Isotope dilution sector-field inductively coupled plasma mass spectrometry combined with extraction chromatography for rapid determination of 241Am in marine sediment samples: A case study in Sagami Bay, Japan

²⁴¹Am is a useful tracer for understanding biogeochemical processes in the marine environment. ²⁴¹Am also poses a potential radiation threat to human health due to the continuous increase of its concentration in the global environment. We report a rapid analytical method for determining ²⁴¹Am in marine sediments using isotope dilution sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) combined with a high-efficiency sample introduction system (APEX-Q). A selective CaF₂ co-precipitation procedure followed by TRU extraction chromatography was employed to effectively remove the major sediment matrix and to pre-concentrate ²⁴¹Am. We achieved an extremely low detection limit of 0.32 fg/g or 0.041 mBq/g (for 1 gram sediment), which is better than that of alpha spectrometry, and it allowed the accurate determination of ²⁴¹Am in low-level marine sediment samples. The accuracy and precision of the developed analytical method was evaluated using a laboratory prepared Am isotope standard solution and Ocean Sediment reference material (IAEA-368). The results were satisfactory. For sediment samples, overall chemical recoveries varied from 60–90%. The developed method was applied to the study of ²⁴¹Am depth distribution in Sagami Bay, Japan, where we observed different depth profiles between ²⁴¹Am activity and ^239+240Pu activity.     

Implementation of an integrated vulnerability and risk assessment model

An integrated vulnerability and risk assessment model (IVR) is proposed. The proposed model is a composite index that assesses the relationships among four critical components, namely exposure, hazard, vulnerability and capacities and measures, and how these can be used to illustrate the integrated risk and vulnerability situation of an area. These factors are assessed using individual variables. Fifty-four variables, objectively decided upon, were used to measure the contribution of each component factor. The general characteristics of vulnerability, risk, exposure and capacities and measures are well known; however, the relative importance of each variable and their inter-relatedness, in measuring risk and vulnerabilities, as demonstrated by the IVR; and how these in turn affect the impacts of natural hazards, are still debatable. The IVR though provides a valid, reliable and sensitive tool, which can be used to further explore these relationships. Through robust testing and application, subjectivity in the selection of variables can be reduced. Moreover, through the establishment of a database for data collection and storage, objectivity (reliability) can be achieved as well as, availability of the requisite data inputs. The sensitivity of the model allows for the quick identification of strategic action, which will ultimately lead to hazard loss reduction. The values yielded for each component factor can help decision-makers in the allocation of scarce developmental funds as well as identify trends in levels of risk, vulnerability, exposures and capacities and measures as well as determine areas where mitigation strategies are needed most.     

Assessment of nitrogen loading from the Kiso-Sansen Rivers into Ise Bay using stable isotopes

Concentrations of particulate organic nitrogen (PN), dissolved inorganic nitrogen (DIN), and their nitrogen isotope ratios (δ ¹⁵N) in the Kiso-Sansen Rivers were determined from monthly observations over the course of a year to assess variations in the form and sources of riverine nitrogen discharged into Ise Bay. The δ ¹⁵N values of NO₃ ⁻ observed in the Kiso-Sansen Rivers showed a logarithmic decreasing trend from 8 to 0‰, which varied with the river discharge, indicating mixing between point sources with high δ ¹⁵N and non-point sources with low δ ¹⁵N. The influence of isotope fractionation of in situ biogeochemical processes (mainly DIN assimilation by phytoplankton) on δ ¹⁵N of NO₃ ⁻ was negligible, because sufficient concentrations of NH₄ ⁺ for phytoplankton demand would inhibit the assimilation of NO₃ ⁻. A simple relationship between river discharge and δ ¹⁵N of NO₃ ⁻ showed that the fraction of total NO₃ ⁻ flux arising from point sources increased from 4.0–6.3% (1.1–1.8 tN day⁻¹) during higher discharge (>600 m³ s⁻¹) to 30.2–48.3% (2.6–4.1 tN day⁻¹) during lower discharge (<300 m³ s⁻¹). Riverine NO₃ ⁻ discharge from the Kiso-Sansen Rivers can explain 75% of the variations in surface NO₃ ⁻ at the head of Ise Bay over the year.     

Radium isotopes in coastal and open ocean surface waters of the Western North Pacific

Using manganese-impregnated fiber extraction and high-efficiency gamma counting techniques, we measured the distribution of ²²⁸Ra and ²²⁶Ra in surface waters near the coast of Japan and in the western North Pacific. There is no evidence in our data that any significant amount of ²²⁸Ra is added to open ocean surface waters from the coastal waters around Tokyo Bay. High ²²⁸Ra concentrations (> 10 dpm/10³ kg), were observed along the Kuroshio Current as compared to < 2.5 dpm/10³ kg between 10° and 30°N of the central gyre, and hence the major source of ²²⁸Ra in the surface water is likely to be the East Asian continental shelf zones. A simple one-dimensional eddy diffusion and advection model is used to explain the observed decrease of ²²⁸Ra from coast to the open ocean. The model results indicate two mixing regimes across the Kuroshio Current System with apparent eddy diffusion coefficients of K_y = 4 × 10⁵ cm² s⁻¹ at distance y < 200 km from the coast, and K_y = 4 × 10⁷ cm² s⁻¹ at y > 200 km. Along 40°N where an eastward flow of the ‘Kuroshio Extension’ prevails, an advective flow of > 0.1 knot is consistent with the observation of nearly constant ²²⁸Ra along the track.The geographical distribution pattern of ²²⁸Ra is clearly different from that of atmospherically derived ²¹⁰Pb. Thus the ²²⁸Ra in surface water serves as a useful tracer that accompanies fluvially and coastally derived elements during their subsequent lateral transport toward the central gyre.     

Size distributions of aerosol number concentrations and water-soluble constituents in Toyama, Japan: A comparison of the measurements during Asian dust period with non-dust period

Size-separated aerosol number concentrations and water-soluble constituents were measured in Toyama, the Hokuriku district, near the coast of the Japan Sea, during the spring and summer in 2003. The number concentrations of coarse particles were significantly high in April, which was due to Asian dust events called Kosa in Japanese. Particulate nssCa²⁺, which is mostly present in the coarse-mode particles, was significantly high in April. On the other hand, the concentrations of NH₄⁺ and nssSO₄²⁻, which mainly exist as the accumulation-mode particles were not high in April. The mass-size distributions of water-soluble constituents were compared with the size-separated number concentrations of particles. Backward trajectory analysis was also employed to examine the transport process of the air mass in Toyama.     

Organic geochemistry of the Japan Sea sediments-1: Bulk organic matter and hydrocarbon analyses of Core KH-79-3, C-3 from the Oki Ridge for paleoenvironment assessments

Organic geochemical study of bulk organic matter (OM), hopanoid hydrocarbon and normal hydrocarbon (C₂₃C₃₅) was conducted for a 936-cm-long sediment core sample from the Oki Ridge of the Japan Sea (Core KH-79-3, C-3; 37°03.5 N, 134°42.6E, water depth 935 m). Stable carbon isotopic ratios were also measured for both bulk OM and individual hydrocarbons. The following results were obtained: (1) The weight ratios of total organic carbon to total nitrogen range from 6.2 to 9.4 in the core. The ¹³C values of bulk OM range from –25.1–20.7%.. The ¹³C values of OM in the sections of 140190 cm are lower (–25–24) than those in the other sections (–23–21). This result indicates that OM in the core except for the 140190 cm sections is essentially of marine origin. (2) The ¹³C value of diploptene (a hopanoid hydrocarbon) in the last glacial maximum (LGM), is –66.3 (vs. PDB), which indicates it originating in methanotrophic bacteria. This result provides evidence to support for the previous ideas (Oba et al., 1980, 1984; Masuzawa and Kitano, 1984) that the bottom waters in the Japan Sea were anoxic in LGM. (3) Long chain (C₂₃C₃₅) n-alkanes of higher-plant wax origin were found throughout the core. Their concentration is high in 140190 cm in depth, suggesting that eolian dust load was high in LGM. (4) The n-alkane/TOC ratio increases with decreasing ¹³C values of bulk OM. This result indicates that the load of terrestrial (probably eolian dust-derived) OM to the Japan Sea became higher in colder climates. (5) The CPI values of long-chain n-alkanes are different in different ¹³O stages of paleoclimate, probably reflecting variations in species of terrestrial higher plants as a result of climatological adaptations.     

Chemical composition of primary cosmic rays with energies from 10 to 10 eV

The chemical composition of primary cosmic rays with energies from 10¹⁵ to 10^16.5 eV, so called “knee” region, is examined. We have observed the time structures of air Čerenkov light associated with air showers at Mt. Chacaltaya, Bolivia, since 1995. The distribution of a parameter that characterizes the observed time structures is compared with that calculated with a Monte Carlo technique for various chemical compositions. Then the energy dependence of the average logarithmic mass numbers ln A of the primary cosmic rays is determined. The present result at 10^15.3 eV is almost consistent with the result of JACEE (A12) and shows gradual increase in ln A as a function of the primary energy (A24 at 10¹⁶ eV). Form the comparison of the observational results with several theoretical models, we conclude that the supernova explosion of massive stars is a plausible candidate for the origin of cosmic rays around the “knee” region.     

Study of the effect of water-soluble fractions of heavy-oil on coastal marine organisms using enclosed ecosystems,mesocosms

Mesocosm facilities composed of 4 experimental and 2 reservoir tanks (1.5 m in diameter, 3.0 m in depth and 5 tons in capacity) made of FRP plastics, were constructed in the concrete fish rearing pond in the Fisheries Laboratory, The University of Tokyo. The water-soluble fraction of Rank A heavy residual oil was formed by mixing 500 g of the oil with 10 l of seawater, which was introduced to the 5000 l-capacity tanks. Experimental Run 4 was conducted from May 31 to June 7, 2000. Oil concentrations in the tanks were 4.5 microg/l called LOW, and 13.5 microg/l, called HIGH tank. Bacterial growth rates very quickly accelerated in the HIGH tank just after the loading of oil which corresponded with a high increase of bacterial cells in the same tank after 2 days. Later, bacterial numbers in HIGH tank rapidly decreased, corresponding with the rapid increase of heterotrophic nano-flagellates and virus numbers on the same day. Sediment traps were deployed at the bottom of the experimental tanks, and were periodically retrieved. These samples were observed both under light microscope and epi-fluorescent microscope with UV-excitation. It was observed that the main components of the vertical flux were amorphous suspended matter, mostly originating from dead phytoplankton and living diatoms. It was further observed from the pictures that vertical transport of oil emulsions were probably conducted after adsorption to amorphous suspended matter and living diatoms, and were settling in the sediment traps at the bottom of the tanks. This means that the main force which drives the soluble fraction of oil into bottom sediment would be vertical flux of such amorphous suspended particles and phytoplankton. Further incubation of the samples revealed that the oil emulsions were degraded by the activity of autochtonous bacteria in the sediment in aerobic condition.     

230Th/234U and 14C dating of a lowstand coral reef beneath the insular shelf off Irabu Island, Ryukyus, southwestern Japan

Abstract   High-resolution seismic reflection profiles delineated the distribution of mound-shaped reflections, which were interpreted as reefs, beneath the insular shelf western off Irabu Island, Ryukyus, southwestern Japan. A sediment core through one of the mounded structures was recovered from the sea floor at a depth of −118.2 m by offshore drilling and was dated by radiometric methods. The lithology and coral fauna of the core indicate that the mounded structure was composed of coral–algal boundstone suggesting a small-scaled coral reef. High-precision α-spectrometric ²³⁰Th/²³⁴U dating coupled with calibrated accelerator mass spectrometric ¹⁴C ages of corals obtained reliable ages of this reef ranging from 22.18 ± 0.63 to 30.47 ± 0.98 ka. This proves that such a submerged reef was formed during the lowstand stage of marine oxygen isotope stages 3–2. The existence of low-Mg calcite in the aragonitic coral skeleton of 22.18 ± 0.63 ka provides evidence that the reef had once been exposed by lowering of the relative sealevel to at least −126 m during the last glacial maximum in the study area. There is no room for doubt that a coral reef grew during the last glacial period on the shelf off Irabu Island of Ryukyus in the subtropical region of western Pacific.