Temporal change of clustered distribution of planktonic ciliates in Toyama Bay in summers of 1989 and 1990

Temporal change of clustered distribution in vertical profiles of three nutritional groups of planktonic ciliates, e.g. heterotrophic naked ciliates, mixotrophic naked ciliates and heterotrophic loricated ciliates, was investigated by following a drifting buoy in Toyama Bay on the Japan Sea coast of central Japan in summers of 1989 and 1990. Clustered distribution, represented as the mode of population density in the vertical plane, occurred mainly in the oligotrophic upper layer (0–50 m depth) above the subsurface chlorophyll-a maximum layer. Its clustered degree was stronger when the mode of population density in the vertical plane was formed at shallower depth, while its longevity was shorter as mentioned above. Vertical distribution of ciliates during summer in Toyama Bay is characterized by ephemeral clustered distribution, or in other wards, by rapid alternations of appearance and disappearance of the clustered distribution.     

A new occurrence of retrogressed eclogite from the Sanbagawa belt of southwest Japan and its significance

The present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm^?1, and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P2/n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω₁ > 8.5 cm^?1, corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature (P–T) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone.     

Fluid Inclusion Study and Opaque Mineral Assemblage at the Deep and Shallow Part of the Batu Hijau Porphyry Copper-gold Deposit, Sumbawa, Indonesia

The Batu Hijau deposit is the only porphyry type deposit in production in the Sunda‐Banda arc, Indonesia. This study discusses the reason for the localization of copper grade at the deep part of the deposit based on the observation of opaque mineral assemblage. In addition, the formation condition of quartz veins and opaque minerals is discussed on the basis of the fluid inclusion microthermometry. Samples were selected from drill holes SBD100, SBD168, SBD194, SBD254, and SBD257 to cover the wide vertical range. At the Batu Hijau deposit, quartz veins have been classified mainly into four types called A, B, C and D veins, and the A veins contain mainly bornite, often associated with digenite and chalcocite. In addition, magnetite occurs in A veins. However, at the deep part of the deposit, there are quartz veins associated with magnetite, but few copper sulfides such as bornite and chalcopyrite in quartz veins, as observed in SBD257. Quartz veins at depth in SBD257 have abundant magnetite and pyrite. Pyrite in quartz veins at depth in SBD257 mainly occur at the rim of magnetite grains or interstices between them. In quartz veins in SBD254, there are abundant copper sulfides such as bornite and chalcopyrite in spite of the depth. Bornite and chalcopyrite occur as inclusions in magnetite grains in quartz veins in SBD254. Pyrite which often occurs in low grade zone in quartz veins in SBD254 is also recognized at the rims of copper sulfides. This indicates that pyrite in SBD257 and SBD254 formed later than magnetite. On the other hand, blebs of bornite and chalcopyrite inclusions in magnetite grains, which are recognized in quartz veins in SBD168 at shallow high grade part, suggest that the hydrothermal fluid, from which magnetite was deposited also brought the copper sulfides such as bornite and chalcopyrite to the deep part of the Batu Hijau deposit. Therefore, it is concluded that initially the high grade ore zone extended to depth without localization. However due to the later overprinting hydrothermal activity, copper sulfides and magnetite were replaced or dissolved and pyrite was formed, resulting the low grade zone at the deep part of the deposit. Dissolution temperatures (Td) of halite obtained by from fluid inclusion microthermometry show significant differences between SBD168 and other drill holes. The high Td obtained in SBD168 may indicate larger volume of NaCl crystals in hydrothermal fluid at the time of entrapment of the fluid inclusions and formation of other opaque minerals such as magnetite and copper‐iron sulfides. It suggests that the ratio of vapor to brine is also higher at the shallow part of the deposit. The higher vapor to brine ratio may suggest a higher degree of boiling. Removal of vapor phase separated from brine during boiling increases the concentration of substances dissolved in the brine, and this will result in saturation, as evidenced by the salinity and NaCl saturation. The higher degree of boiling suggested by the higher vapor to brine ratio at shallow part may have increased the copper concentration in the brine that may have lead the saturation, resulted in the deposition of copper‐bearing minerals.     

Correction to: El Niño-related sea surface elevation and ocean bottom pressure enhancement associated with the retreat of the Oyashio southeast of Hokkaido,Japan

Marine Geophysical Research - In the original publication, the Fig. 2 was published incorrectly. The correct version (Fig. 2) is given in this correction. The original article has been...     

Particulate peptidoglycan in seawater determined by the silkworm larvae plasma (SLP) assay

Peptidoglycan (PG) is a biopolymer found exclusively in the cell wall of bacteria. Recent chemical analysis of particulate organic matter suggests that a major amount of the muramic acid, an amino sugar present only in PG, could not be accounted for in terms of bacterial cells (Benner and Kaiser, 2003); however, data on particulate PG is quite sparse. In the present study, conducted in 1996, the PG was examined at 5 sampling sites in the northwestern Pacific Ocean, and in natural seawater cultures. Particulate PG, which was concentrated using a 96-well filtration plate equipped with Durapore filters (pore size, 0.22 μm), was measured by the silkworm larvae plasma (SLP) assay. The PG concentration generally decreased with depth and correlated significantly with bacterial abundance throughout the entire water column. However, the ratio of particulate PG to bacterial abundance varied with depth. The average ratio was 0.61 ± 0.53 (average ± SD, n = 40) between 50 and 2000 m, which agreed with the bacterial cellular PG content from 0.63 to 1.1 fg cell⁻¹ obtained in seawater cultures. On the other hand, the ratios of PG to bacteria from the surface to 50 m (3.7 ± 2.6, n = 29) and below 2,000 m (2.1 ± 1.7, n = 7) were significantly higher than that between 50 and 2,000 m. These results may suggest that, in the surface and deep layers, a significant fraction of particulate PG was present in bacterial detritus, whereas this fraction was reduced in the middle layer.     

Dissecting large earthquakes in Japan: Role of arc magma and fluids

We synthesized information from recent high-resolution tomographic studies of large crustal earthquakes which occurred in the Japanese Islands during 1995–2008. Prominent anomalies of low-velocity and high Poisson's ratio are revealed in the crust and uppermost mantle beneath the mainshock hypocenters, which may reflect arc magma and fluids that are produced by a combination of subducting slab dehydration and corner flow in the mantle wedge. Distribution of 164 crustal earthquakes ( M 5.7–8.0) that occurred in Japan during 1885–2008 also shows a correlation with the distribution of low-velocity zones in the crust and uppermost mantle. A qualitative model is proposed to explain the geophysical observations recorded so far in Japan. We consider that the nucleation of a large earthquake is not entirely a mechanical process, but is closely related to the subduction dynamics and physical and chemical properties of materials in the crust and upper mantle; in particular, the arc magma and fluids.     

Jurassic plume-origin ophiolites in Japan: accreted fragments of oceanic plateaus

The Mikabu and Sorachi–Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geochemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to Fo_93.5, and their Fo–NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature (T _p) up to 1,650 °C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi–Yezo belts can be divided into the same types as the Mikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T _p mantle. Evidence for the high T _p mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that the Mikabu and Sorachi–Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E- and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary magma generated from Fe-rich peridotite mantle source, and the HE-type magma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed later and in lower temperatures than the E- and D-type rocks. The Mikabu and Sorachi Plateaus were formed in a low-latitude region of the Late Jurassic Pacific Ocean possibly near a subduction zone, partially experienced high P/T metamorphism during subduction, and then uplifted in association with (or without, in case of Mikabu) the supra-subduction zone ophiolite. The Mikabu and Sorachi Plateaus may be the Late Jurassic oceanic LIPs that could have been formed in brotherhood with the Shatsky Rise.     

Sea navigation, challenges and potentials in South East Asia: an assessment of suitable sites for a shipping canal in the South Thai Isthmus

This paper discusses the situation of sea navigation in south East Asia focusing on the Strait of Malacca. The strait links the Indian and Pacific oceans, which is considered one of the busiest in several narrow channels around the world. The paper highlights the significance of the strait to global maritime trade, volume of traffic, and rising environmental and social consequences in the strait. A feasibility study of constructing a new shipping canal in the South Thai Kra Isthmus as an alternative option of Malacca route had been studied since 19th century. The paper explores suitable sites for a potential shipping canal in the Kra Isthmus using physiographic spatial data i.e., elevation, sea charts, geology, soils and river systems. Each spatial data was considered as a separate decision variable for site evaluation. Separate evaluation criterions were prepared for each variable based on shipping canal requirements. Overlaying the maps in GIS environment, the variables were carefully evaluated, and five geographic sites for the canal were derived. The length of the shipping canal over sea and land was computed for each site. Site B located in south of Ranong and Chumphon provinces, was the shortest one, whereas site C in Surat Thani, Pangnna and Krabi provinces was the longest. However, each site consisted of benefits and constraints.     

Investigation of local time dependence of Mercury's sodium exosphere based on a numerical simulation

Mercury has a surface-bounded exosphere (SBE) similar to that of the Moon. One of the atmospheric species, sodium, was found by ground-based observations to be the most prominent component. Mercury's sodium SBE is known to be non-uniform with respect to local time (LT) in low-latitude regions: the sodium column density in the dawn-side region is larger than that in the dusk-side region, and the sodium abundance is the largest in the morning-noon region. To reveal the production processes for the exosphere near Mercury's surface, the LT dependence of the exosphere was investigated through a numerical simulation. Three data sets of sodium column densities observed for the dawn-side hemisphere, observed by Sprague et al. [1997. Distribution and abundance of sodium in Mercury's atmosphere, 1985-1988. Icarus 12, 506-527], were compared with results simulated by a 3D Monte Carlo method, and the source rates and density of sodium of the planetary surface were estimated. In the simulation, the photon-stimulated desorption (PSD) and thermal desorption (TD) processes were assumed as the release mechanisms. The sodium source rates for the three data sets, at respective heliocentric distances of about 0.33, 0.42, and 0.44 AU, were estimated as 1-4×10⁸ Na/cm²/s with weak LT dependence. In contrast, the expected sodium surface density showed clear dependence on LT and the heliocentric distance. The sodium surface density decreases from early morning to noon by a few orders, and, particularly for large heliocentric distances, the surface is in a condition of sodium excess and depletion with respect to the surface sodium density assumed by Killen et al. [2004. Source rates and ion recycling rates for Na and K in Mercury's atmosphere. Icarus 171, 1-19] in the early morning and morning-noon regions, respectively. This study implies that the decrease in sodium surface density from the early morning to noon regions might produce the characteristic LT dependence in the low-latitude dawn-side region.