Vertical diffusion in coastal areas—I

The suspended matter in seawater near the bottom is distributed by the effects of sinking and diffusion. The author developed a method of estimation of the vertical eddy diffusivity near the sea bottom and the particle size distribution of bottom sediment, in the case of equilibrium state. This estimation was made by the comparison of measured and computed vertical distribution of beam attenuation. The parameters which were used for the computation were (1) median of the particle size distribution of bottom sediment, assuming that the particle size was a log-normal distribution, and (2) the proportional constant of vertical eddy diffusivity which was proportional to the height from the bottom.As the suspended matter in seawater contains particles of different sizes, the computation of the diffusion and beam attenuation was made for each particle size, and summarizing the results, the vertical distribution of beam attenuation coefficient was computed.In order to estimate the beam attenuation in high particle concentration, an equation by which the effect of overlapping of particle shadow in the light beam was eliminated, was used.     

A Generation Mechanism for Mesoscale Eddies in the Kuril Basin of the Okhotsk Sea: Baroclinic Instability Caused by Enhanced Tidal Mixing

Mesoscale eddies, particularly anticyclonic ones, are dominant features in the Kuril Basin of the Okhotsk Sea. In 1999, both surface drifter and hydrographic observations caught the same anticyclonic eddy northwest of Bussol’ Strait, which has a diameter of ∼100 km, typical surface velocity of 0.2–0.3 m s⁻¹, and less dense core extending to a depth of ∼1200 m. Based on an idea that the generation of mesoscale eddies is caused by strong tidal mixing in and around Kuril Straits, we have conducted a series of three-dimensional numerical model experiments, in which strong tidal mixing is simply parameterized by increasing coefficients of vertical eddy viscosity and diffusivity along the eastern boundary. Initially, a regular series of disturbances with a wavelength of ∼70 km starts to develop. The disturbances can be clearly explained by a linear instability theory and regarded as the baroclinic instability associated with the near-surface front formed in the region between the enhanced mixing and offshore regions. In the mature phase, the disturbances grow large enough that some eddies pinch off and advect offshore (westward), with the scale of disturbances increasing gradually. Typical eddy scale and its westward propagation speed are ∼100 km and ∼0.6 km day⁻¹, respectively, which are consistent with the observations by satellites. The westward propagation can be explained partly due to nonlinear effect of self-offshore advection and partly due to the β-effect. With the inclusion of the upper ocean restoring, the dominance of anticyclonic eddy, extending from surface to a depth of ∼1200 m, can be reproduced.     

A new expression for the production rate of sea water droplets on the sea surface

A new set of empirical formulas for the production rate and the number concentration of sea-water droplets on the sea surface are proposed, synthesizing past observation data of sea-salt particles in the sea and water droplets in wind-wave tanks. A new levelz _c is introduced as the effective wind-sea surface where seawater droplets are produced. The new formulas are expressed in linear functions in logarithmic scales ofu*²/v _p , a parameter to describe overall conditions of airsea boundary processes, whereu _* is the friction velocity of air,v the kinematic viscosity of air and _p the peak angular frequency of wind-wave part of wave spectra. A model of coexistence of spray droplets and suspended particles near the sea surface is proposed. As for the independent parameter, a comparison between the uses ofu*²/v _p and ofu _* ³ which was the traditional way of parameterization excluding wave measure, shows that the advantage of usingu*²/v _p is statistically significant with a confidence limit 89% in F-test.     

Underplating of mélange evidenced by the depositional ages: U–Pb dating of zircons from the Shimanto accretionary complex, southwest Japan

Abstract   Growth of an accretionary prism is effected by frontal accretion and deep subsurface underplating at the base of the prism. A systematic oceanward and structurally downward younging of underplated sequences is expected as the prism thickens and grows. To test this hypothesis and explore the processes of underplating, the U–Pb ages of zircon grains contained in underplated mélange sequences or packages of the Late Cretaceous and early Paleogene accretionary complex of the Shimanto Belt, southwest Japan, were determined using LA–ICP–MS laser technology. The results document systematic but intermittent younging ages within a single underplated mélange package. This finding suggests that underplating took place episodically during a period of several million years and that between episodes of underplating, a large amount of sediment was subducted to depths much greater than where underplating was occurring.     

Recognizing cryptic environmental changes by using paleoecology and taphonomy of Pleistocene bivalve assemblages in the Oga Peninsula,northern Japan

Multivariate analyses applied to Pleistocene bivalve assemblages from the Oga Peninsula (northern Japan) discriminate three distinct assemblages. The assemblages and their taphonomy were used to recognize environmental settings and changes. The Astarte–Cyclocardia–Glycymeris assemblage indicates shelf environment (below the storm wave base) where gravels and shells are transported from shallower settings. Supply of the exotic coarse sediment probably enabled epifaunal bivalves to inhabit the sea floor. The Glycymeris assemblage is characterized by dominance of G. yessoensis and represents current-swept shoreface environment (above the storm wave base). The Moerella assemblage is characterized by bivalves inhabiting bay to open-marine conditions and diverse deposit-feeders, indicating a moderately land-locked environment, such as an open bay or a bay mouth. Fine-grained substrata rich in organic matters in the bay were probably suitable for the deposit-feeders. Ordination also shows the assemblages along two environmental gradients, a bathymetrical one and the other related to open-marine and bay conditions. The environmental changes are explained mainly by glacio-eustatic sea-level changes and alternation of coastal geomorphology caused by local crustal movements. This study also suggests that fossil assemblages can be a powerful tool to reconstruct environments and depositional dynamics even in intensely bioturbated sedimentary facies.     

Water content of primitive low-K tholeiitic basalt magma from Iwate Volcano,NE Japan arc: implications for differentiation mechanism of frontal-arc basalt magmas

The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78–85) correlates positively with the An content of coexisting plagioclase phenocrysts (An_85–92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4–5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa.     

Contents of hepatic and renal metallothioneins in Hyposarcus pardalis: for construction of biomarker for heavy metal contamination in environments

The pollution of water by heavy metals is one of the most serious problems in the developing countries, where watercourses play important roles in transport and economic activities. The aim of this study was to examine whether Hyposarcus pardalis, a fish species widespread in the freshwater environment in Indonesia, could be used as a biomarker for environmental pollution by metals. To this effect, the concentrations of metallothioneins and metals in the livers and kidneys of H. pardalis were measured. In addition, to clarify the relationship between metallothionein concentrations and metal exposure, the concentrations of metallothioneins and metals were determined in the liver and the kidney of fish exposed to 50 and 500 ppb Cu and 500 ppb Mn, compared with those kept in clean water. Sufficient concentrations of metallothionein were detected in fish captured from Lake Rawakalong located in an industrial area in the suburbs of Jakarta. The results of exposure experiments suggested that H. pardalis retained a history of pollution in its organs for a long duration, and the metals bound to metallothioneins in the liver and kidney could be replaced with Cu following exposure. In conclusion, the hepatic and renal metallothioneins in H. pardalis are a useful candidate biomarker for monitoring heavy metal contamination.     

Geochemistry and petrogenesis of volcanic rocks in the Kuril island arc

Newly obtained precise analytical data on trace elements and radiogenic Sr, Nd, and Pb isotopes testify to anomalous geochemical characteristics of mafic and intermediate Quaternary lavas in Paramushir (in the north of the Kuril arc), Kunashir and Iturup (in the south) islands, which are the largest three islands of the Kuril island arc. The high K and LREE concentrations in the volcanic products in Paramushir Island resulted from the southward expansion of the mantle thermal anomaly of the Kamchatka Peninsula and the involvement of melts related to the melting of oceanic sediments in magma generation. The depleted characteristics of the mafic volcanics are explained by the relatively young tectono-magmatic events during the opening of the Kuril backarc basin. The Kuril island-arc system developed on a heterogeneous basement. The northern islands are a continuation of the volcanic structures of southern Kamchatka, which were formed above an isotopically depleted and hot lithospheric mantle domain of composition close to that of the Pacific MORB type. The southern islands were produced above an isotopically enriched and cold lithospheric domain of the Indian-Ocean MORB type, which was modified in relation to relatively young backarc tectono-magmatic processes. Although issues related to the genesis of the transverse geochemical zoning were beyond the originally formulated scope of our research, the homogeneous enough isotopic composition of the rear-arc lavas in the absence of any mineralogical and geochemical lines of evidence of crustal contamination suggests an independent magmatic source.     

SHRIMP U–Pb zircon geochronology and Sr–Nd isotopic systematic of the Neoproterozoic Ghimbi-Nedjo mafic to intermediate intrusions of Western Ethiopia: a record of passive margin magmatism at 855 Ma?

The reworked Pre-Neoproterozoic and juvenile Neoproterozoic terrane of the Western Ethiopian Shield (WES) consists of three N–S trending terranes. These are the western migmatitic gneissic terrane, the central metavolcano sedimentary terrane (CVST) and the eastern migmatitic gneissic terrane. The eastern part of the CVST mostly consists of suture-related ultramafic-metasedimentary complexes, whereas metavolcanics predominate in the western part. Gabbroic to granitic intrusions frequently occur in the CVST and in adjacent areas. New zircon SHRIMP U–Pb ages for two gabbros and three diorites in the Ghimbi-Nedjo region of the WES indicate magmatic crystallization ages. Two pulses of magmatism, at 860–850 and 795–785 Ma, are documented with the former for the first time. The tholeiitic Kemashi diorite and Bikilal-Ghimbi gabbros have oceanic affinities and yield U/Pb zircon ages of 856.3 ± 9.8 and 846.0 ± 7.6 Ma, respectively. The calc-alkaline Gebeya Kemisa pyroxene diorite, and the Senbet Dura hornblende diorite plus the tholeiitic Wayu Meni gabbro, which collectively have arc-back arc characteristics are indistinguishable at ages of 794.3 ± 9.4, 787.7 ± 8.8 and 778.1 ± 6.3 Ma, respectively. Positive εNd (4.5–7.0) and low initial ⁸⁷Sr/⁸⁶Sr (0.7029 ± 0.0002) and a mean T _DM model age of 0.95 Ga for the Ghimbi-Nedjo region (mean T _DM model age of 0.95 Ga for the WES overall) indicate that the magmas were generated from juvenile Neoproterozoic depleted mantle sources, with no discernable involvement of pre-Neoproterozoic continental crust. The occurrence of gabbros and diorites with oceanic tholeiite affinities combined with the new ages suggests that the intrusions were emplaced in the earliest stages of the rifting of Rodinia. This event in the WES led to the development of a passive margin and associated plume-type magmatism at ~855 Ma. The two intrusive groups with differing magma chemistry and ages suggest that the earliest magmatism was tholeiitic and associated with the passive margin system followed by continental breakup to form the Mozambique Ocean. The combination of tholeiitic and calc-alkaline magmatism was related to arc and back-arc basin formation and later terrane accretion (~830–690 Ma).