Equivalent forcing depth in tropical oceans

The forcing efficiency for the first and the second baroclinic modes by the wind stress in tropical oceans has been discussed by calculating equivalent forcing depth from annual mean, seasonal, and pentadal density profiles of the observational data. In the annual mean field, the first mode is forced preferentially in the western Pacific and the Indian Ocean, whereas the second mode is more strongly excited in the Atlantic and the eastern Pacific. This difference is mostly due to the pycnocline depth; the second mode is more dominantly forced where the pycnocline depth is shallower. We also revealed large seasonal variations of the second mode's equivalent forcing depth in the western Indian Ocean. The first mode is more dominantly forced during boreal spring and fall in the western Indian Ocean, while the second mode becomes more dominantly forced during boreal summer and winter. Those are due to seasonal variations of both the zonal wind and the pycnocline depth. Moreover, we show that the excitation of the second mode in the western Pacific increases after the late 1970s, which is associated with the decreasing trend of the zonal pycnocline gradient. Revealing the variation of the equivalent forcing depth will be useful for understanding the oceanic response to winds in tropical oceans and the improvement in the predictability of air-sea coupled climate variability in the tropics.     

Assessing land-use and carbon stock in slash-and-burn ecosystems in tropical mountain of Laos based on time-series satellite images

In the tropical mountains of Southeast Asia, slash-and-burn (S/B) agriculture is a widely practiced and important food production system. The ecosystem carbon stock in this land-use is linked not only to the carbon exchange with the atmosphere but also with food and resource security. The objective of this study was to provide quantitative information on the land-use and ecosystem carbon stock in the region as well as to infer the impacts of alternative land-use and ecosystem management scenarios on the carbon sequestration potential at a regional scale. The study area was selected in a typical slash-and-burn region in the northern part of Laos. The chrono-sequential changes of land-use such as the relative areas of community age and cropping (C) + fallow (F) patterns were derived from the analysis of time-series satellite images. The chrono-sequential analysis showed that a consistent increase of S/B area during the past three decades and a rapid increase after 1990. Approximately 37% of the whole area was with the community age of 1–5 years, whereas 10% for 6–10 years in 2004. The ecosystem carbon stock at a regional scale was estimated by synthesizing the land-use patterns and semi-empirical carbon stock model derived from in situ measurements where the community age was used as a clue to the linkage. The ecosystem carbon stock in the region was strongly affected by the land-use patterns; the temporal average of carbon stock in 1C + 10F cycles, for example, was greater by 33 MgC ha⁻¹ compared to that in 1C + 2F land-use pattern. The amount of carbon lost from the regional ecosystems during 1990–2004 periods was estimated to be 42 MgC ha⁻¹. The study approach proved to be useful especially in such regions with low data-availability and accessibility. This study revealed the dynamic change of land-use and ecosystem carbon stock in the tropical mountain of Laos as affected by land-use. Results suggest the significant potential of carbon sequestration through changing land-use and ecosystem management scenarios. These quantitative estimates would be useful to better understand and manage the land-use and ecosystem carbon stock towards higher sustainability and food security in similar ecosystems.     

Accurate Isotopic and Concentration Analyses of Small Amounts of Pb Using Isotope Dilution Coupled with the Double Spike Technique

A new method has been developed for the simultaneous determination of Pb abundance and Pb isotopic composition with high precision and accuracy for small test portion masses by thermal ionisation mass spectrometry. In this method, a ²⁰⁵pb-²⁰⁴pb double spike is added to samples prior to the chemical separation of Pb, and the isotopic composition of the spike-sample mixture is determined rigorously by the double spike technique using a ²⁰⁷Pb-²⁰⁴Pb spike. The isotopic composition and concentration of Pb in the sample are then obtained by utilising the principle of isotope dilution. Using this technique, replicate determinations of Pb from NIST SRM 981 and GSJ JP-1 (peridotite; 0.07 μg g⁻¹ Pb) were performed. The measured concentration and isotopic data were identical, within uncertainty, to published data or to data that were determined independently in this study. The application of this method to U-Pb dating and the determination of the "initial" Pb isotopic composition was also tested. Lead isotopic compositions and the concentrations of Pb, Th and U were determined for a single batch of samples, through the addition of ²⁰⁵pb-²⁰⁴pb, ²³⁰Th and ²³⁵U spikes to samples prior to chemical separation. Also in these experiments, we confirmed that this routine gives accurate data for Pb, Th and U concentrations and Pb isotopic compositions.     

A preparation zone for volcanic explosions beneath Naka-dake crater,Aso volcano,as inferred from magnetotelluric surveys

The 1st crater of Naka-dake, Aso volcano, is one of the most active craters in Japan, and known to have a characteristic cycle of activity that consists of the formation of a crater lake, drying-up of the lake water, and finally a Strombolian-type eruption. Recent observations indicate an increase in eruptive activity including a decrease in the level of the lake water, mud eruptions, and red hot glows on the crater wall. Temporal variations in the geomagnetic field observed around the craters of Naka-dake also indicate that thermal demagnetization of the subsurface rocks has been occurring in shallow subsurface areas around the 1st crater. Volcanic explosions act to release the energy transferred from magma or volcanic fluids. Measurement of the subsurface electrical resistivity is a promising method in investigating the shallow structure of the volcanic edifices, where energy from various sources accumulates, and in investigating the behaviors of magma and volcanic fluids. We carried out audio-frequency magnetotelluric surveys around the craters of Naka-dake in 2004 and 2005 to determine the detailed electrical structure down to a depth of around 1 km. The main objective of this study is to identify the specific subsurface structure that acts to store energy as a preparation zone for volcanic eruption. Two-dimensional inversions were applied to four profiles across the craters, revealing a strongly conductive zone at several hundred meters depth beneath the 1st crater and surrounding area. In contrast, we found no such remarkable conductor at shallow depths beneath the 4th crater, which has been inactive for 70 years, finding instead a relatively resistive body. The distribution of the rotational invariant of the magnetotelluric impedance tensor is consistent with the inversion results. This unusual shallow structure probably reflects the existence of a supply path of high-temperature volcanic gases to the crater bottom. We propose that the upper part of the conductor identified beneath the 1st crater is mainly composed of hydrothermally altered zone that acts both as a cap to upwelling fluids supplied from deep-level magma and as a floor to infiltrating fluid from the crater lake. The relatively resistive body found beneath the 4th crater represents consolidated magma. These results suggest that the shallow conductor beneath the active crater is closely related to a component of the mechanism that controls volcanic activity within Naka-dake.     

Ground temperature change observed at Unzen Volcano associated with the 1990–1995 eruption

Ground temperature associated with eruptive activity of Unzen Volcano, southwestern Japan, was observed in a cave about 680 m west of the central vent. Based on the data during 1991–1996, eliminating the effects of seasonal change, the temperature rose to the highest level probably around the first half of 1992. Hereafter the temperature decreased gradually year by year at least up to the autumn of 1996, beyond the level recovered at the beginning of the lava extrusion in May 1991. Although it is not necessarily conclusive, the present observation suggests a possible rise in ground temperature prior to the commencement of the first phreatic eruption in November 1990. If so, a careful observation may help to predict future eruptions by detecting a precursory rise of the ground temperature in this volcano.     

Adjustable vertical vibration isolator with a variable ellipse curve mechanism

This paper presents a passive vertical quasi‐zero‐stiffness vibration isolator intended for relatively small objects. The present isolator has features of compactness, long stroke, and adjustability to various load capabilities. To realize these features, we use constant‐force springs, which sustain constant load regardless of their elongation, and propose a variable ellipse curve mechanism that is inspired by the principle of ellipsographs. The variable ellipse curve mechanism can convert the restoring force of the horizontally placed constant‐force springs to the vertical restoring force of the vibration isolator. At the same time as converting the direction, the vertical restoring force can be adjusted by changing the ratio of the semi‐minor axis to the semi‐major one of the ellipse. In this study, a prototype of a class of quasi‐zero‐stiffness vibration isolator with the proposed variable ellipse curve mechanism is created. Shaking table tests are performed to demonstrate the efficacy of the present mechanism, where the prototype is subjected to various sinusoidal and earthquake ground motions. It is demonstrated through the shaking table tests that the prototype can reduce the response acceleration within the same specified tolerance even when the mass of the vibration isolated object is changed. Copyright © 2017 John Wiley & Sons, Ltd.     

High-Magnesian Andesite Produced by Two-Stage Magma Mixing: a Case Study from Hachimantai, Northern Honshu, Japan

High-magnesian andesite occurs at Hachimantai, northern Honshu,Japan. Disequilibrium zoning features indicate that the phenocrystminerals were derived from three different magmas. Chemicalcompositions and zoning profiles are accounted for by two-stagemagma mixing: the first mixing occurred between a crystal-freebasalt magma and a more differentiated olivine basalt magma;the second stage occurred by mixing between the resultant ofthe first-stage mixing and a hypersthene–augite andesitemagma. Mass balance of phenocryst crystals shows that end-membercompositions were c. 52·0 wt % SiO₂ and 10·1 wt% MgO for the mafic end-member and 57·0 wt % SiO₂ forthe felsic end-member of the second-stage mixing. Phenocrystminerals of the first-stage mixing end-member indicate the similarityof the end-member composition to that of basalts from nearbyvolcanoes. The counterpart aphyric magma in the first-stagemixing was more magnesian than the estimated mafic end-member.Calculations of the phase equilibria of similar basalts fromnearby volcanoes and comparison of results with previous phaseequilibrium experiments showed that the olivine basalt end-memberof the first stage was hydrous and situated at a depth wherethe pressure was less than 2 kbar. Two-pyroxene thermometryestimates are about 1050°C for the pyroxenes derived fromthe felsic end-member of the second-stage mixing, and about1180°C for groundmass pyroxenes. Crystallization temperaturesof 1170–1230°C are estimated for minerals from themafic end-member of the second-stage mixing based on phase equilibriumcalculations. These similar temperature estimates between thegroundmass and the mafic end-member imply achievement of thermalequilibrium between end-members preceding crystallization. Themagma plumbing system of the eastern Hachimantai is illustratedby a recent volcanic event, involving lateral dike intrusiontoward a pressure source. The encounter of a laterally migratingbasalt dike and an andesite magma chamber triggered the magmamixing that produced the high-magnesian andesite. The modelcan account for the relation between the petrological modeland surface distribution of volcanic rocks. The infrequencyof such mixing-derived high-magnesian andesite stems from therarity of high-magnesian basalt as a potential mixing end-memberin northern Honshu. KEY WORDS: high-magnesian andesite; Hachimantai; Northern Honshu; high-magnesian basalt; two-stage magma mixing