Incessant excitation of the Earth's free oscillations: global comparison of superconducting gravimeter records

Records of superconducting gravimeters (SGs) at Canberra (Australia), Esashi (Japan), Metsähovi (Finland) and Syowa Stations (Antarctica) were analyzed to search for further evidence of background free oscillations of the Earth. Spectrograms for 1-year period and averaged power spectra for seismically quiet periods were obtained for each of the stations. Anomalous features of the oscillations observed at Syowa Station, such as an apparent seasonal variation and a high intensity at frequencies between 3 and 4 mHz, were absent at the other SG stations. Among the SG stations used in this study, the background free oscillations were detected most consistently and distinctly at Canberra, where the noise level was comparable to that at the IDA quietest station, while that at Syowa Station was close to the critical limit for detecting the oscillations. The background free oscillations provide a good reference to evaluate the noise level in the milliHertz band.     

Rapid shoreline progradation followed by vertical foredune building at Pedro Beach,southeastern Australia

At Pedro Beach on the southeastern coast of Australia a series of foredune ridges provides an opportunity to explore the morphodynamic paradigm as it applies to coastal barrier systems using optically stimulated luminescence (OSL) dating, ground penetrating radar (GPR) and airborne LiDAR topography. A series of sandy dune-capped ridges, increasing in height seawards, formed from c. 7000 years ago to c. 3900 years ago. During this time the shoreline straightened as the embayment filled and accommodation space for Holocene sediments diminished. Calculation of Holocene sediment accumulation above mean sea level utilising airborne LiDAR topography shows a decline in average sediment supply over this time period coupled with a decrease in shoreline progradation rate from 1.2 m/yr to 0.38 m/yr. The average ridge ‘exposure lifetime’ during this period increases resulting in higher ridges as dune-forming processes have longer to operate. Increasing exposure to wave and wind energy also appears to have resulted in higher ridges as the sheltering effect of marginal headlands was diminished. An inherited disequilibrium shoreface profile will drive onshore accumulation of sandy sediments forming a prograded barrier; however, if there is no longer ‘accommodation space’ for sediment, this will be an overriding factor causing the cessation of progradation, as occurred c. 3900 years ago at Pedro Beach. Excess sediment in the nearshore zone after 3900 years ago may have been moved northward to nourish downdrift beaches in the compartment. A high outer foredune has formed through vertical accretion after 500 years ago, evidenced by GPR subsurface structures and OSL ages, with a distinct period of vertical and lee slope accretion and dated to the period 1890–1930 AD. The increased dune sediment transport resulting in foredune building is attributed to recent human disturbance. © 2018 John Wiley & Sons, Ltd.     

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.     

近岸波生流数值计算的一种新方法

运用Hardy-Cross法的基本原理建立了一种波生流计算的数值模型.本方法在维持计算稳定性方面表现出色,解决了以往的计算方法所存在的计算不稳定的根本缺陷,但是运用这一方法进行实际问题计算时还明显存在计算收敛缓慢的现象,对此,在探讨计算收敛原因的基础上,提出了同心矩形回路补正、粗网格计算初始值等对策.运用这些对策成功地提高了计算的效率和精度.最后通过计算例验证表明,运用本方法既能有效地维持计算的稳定,又能有效地解决局部缓和法所固有的计算效率低的问题,从而为研究和预测海岸工程对地形及环境变化影响问题提供了合理、实用的方法.     

Food sources of the pearl oyster in coastal ecosystems of Japan: Evidence from diet and stable isotope analysis

We estimated the composition of two food sources for the cultured pearl oyster Pinctada fucata martensii using stable isotopes and stomach content analysis in the coastal areas of the Uwa Sea, Japan. The δ¹³C values of oysters (−17.5 to −16.8‰) were intermediate between that of particulate organic matter (POM, −20.2 to −19.1‰) and attached microalgae on pearl cages (−13.0‰). An isotope mixing model suggested that oysters were consuming 78% POM (mainly phytoplankton) and 22% attached microalgae. The attached microalgal composition of the stomach content showed a strong resemblance to the composition of that estimated through the isotope mixing model, suggesting preferential utilization of specific components is unlikely in this species. These results indicate that P. fucata martensii feed on a mixture of phytoplankton and attached microalgae, and that the attached microalgae on pearl cages can serve as an important additional food source.     

A near-infrared survey of Miras and the distance to the Galactic Centre

We report the results of a near-infrared survey for long-period variables in a field of view of 20× 30 arcmin² towards the Galactic Centre (GC). We have detected 1364 variables, of which 348 are identified with those reported in Glass et al. We present a catalogue and photometric measurements for the detected variables and discuss their nature. We also establish a method for the simultaneous estimation of distances and extinctions using the period–luminosity relations for the JHK _s bands. Our method is applicable to Miras with periods in the range 100–350 d and mean magnitudes available in two or more filter bands. While J band means are often unavailable for our objects because of the large extinction, we estimated distances and extinctions for 143 Miras whose H - and   K _s  -band mean magnitudes are obtained. We find that most are located at the same distance to within our accuracy. Assuming that the barycentre of these Miras corresponds to the GC, we estimate its distance modulus to be  14.58 ± 0.02 (stat.) ± 0.11 (syst.) mag  , corresponding to  8.24 ± 0.08 (stat.) ± 0.42 (syst.) kpc  . We have assumed the distance modulus to the Large Magellanic Cloud to be 18.45 mag, and the uncertainty in this quantity is included in the above systematic error. We also discuss the large and highly variable extinction. Its value ranges from 1.5 mag to larger than 4 mag in     except towards the thicker dark nebulae and it varies in a complicated way with the line of sight. We have identified mid-infrared counterparts in the Spitzer /IRAC catalogue of Ramírez et al. for most of our variables and find that they follow rather narrow period–luminosity relations in the 3.6–8.0 μm wavelength range.     

Turbulent Airflow and Wave-Induced Stress Over the Ocean

We examine the structure of turbulent airflow over ocean waves. Based on an analysis of wind and wave observations derived from a moored and floating Air–Sea Interaction Spar buoy during the Shoaling Waves Experiment field campaign, we show that the cospectra of momentum flux for wind–sea conditions follow established universal scaling laws. Under swell-dominant conditions, the wave boundary layer is extended and the universal cospectral scaling breaks down, as demonstrated previously. On the other hand, the use of peak wave frequency to reproduce the universal cospectra successfully explains the structure of the turbulent flow field. We quantify the wave-coherent component of the airflow and this clarifies how ocean waves affect momentum transfer through the wave boundary layer. In fact, the estimated wave-induced stresses for swell-dominant conditions explain the anomalous cospectral shapes observed near the peak wave frequency.     

Oxygen isotope study of metamorphic and granitic rocks of the Yanai district in the Ryoke belt,Japan

¹⁸O/¹⁶O ratios have been obtained for 134 whole-rocks and minerals from metamorphic and granitic rocks of the Yanai district in the Ryoke belt, Southwest Japan. The ¹⁸O/¹⁶O ratios of pelitic rocks of the marginal metamorphic zone decrease progressively with increasing metamorphic grade. In the gneiss-granite complex (zone of migmatite [1]), the most characteristic feature of the rocks is that oxygen isotopic homogenization proceeds on both local and regional scales in parallel with “granitization” or chemical homogenization. Granitic rocks of various origin are fairly uniform in isotopic composition with δ ¹⁸O of quartz of 12 to 14‰ (SMOW) and δ ¹⁸O of biotite of 7 to 9‰ and are about 3 to 4‰ enriched in ¹⁸O compared to other Cretaceous granites of non-metamorphic terranes in Japan. The high ¹⁸O/¹⁶O ratios of granitic rocks of this district were discussed in relation to the ¹⁸O-depletion in metasediments. Oxygen isotopic fractionations among coexisting minerals from various rock-types of the gneiss-granite complex indicate that these minerals were formed under near isotopic equilibrium at a temperature of about 600 to 700° C. Some abnormal fractionations of quartz-biotite pairs also were obtained for rocks which had undergone a progressive ¹⁸O-depletion or ¹⁸O-enrichment. This is due to high resistivity of quartz and contrastive susceptibility of biotite to isotopic exchange during metamorphism and “granitization”.     

Mantle hornblendites of Naein ophiolite (Central Iran): Evidence of deep high temperature hydrothermal metasomatism in an upper mantle section

The Naein ophiolite is the most complete ophiolitic exposure in Cental Iran and considered as a remnant of the Mesozoic Central East Iranian microcontinent (CEIM) confining oceanic crust. In the northeastern part of this ophiolite (Darreh Deh area) within the mantle peridotites, a few hundred meters below the top of the Moho transition zone (MTZ), the hornblendites are present as dykes (former cracks and joints) from a few millimeters to nearly 50 cm wide. They have sharp boundaries with the surrounding mantle harzburgites and dunites. These hornblendites are pale green and coarse-grained in hand specimen and composed of magnesio-hornblende (Mg# = 0.93), chlorite (penninite and clinochlore, Mg# = 0.95), Cr-spinel (chromite, Cr# = 0.67 and Mg# = 0.55), tremolite, calcite and dolomite. Tremolites were formed by retrograde metamorphism of hornblendes. Calcite and dolomite occur as late-stage veins. Very high amount of primary hydrous phases (~94 vol % hornblende and chlorite), as well as peculiar mineralogical and chemical characteristics of the Naein ophiolite mantle hornblendites, do not match a magmatic origin. They are possibly products of the reaction between mantle peridotites and seawater-originated supercritical fluids, rich in silicate components. The presence of primary hydrous phases (hornblende and chlorite) may reveal high activity of H₂O in the involved solution. The chemical composition of chromite in the hornblendites is near to the average chromite composition from the surrounding harzburgite and dunite. This suggests that the main source of Cr should be chromites of nearby peridotites, which were totally or partly dissolved by hydrothermal fluids. The positive anomaly of Eu in the chondrite-normalized REE patterns of hornblendes, high modal abundance of Ca-rich hornblende, as well as presence of calcite and dolomite, point to seawater ingression through the gabbros in to the uppermost mantle peridotites. The higher value of MgO than CaO, presence of high-Cr chromite and Cr-enrichment of hornblendes and chlorites indicate a higher contribution of peridotites rather than gabbros to the chemical characteristics of the involved fluids. This study shows that circulation of possibly seawater-derived high temperature hydrous fluids in the upper mantle can leach and provide necessary elements to form hornblendite in joints and cracks of the uppermost mantle.