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 Ky = 4 × 105 cm2 s−1 at distance y < 200 km from the coast, and Ky = 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. 相似文献
Concentrations of U and Th isotopes in Okinawa Trough and East China Sea sediment cores were determined by isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) to investigate the behavior of redox sensitive uranium in suboxic hemipelagic sediments and determine their significance in oceanic uranium balance. 238U concentrations and 238U/232Th activity ratios in the East China Sea sediments showed no remarkable variation with depth. However, 238U and 238U/232Th ratios in the Okinawa Trough sediments were low in the surface oxidizing layer but increased where the suboxic condition was encountered. The distribution profiles of 230Th and 232Th concentrations were relatively constant with depth in both the Okinawa Trough and East China Sea sediment cores. These results suggested that there has been post-depositional precipitation of authigenic uranium within the suboxic Okinawa Trough sediment column. The post-depositional precipitation rates of authigenic uranium were estimated to be 47 ± 5 to >62 ± 8 ng cm−2 yr−1; these rates were comparable to those previously reported for several anoxic sediments. A mechanism controlling precipitation of uranium may be the downward diffusion of uranium U(VI), reduction to U(IV) and finally precipitation onto the solid phase. The accumulation rate of uranium for the Okinawa Trough sediments was approximately eight times higher than the world average rate reported for suboxic sediments. This removal of uranium in the oceanic budget increases the importance of the suboxic sediment sink. 相似文献
We investigate the total kinetic powers (Lj) and ages (tage) of powerful jets of four FR II radio sources (Cygnus A, 3C 223, 3C 284, and 3C 219) by the detail comparison of the dynamical
model of expanding cocoons with observed ones. It is found that these sources have quite large kinetic powers with the ratio
of Lj to the Eddington luminosity (LEdd) resides in 0.02<Lj/LEdd<10. Reflecting the large kinetic powers, we also find that the total energy stored in the cocoon (Ec) exceed the energy derived from the minimum energy condition (Emin ): 2<Ec/Emin <160. This implies that a large amount of kinetic power is carried by invisible components such as thermal leptons (electron
and positron) and/or protons. 相似文献
A precipitation system developed continuously along the western coastline of the Korean Peninsula and created considerable precipitation both along the coast and inland on 26 July 2011. In this study, the causes for this nearshore convective system are investigated from observations and the results of model experiments. Three-dimensional radar fields clearly show that a change of wind at the surface border played an important role in the development of the nearshore convection system. The simulation results, which are very similar to the observations, show that the surface border generated and maintained the convergence zone. The roughness change enhanced the convergence, and the interaction between the deepening cold pool and downward flow maintained the convergence zone. The surface mechanical discontinuity affected by the roughness change between sea and land formed the convergence (gradient of wind stress),which induced momentum transfer to the upper layer. The cold pool created a steep gradient of potential temperature and provided the reason for the propagated convergence zone with the downward flow. The maximum value of the surface change factor, which comprises the influencing factors for the long-lasting convective system, reflects the enhancement of the system at the coast. 相似文献
Coral reef ecosystems worldwide are now being harmed by various stresses accompanying the degradation of fish habitats and thus knowledge of fish-habitat relationships is urgently required. Because conventional research methods were not practical for this purpose due to the lack of a geospatial perspective, we attempted to develop a research method integrating visual fish observation with a seabed habitat map and to expand knowledge to a two-dimensional scale. WorldView-2 satellite imagery of Spermonde Archipelago, Indonesia obtained in September 2012 was analyzed and classified into four typical substrates: live coral, dead coral, seagrass and sand. Overall classification accuracy of this map was 81.3% and considered precise enough for subsequent analyses. Three sub-areas (CC: continuous coral reef, BC: boundary of coral reef and FC: few live coral zone) around reef slopes were extracted from the map. Visual transect surveys for several fish species were conducted within each sub-area in June 2013. As a result, Mean density (Ind. / 300 m2) of Chaetodon octofasciatus, known as an obligate feeder of corals, was significantly higher at BC than at the others (p < 0.05), implying that this species’ density is strongly influenced by spatial configuration of its habitat, like the “edge effect.” This indicates that future conservation procedures for coral reef fishes should consider not only coral cover but also its spatial configuration. The present study also indicates that the introduction of a geospatial perspective derived from remote sensing has great potential to progress conventional ecological studies on coral reef fishes.
The stratigraphic architecture of sedimentary basin provides important constraints on the rheological structure of the upper mantle and the crust, eustatic events and tectonic movements. In this study, we examined the convective coupling between the uppermost mantle and ductile lower crust as a formation mechanism of sedimentary basins. In this mechanism, the lower crust is squeezed by the upper mantle flow associated with mantle diapiric upwelling, resulting in the surface subsidence and formation of sedimentary basin. We investigated the stratigraphic architecture by taking into account the horizontal migration of the upwelling in time and spatial domains. The sedimentary basin is characterized by both the thickness and formation period for its gross feature and the sequence stratigraphy. The decay of the upwelling causes the surface uplift, and some parts of sediments deposited during the surface subsidence are consequently eroded. The subsidence area moves as the activity of upwelling horizontally migrates, resulting in the formation of unconformity for the uplifted and eroded area over the previous upwelling. We also incorporated the effects of third order eustasy, with amplitude of 100 m and period of 1 Myr, into convective coupling model. An application of our model to Karatsu-Sasebo coalfield in the Tertiary of the northwest Kyushu, Japan, indicates that the stratigraphic architecture of sedimentary basin including two effects, i.e. convective coupling and eustasy, may provide important information about the viscosity structure of the lower crust and uppermost mantle and spatio-temporal growth and decay histories of the mantle diapiric upwelling. 相似文献
By means of nanoscale surface observation, we have proposed a new approach for investigating fine crystals of cosmic materials to reveal their origin and growth conditions. Several different morphologies of polyhedral fine olivines with faceted faces have been found in Allende carbonaceous chondrite (4.5 byr in geochronological age). In the present work, molecular level topography of the faceted matrix olivine by Atomic Force Microscopy (AFM) has successfully been performed. The matrix olivine found to have preserved growth step pattern on its surface even though quite long time has passed since they formed in the early Solar System. The surface pattern suggests that the faceted matrix olivine could have been condensed from the gas phase, and possibly that these olivine crystals had continued to grow under a rapid cooling condition (0.1-1 K s−1). The estimated cooling rate agrees well with predictions based on hypothetical rapid heating and cooling events such as shock wave heating. 相似文献