Behavior of Cu, Ni and Cd during nutrient depletion in a spring bloom in Funka Bay

The concentrations of Cu, Ni and Cd were determined in Funka Bay during a spring phytoplankton bloom, consisting of diatoms. Just after the bloom, both dissolved Cd and nutrients were removed in the euphotic zone. However, the removal ratio of Cd to phosphate was very different from that in seawater. The removal of Cd took place at a Cd/phosphate ratio of 0.07×10⁻³, which was lower than in seawater before the bloom (0.25×10⁻³), leading to an increase in this ratio in seawater exceeding 0.7×10⁻³ at the end of the bloom. Elevated concentrations of Cd and phosphate were observed in the deeper layer after the bloom due to the decomposition of detrital materials produced in the bloom. The ratio of Cd/phosphate in the regeneration step was 0.24×10⁻³ which was different from the removal ratio of 0.07×10⁻³. These observations suggest that the high Cd/phosphate ratio in the regeneration would reflect a relatively high regeneration rate of Cd than that of phosphate. No significant decrease in Cu and Ni concentrations was observed during the development of the bloom, suggesting that biological removal of these metals was not so significant during the spring bloom. The concentrations of Cd, Cu and silicate in surface waters increased after the bloom with decreasing salinity due to the influence of a spring thaw.     

Determination of plastic regions around underground openings by a coupled boundary-element-characteristics method

A two-dimensional hybrid method for solving elastoplastic problems in engineering is presented by coupling two existing methods, namely, the boundary element method and the characteristics method. The formulation of this method is presented, as well as an excellent procedure for the determination of the boundary between elastic and plastic regions. It is shown not only that this method is a powerful and accurate method for evaluating the shape and extent of the plastic region around rock caverns, which is of prime importance for the construction of rock caverns, but also applicable to a given range of the initial stress field ratio where only compressive failure occurs. Then, some typical examples are solved in order to check the accuracy of the solution by this method. Furthermore, its successful applications are presented and discussed to determine the shape and the extent of the plastic regions around parallel, circular and rectangular openings.     

The phase transitions of superdense matter and supernova explosions

Effects of the phase transitions of superdense matter on supernova explosions are investigated with the aid of an idealized equation of state on the assumptions of adiabatic collapse. It is found that in the case of strong phase transitions explosions become weaker, while in the case of weak phase transitions explosions become stronger. However, the increment of the ejected energy is not so large as suggested by Migdalet al. (1979).Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Downstream transition of the Tsushima Current west of Kyushu in summer

In order to clarify detailed current structures west of Kyushu, ADCP measurements were carried out in July and September 1990 by the quadrireciprocal method (Katoh, 1988) for removing diurnal and semidiurnal tidal flows from observed flows. On the basis of these results, together with data of routine oceanographic observations, we study the downstream transition of the Tsushima Current west of Kyushu in summer. In the southwest of the Goto Islands, a northward current identified as the Tsushima Current was clearly found. In the south of Cheju, a westward current bifurcated from the Tsushima Current. In the northwest of the Goto Islands, the Tsushima Current narrowed and its velocity became strengthened. Salinity of the Tsushima Current water was much diluted by a current from the Cheju Strait. Near the west coast of the Goto Islands, a countercurrent bifurcating from the Tsushima Current often occurred. The volume transport of the Tsushima Current was 2.3 Sv (1 Sv=10⁶ m³s^–1) on the northern side of latitude 31°N. The substantial bifurcation of the Tsushima Current toward the Eastern and Western Channels of the Tsushima Strait occurred in the vicinity of Tsushima. The volume transport through the Western Channel was two to three times larger than that through the Eastern Channel. The baroclinic component in volume transport of the Tsushima Current west of Kyushu was much smaller than that in the Japan Sea.     

Anomalous distribution of suspended matter and some chemical compositions in seawater near the seabed: Transport processes

Vertical fine distributions of suspended matter and some chemical compositions in seawater were measured in the layer near the seabed. Distributions of organic suspended matter were almost uniform throughout the layer, but others were anomalous. However, with respect to their peak heights and distribution patterns, the tendency of profiles was the same on the whole. We suggested that these anomalous distributions were primarily caused by the destruction of the large flocculent particle on the seabed and by upward diffusion of disintegrated particles. The similarity of profiles between chemical compositions of dissolved substances and suspended matter of non-dissolved substances, was explained by assuming that pore water in the large particle was simultaneously transported with disintegrated particles after the destruction. An equation for the vertical distribution of suspended matter near the seabed was derived, provided that the rate of destruction of large flocculent particle was in proportion to the current velocity on the seabed. The equation represented the existence of the anomalous distribution, which was continuously changing its pattern. Measurements of vertical profiles of suspended matter showed almost the same tendency with the theory. From the characteristics of the theoretical equation, it was expected that the eddy diffusivity near the seabed was 1050 cm²/sec.     

Dissociation of CAS phase in the uppermost lower mantle

The high-pressure stability limit of calcium aluminosilicate (CAS) phase has been examined in its end-member CaAl₄Si₂O₁₁ composition at 18–39 GPa and 1,670–2,300 K in a laser-heated diamond-anvil cell (LHDAC). The in-situ synchrotron X-ray diffraction measurements revealed that the CAS phase decomposes into three-phase assemblage of cubic Al-bearing CaSiO₃ perovskite, Al₂O₃ corundum, and SiO₂ stishovite above 30 GPa and 2,000 K with a positive pressure–temperature slope. Present results have important implications for the subsolidus mineral assemblage of subducted sediment and the melting phase relation of basalt in the lower mantle.     

Episyenite formation in the Toki granite,central Japan

Episyenite is a quartz-depleted vuggy rock resulting from hydrothermal alteration of granitic rocks. This is the first report of its existence in an island arc, which is identified in a deep drill core of the Toki Cretaceous granite distributed in central Japan. In order to understand the petrographical features of the episyenite, neutron porosity measurement, geochemical analysis, microscopic observation, and X-ray computed tomography scanning were carried out. The results show remarkably high porosity (35.4 %) due to interconnecting vugs and the removal of quartz, plagioclase, and biotite. The Rb–Sr isotopic results and the paragenetic sequence of secondary minerals in the vugs suggest that the hydrothermal alteration process can be divided into an episyenitization stage and a later hydrothermal stage. At the episyenitization stage (70.6 ± 3.1 Ma) ca. 6 million years after the emplacement of the unaltered granite (76.3 ± 1.5 Ma), dissolution of quartz, biotite, and plagioclase occurred and was followed by the precipitation of albite, vermicular chlorite, and platy calcite. The episyenitization is considered as a local alteration of the Toki granite in an isotopically closed system. At the later hydrothermal stage, illite and secondary quartz precipitated from circulating meteoric-derived water in the dissolution vugs. Superimposing alteration at the later hydrothermal stage is limited, which results in the preservation of the episyenite in an almost primitive condition.     

Moho depth variation beneath southwestern Japan revealed from the velocity structure based on receiver function inversion

The Philippine Sea plate is subducting under the Eurasian plate beneath the Chugoku-Shikoku region, southwestern Japan. We have constructed depth contours for the continental and oceanic Mohos derived from the velocity structure based on receiver function inversion. Receiver functions were calculated using teleseismic waveforms recorded by the high-density seismograph network in southwestern Japan. In order to determine crustal velocity structure, we first improved the linearized time-domain receiver function inversion method. The continental Moho is relatively shallow ( 30 km) at the coastline of the Sea of Japan and at the Seto Inland Sea, and becomes deeper–greater than 40 km–around 35°N and 133.8°E. Near the Seto Inland Sea, a low-velocity layer of thickness 10 km lies under the continental Moho. This low-velocity layer corresponds to the subducting oceanic crust of the Philippine Sea plate. The oceanic Moho continues to descend from south to northwest and exhibits complicated ridge and valley features. The oceanic Moho runs around 25 km beneath the Pacific coast and 45 km beneath the Seto Inland Sea, and it extends to at least to 34.5°N. The depth variation of the Moho discontinuities is in good qualitative agreement with the concept of isostasy. From the configurations of both the continental and oceanic Mohos, we demonstrate that the continental lower crust and the subducting oceanic crust overlap beneath the southern and central part of Shikoku and that a mantle wedge may exist beneath the western and eastern part of Shikoku. The southern edge of the overlapping region coincides with the downdip limit of the slip area of a megathrust earthquake.     

Sylvite and halite on particles recovered from 25143 Itokawa: A preliminary report

We observed cross sectional ultra‐thin sections near the surface of 12 particles recovered from the S‐type asteroid Itokawa by the Hayabusa spacecraft in 2010, using spherical aberration–corrected STEM and conventional TEM. Although their mineralogy is almost identical to the equilibrated LL chondrites and therefore basically anhydrous, micrometer‐to‐submicrometer‐sized sylvite was identified on the surface of Itokawa particle RA‐QD02‐0034. Separately, micrometer‐sized halite was also identified on the surface of Itokawa particle RA‐QD02‐0129. Detailed inspection of the sample processing procedures at the JAXA's Planetary Materials Sample Curation Facility and textural observation of the sylvite and halite indicate that they were clearly present on two Itokawa particles before they were removed from Clean Chamber #2 at JAXA. However, there is no direct evidence for their extraterrestrial origin at present. If the sylvite and halite are extraterrestrial, their presence suggests that they may be more abundant on the surface of S‐type asteroids than previously thought.     

Exploring sustainability of aquifers based on predictive modeling of sorption characteristics of arsenic enriched Holocene sediments in Bangladesh

The importance of accessing safe aquifers in areas with high As is being increasingly recognized. The present study aims to investigate the sorption and mobility of As at the sediment-groundwater interface to identify a likely safe aquifer in the Holocene deposit in southwestern Bangladesh. The upper, shallow aquifer at around 18 m depth, which is composed mainly of very fine, grey, reduced sand and contains 24.3 μg/g As, was found to produce highly enriched groundwater (190 μg/L As). In contrast, deeper sediments are composed of partly oxidized, brownish, medium sand with natural adsorbents like Fe- and Al-oxides; they contain 0.76 μg/g As and impart low As concentrations to the water (4 μg/L). These observations were supported by spectroscopic studies with SEM, TEM, XRD and XRF, and by adsorption, leaching, column tests and sequential extraction. A relatively high in-situ dissolution rate (R_r) of 1.42 × 10⁻¹⁶ mol/m²/s was derived for the shallower aquifer from the inverse mass-balance model. The high R_r may enhance As release processes in the upper sediment. The field-based reaction rate (K_r) was extrapolated to be roughly 1.23 × 10⁻¹³ s⁻¹ and 6.24 × 10⁻¹⁴ s⁻¹ for the shallower and deeper aquifer, respectively, from the laboratory-obtained adsorption/desorption data. This implies that As is more reactive in the shallower aquifer. The partition coefficient for the distribution of As at the sediment-water interface (K_d-As) was found to range from 5 to 235 L/kg based on in-situ, batch adsorption, and flow-through column techniques. Additionally, a parametric equation for K_d-As (R² = 0.67) was obtained from the groundwater pH and the logarithm of the leachable Fe and Al concentrations in sediment. A one-dimensional finite-difference numerical model incorporating K_d and K_r showed that the shallow, leached As can be immobilized and prevented from reaching the deeper aquifer (∼150 m) after 100 year by a natural filter of oxidizing sand and adsorbent minerals like Fe and Al oxides; in this scenario, 99% of the As in groundwater is reduced. The deeper aquifer appears to be an adequate source of sustainable, safe water.