Clinoenstatite in volcanic rocks from the Bonin Islands

Multiply-twinned clinoenstatite has been found in andesitic rocks from Chichi-jima and Mukojima in the Bonin Islands.The clinoenstatite occurs as (1) reaction rims around olivine, (2) composite crystals with bronzite, and (3) anhedral phenocrysts rimmed or included by bronzite.The clinoenstatite is chemically characterized by low contents of Ca (less than 0.5 wt.% oxide), Fs (9.2–11.5) and Al (less than 0.6 wt.% oxide) relative to the coexisting bronzite. The partition coefficient, K _{D op} ^cp =(Mg/Fe²⁺)^cp/(Mg/Fe²⁺)^op between coexisting clinoenstatite and bronzite, ranges from 1.02 to 1.32 (average 1.17). The Fe²⁺/Mg+Fe²⁺ ratios of coexisting clinoenstatite and bronzite are different in different rock types, which suggests variable inversion temperatures of protoenstatite to orthopyroxene in boninite.Bronzite phenocrysts in a specimen, ranging from Fs15 to Fs30, coexist with augite phenocrysts, whereas those in the other specimens, ranging from Fs12 to Fs18, do not coexist with augite phenocrysts. These differences in petrographical nature and mineral chemistry among the specimens examined may be due to variations in their quenching stages.The boninite clinoenstatite and bronzite are relatively rich in Ca and Mg, compared with the Papuan and Mariana pyroxenes, which seems to depend upon the rock chemistry.     

Swell-dominant surface waves observed by a moored buoy with a GPS wave sensor in Otsuchi Bay,a ria in Sanriku,Japan

Real-time monitoring of wind and surface waves in Otsuchi Bay, Iwate, Tohoku, Japan, commenced in October 2012, using a mooring buoy with an ultrasonic anemometer and a single-mode GPS wave sensor. Wind and wave data are distributed hourly in real time via the Internet along with a chart of their time series. We analyzed data monitored in the first 3 months in order to assess the variability and occurrence of wind and waves and to elucidate the main reasons for wave variation in Otsuchi Bay. The monitoring data revealed that surface waves in the bay were predominantly affected by swells propagated from the northeastern offshore region and that the wave height was significantly correlated with the component of wind velocity toward Otsuchi Bay in the northeastern offshore region that faces the bay mouth. The offshore wind field was expected to provide information useful for predicting coastal waves in a ria bay in Sanriku such as Otsuchi Bay. However, it should be emphasized that the horizontal distribution of the offshore wind field which has a significant effect on the surface waves in a ria bay depends heavily on the topographic shape of the bay.     

Temporal variations of the winter mixed layer south of the Kuroshio Extension

A monthly mean time series of the temperature profile in the recirculation gyre south of the Kuroshio Extension has been produced for the period 1971–2007 to examine temporal variations of the winter mixed layer. The winter mixed layer depth (MLD) shows both interannual and decadal variations and is significantly correlated with variation of the mean net surface heat flux in late autumn to early winter. There is also a close relation with the strength of pre-existing subsurface stratification, measured as vertical temperature gradients in the preceding summer. Linear multiple regression analysis shows that a significant fraction of the variations in the winter MLD is explained by the surface heat flux and the strength of the stratification. The contribution of the two factors is comparable.     

Trace-element characteristics of east–west mantle geochemical hemispheres

Recent statistical analyses on the isotopic compositions of oceanic, arc, and continental basalts have revealed that the Earth's mantle is broadly divided into eastern and western hemispheres. The present study aimed to characterize the isotopically defined east–west geochemical hemispheres using trace-element concentrations. Basalt data with Rb, Sr, Nd, Sm, Pb, Th, and U in addition to the isotopic ratios ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb were selected mostly from the GEOROC and PetDB databases. A total of 4787 samples were used to investigate the global geochemical variations. The results show that the wide trace-element variations are broadly explained by the melting of melt-metasomatized and fluid-metasomatized mantle sources. The larger amount of the fluid component derived from subducted plates in the eastern hemisphere than that in the western hemisphere is inferred from the basalts. These characteristics support the hypothesis that focused subduction towards the supercontinent created the mantle geochemical hemispheres.     

A multi‐geochronological study of the Hakusan volcano,central Japan

The Hakusan volcano, central Japan, is located in a region where two subducting plates (the Pacific Plate and the Philippine Sea Plate) overlap near the junction of four plates adjacent to the Japanese Islands (the Pacific Plate, the Philippine Sea Plate, the Eurasia Plate, and the North American Plate). The Hakusan volcano consists of products from four major volcanic episodes: Kagamuro, Ko‐hakusan, and Shin‐Hakusan I and II. To date the eruption events of the Hakusan volcano we applied thermoluminescence and fission track methods. ²³⁸U(²³⁴U)–²³⁰Th disequilibrium and ²⁰⁶Pb/²³⁸U methods were applied to date the zircon crystallization ages for estimating the magma residence time before the eruptions. The eruption ages we obtained are ca 250 ka for Kagamuro, ca 100 ka and ca 60 ka for Ko‐Hakusan, ca 50 ka for Shin‐Hakusan I, and <10 ka for Shin‐Hakusan II. They are concordant with previous reports based on K–Ar dating. Some of the pyroclastic rocks, possibly originating from Shin‐Hakusan II activities, are dated to be ca 36 ka or 50 ka, and belong to the Shin‐Hakusan I activity. The zircon crystallization ages show several clusters prior to eruption. The magma residence time was estimated for each volcanic activity by comparing the major crystallization events and eruption ages, and we found a gradual decrease from ca. 500 ky for the Kagamuro activity to ca. 5 ky for the Shin‐Hakusan II activity. This decrease in residence time may be responsible for the decrease in volume of erupted material estimated from the current topography of the region. The scale of volcanic activity, which was deduced from the number of crystallized zircons, is more or less constant throughout the Hakusan volcanic activity. Therefore, the decrease in magma residence time is most likely the result of stress field change.     

Wind- and density-driven circulation in a bay on the Sanriku ria coast,Japan: study of Shizugawa Bay facing the Pacific Ocean

Flow fields in Shizugawa Bay on the Sanriku ria coast, which faces the Pacific Ocean, were investigated using hydrographic observations for the purpose of understanding oceanographic conditions and the process of water exchanges in the bay after the 2011 earthquake off the Pacific coast of Tohoku. In spring to summer, density-driven surface outflow is part of estuarine circulation and is induced by a pressure gradient force under larger longitudinal gradients in density along with lower salinity water in the innermost part of the bay, regardless of wind forcing. In winter to summer, another density-driven current with a thermal structure is induced by a pressure gradient force under the smaller longitudinal density gradients in calm wind conditions. Particularly in winter, Tsugaru Warm Current water can be transported in the surface layer inside the bay. Wind-driven bay-scale circulation with downwind and upwind currents in the surface and deeper layers, respectively, is induced by strong longitudinal wind forcing under the smaller longitudinal density gradients, irrespective of season. Particularly in fall to spring, this circulation can cause the intrusions of oceanic water associated with Oyashio water and Tsugaru Warm Current water in the deeper layer. These results suggest that wind- and density-driven currents can produce the active exchange of water from inside and outside the bay throughout the year.     

Seasonal transition of hydrological processes in a slow‐moving landslide in a snowy region

A comprehensive understanding of seasonal hydrological dynamics is required to describe the influence of pore‐water pressure on the stability of landslides in snowy regions. This study reports on the results of continuous meteorological and hydrological observations over 2 years on a landslide body comprising Neogene sedimentary rocks in northern Japan, where a thick (3–5 m) seasonal snowpack covers the land surface. Monitoring of the volumetric water content in shallow unsaturated zones (<0.8 m depth) and pore‐water pressure in saturated bedrock at depths of 2.0 and 5.2 m revealed clear seasonality in hydrological responses to rainfall and meltwater supply. During snow‐free periods, both the shallow soil moisture and deep pore‐water pressure responded rapidly to intense rainwater infiltration. In contrast, during snowmelt, the deep pore pressure fluctuated in accordance with the daily cycle of meltwater input, without notable changes in shallow moisture conditions. During occasional foehn events that cause intense snow melting in midwinter, meltwater flows preferentially through the layered snowpack, converging to produce a localized water supply at the ground surface. This episodically triggers a significant rise in pore‐water pressure. The seasonal differences in hydrological responses were characterized by a set of newly proposed indices for the magnitude and quickness of increases in the pressure head near the sliding surface. Under snow‐covered conditions, the magnitude of the pressure increase tends to be suppressed, probably owing to a reduction in infiltration caused by a seasonal decrease in the permeability of surface soils, and effective pore‐water drainage through the highly conductive colluvial layer. Deep groundwater flow within bedrock remained in a steady upwelling state, enhanced by increasing moisture in shallow soils under snow cover, reflecting the convergence of subsurface water from surrounding hillslopes.