Spatial changes in a macrozoobenthic community along environmental gradients in a shallow brackish lagoon facing Sendai Bay, Japan

The spatial distribution, abundance, and assemblage structure of macrozoobenthos were examined at 45 stations in a brackish lagoon (Idoura Lagoon, Japan) to examine the animal–environmental relations in estuarine soft-bottom habitats. We found a total of 23 taxa; the polychaetes Heteromastus sp., Hediste spp., and Prionospio japonica and the isopod Cyathura muromiensis numerically dominated the community. Cluster analysis and one-way analysis of similarity (ANOSIM) identified seven groups of stations that had significantly different macrozoobenthic communities; these were subsequently consolidated into five habitat groups according to their association with environmental characteristics. Canonical correspondence analysis (CCA) showed that salinity, silt-clay content, and the oxidation–reduction potential (ORP) of the sediment strongly affected the macrozoobenthos distribution pattern in the lagoon, whereas other factors (e.g., relative elevation of the habitat and sediment organic content) had much weaker effects. Similarity percentages (SIMPER) procedures indicated that the polychaete Notomastus sp. and the bivalve Macoma contabulata were specific to habitats with low salinity and reduced mud, whereas the bivalve Nuttallia olivacea was specific to sandy bottoms. Heteromastus sp. and Hediste spp. achieved their highest densities in rather oxidized sediments. The acid-volatile sulfide (AVS) content in the sediment was suggested as another possible factor affecting macrozoobenthic density. Our results clearly demonstrate that macrozoobenthic assemblages in estuarine soft-bottoms have high spatial heterogeneity on a small scale (e.g., hundreds of meters) related to physical and chemical environmental changes. Our data also suggested the importance of sediment redox condition (e.g., ORP and AVS content) and sediment grain size as structuring factors in estuarine soft-bottom communities as well as the salinity in the habitat.     

Variations in turbulent energy dissipation and water column stratification at the entrance of a tidally energetic strait

We observed tidal currents, turbulent energy dissipation and water column stratification at the entrance of a narrow strait (Neko Seto) in the Seto Inland Sea, Japan, using a free-falling turbulent microstructure profiler (TurboMAP) and acoustic Doppler current profiler (ADCP). The variation in turbulent energy dissipation at the entrance of the strait was not at quarter-diurnal frequency but at semi-diurnal frequency; turbulent energy dissipation was enhanced during the ebb tide, although it was moderate during the flood tide. This result is consistent with the results of Takasugi (1993), which showed the asymmetry of tidal energy loss during a semidiurnal tidal cycle using control volume analysis. It is suggested that significant turbulent energy dissipation is generated in the strait, which influences the properties of water outside the strait when tidal currents flow out from the strait.     

Eigen oscillation of a fluid sphere and source mechanism of harmonic volcanic tremor

The eigen oscillation of a fluid sphere embedded in an infinite elastic medium is analyzed to understand the source mechanism of volcanic tremor that vibrates nearly monotonically and attenuates slowly. The dimensionless eigen frequencies of the sinusoidal oscillation are calculated in a complex form with the attenuation factor in its imaginary part for various combinations of the three parameters: the contrasts of P-wave velocity, density and rigidity between the fluid and the country rock. Eigen oscillations consist of a high attenuation mode with a rapidly decaying pulsive wave at a low frequency and infinite number of regular modes with slowly decaying vibrations. For regular modes, the frequency of oscillation obtained from the real part of an eigen value is distributed in approximately regular intervals while the attenuation factor from the imaginary part is almost constant independent of the mode. Each eigen frequency of regular and high attenuation modes is degenerated with two independent eigen functions describing different distributions of displacement, velocity and stress. The theory is applied to harmonic volcanic tremor observed at Kusatsu-Shirane Volcano, central Japan. Observed spectral peaks of the tremor are explained by the eigen frequencies and attenuation factors of several lowest regular modes if the spherical fluid oscillator has a radius of dozens of meters and a P-wave velocity of about several hundred meters per second.     

Estimates of the turbulent kinetic energy budget in the oceanic convective boundary layer

The terms of the steady-state turbulent kinetic energy (TKE) budget in the oceanic convective boundary layer (CBL) are estimated by use of microstructure data obtained over the continental shelf of the East China Sea. The dissipation term is calculated from the micro-scale vertical shear of horizontal velocity measured directly using a freely-falling microstructure profiler, whereas the buoyancy flux and shear production terms are estimated indirectly by integrating vertically the one-dimensional conservation equation of density and by applying similarity theory, respectively. The transport term, calculated as the residual of the other three terms, vertically redistributes the TKE from the upper half of the CBL to the lower half, consistent with the TKE budgets in the atmospheric CBL and in shear-free and slightly-sheared CBLs simulated by large eddy-simulation models. The relatively large contribution of the transport term to the TKE budget shows that a local equilibrium form of the TKE equation is not appropriate for the TKE budget in the oceanic CBL.     

Stress field change around the Mount Fuji volcano magma system caused by the Tohoku megathrust earthquake, Japan

Crustal deformation by the M _w 9.0 megathrust Tohoku earthquake causes the extension over a wide region of the Japanese mainland. In addition, a triggered M _w 5.9 East Shizuoka earthquake on March 15 occurred beneath the south flank, just above the magma system of Mount Fuji. To access whether these earthquakes might trigger the eruption, we calculated the stress and pressure changes below Mount Fuji. Among the three plausible mechanisms of earthquake–volcano interactions, we calculate the static stress change around volcano using finite element method, based on the seismic fault models of Tohoku and East Shizuoka earthquakes. Both Japanese mainland and Mount Fuji region are modeled by seismic tomography result, and the topographic effect is also included. The differential stress given to Mount Fuji magma reservoir, which is assumed to be located to be in the hypocentral area of deep long period earthquakes at the depth of 15 km, is estimated to be the order of about 0.001–0.01 and 0.1–1 MPa at the boundary region between magma reservoir and surrounding medium. This pressure change is about 0.2 % of the lithostatic pressure (367.5 MPa at 15 km depth), but is enough to trigger an eruptions in case the magma is ready to erupt. For Mount Fuji, there is no evidence so far that these earthquakes and crustal deformations did reactivate the volcano, considering the seismicity of deep long period earthquakes.     

Nitrogen and phosphorus distributions across the thermohaline front in Kii Channel in winter

We have studied nitrogen and phosphorus distributions across the thermohaline front in Kii Channel in winter by using engine-cooling sea water of a ferry boat. On Dec. 1986 and Jan. 1987, differences of PO₄–P and DIN across the front are recognized. Especially in the latter case, differences of nutrients concentrations across the front are very obvious. But differences of nutrients across the front on Feb. 1986, Feb. and Mar. 1987 are not obvious. Inspite of winter,Akashiwo had happened in Osaka Bay, nutrients mostly have already been utilized by phytoplankton in inner part of Osaka Bay. Consequently, differences of nutrients concentrations across the front are nearly zero.     

Magma discharge variations during the 2011 eruptions of Shinmoe-dake volcano, Japan, revealed by geodetic and satellite observations

We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption.     

Observations of turbulence under weakly and highly stratified conditions in the Ariake Sea

Observations of turbulence, stratification, and mean current were made using a microstructure profiler and an acoustic Doppler current profiler (ADCP) during four cruises at a central location in the Ariake Sea, under weakly and strongly stratified conditions. Continuous measurements of the dissipation rate of turbulent kinetic energy (TKE), ε, were made. These revealed that frictional bed turbulence with quarterdiurnal variation in the bottom boundary layer (BBL) was one of the most energetic sources of vertical mixing in the sea. Thickness of the BBL was strongly confined by the stable stratification. We investigate a relationship between the BBL height h and the Ozmidov scale. We present a systematic argument that describes the vertical structure and characteristic scales of velocity and turbulence inside the frictional BBL, where the stratification persisted. Considerable deviation of observed vertical shear from the law of the wall indicated a modification of turbulent scales by the stratification. Shear stress calculated from the velocity data using vertical integration of the equation of motion was found to decrease approximately linearly with height. The TKE production rate P, estimated using the shear stress, was highly correlated with the dissipation rate. The buoyancy contribution to TKE balance in the BBL was quantified in terms of the flux Richardson number R _f as R _f?=?0.12.     

Spatio-temporal variations in macrozoobenthic assemblage structures in a river-affected lagoon (Idoura Lagoon,Sendai Bay,Japan): Influences of freshwater inflow

Spatio-temporal changes in macrozoobenthic assemblage structure were monitored at 8 stations in a river-affected temperate lagoon (Idoura Lagoon, Japan) from May 1997 to May 1998 to assess animal–environment relationships in estuarine soft-bottom habitats. A total of 29 taxa occurred with a predominance of a few polychaete species including Heteromastus cf. similis, Hediste spp., Pseudopolydora cf. kempi and Prionospio japonica. The community structure differed significantly among four habitat types that were characterized by different salinity, sediment characteristics, and/or bottom elevation. For each habitat, macrozoobenthic community structure changed significantly with time, which was discriminated mostly by the lower density of polyhaline opportunistic species such as P. cf. kempi in warmer period. The salinity in the study area was much lower from May to September, mainly due to irrigation discharge from paddy fields, which seemed to be a lethal factor for the polyhaline species. C/N ratio and δ¹³C value of the sediment implied the accumulation of river-derived organic matters during the high river flow period. Present results showed the significant effects of fluctuating river discharge on the estuarine soft-bottom habitats, suggesting a linkage among terrestrial, riverine and estuarine ecosystems.     

Determination of the Fe oxidation state of the Chassigny kaersutite: A microXANES spectroscopic study

Abstract— In order to elucidate the formation of low‐H kaersutites in Martian meteorites, the Fe³⁺/ΣFe ratio of Chassigny kaersutites in magmatic inclusions was directly determined by the synchrotron microXANES analysis. XANES analysis for standard kaersutites with known Fe³⁺/ΣFe ratios shows a linear relationship between centroid energy positions of XANES pre‐edge spectra and the Fe³⁺/ΣFe ratio. Based on the linear relationship, the Fe³⁺/ΣFe ratio of Chassigny kaersutites is estimated to be about 0.05. The low Fe³⁺/ΣFe ratio clearly suggests that low‐H kaersutites in Chassigny are not likely to be formed by the oxidation‐dehydrogenation reactions. The low‐H content of the Chassigny kaersutites is mainly due to the presence of a Ti oxy‐component.