine sediments in Jakarta Bay,Indonesia due to increasing environmental regulations

The ²¹⁰Pb geochronology, heavy metal concentrations (Zn, Cu, and Pb), and stable Pb isotope ratios (²⁰⁶Pb/²⁰⁷Pb) of three sediment cores collected from Jakarta Bay were analyzed to decipher the history of heavy metal contamination in the period 1900–2006. The chemical and isotopic analyses clearly suggest that anthropogenic metal accumulation in the sediments began in the 1920s and increased greatly from the 1970s until the end of the 1990s. From the end of the 1990s to 2006, accumulation rates were constant or decreased for Zn and Pb near the coastal industrialized area. Comparison of economic data and sociological information suggests that the decline in the concentrations of heavy metals could be attributed to the stricter environmental regulations which were enforced at the end of 1990s. However, metal contamination is currently still an important cause of concern in dealing with environmental preservation and protection in Jakarta Bay.     

Rayleigh scattering by aqueous colloidal silica as a cause for the blue color of hydrothermal water

Thermal waters in hydrothermal ponds, bathing pools and the brines of geothermal electric power plants commonly have a characteristic blue color. Although many researchers have assumed that the blue color is due to a colloidal suspension and/or absorption by dissolved ferrous iron or by water itself, there has been no specific effort to identify the physical nature of this phenomenon. We have tested, in synthetic and natural solutions, whether aqueous colloidal silica is responsible for the blue color. Aqueous colloidal silica is formed by silica polymerization in thermal waters of the neutral-chloride type which contain initially monomeric silica in concentrations up to three times above the solubilities of amorphous silica. The hue of the blue thermal waters in the pools tested agrees with that of a synthesized colloidal silica solution. Grain-size analyses of aqueous colloidal silica in the blue-colored thermal waters demonstrate that the color is caused by Rayleigh scattering from aqueous colloidal silica particles with diameters (0.1–0.45 μm) smaller than the wavelengths of visible radiation.     

Ongoing partial reconnection in a limb flare

The limb flare on August 2, 1993 08:30 UT, consisting of two crossed loops, shows evidence of partial X-type reconnection. The observations suggest that partial reconnection was an ongoing process during about 10 min until flare ignition.Visiting Fellow of the Japan Society for the Promotion of Science, on leave from SRON Laboratory for Space Research, Sorbonnelaan 2, Utrecht, The Netherlands.