A new scheme of nonlinear energy transfer among wind waves: RIAM method-algorithm and performance-

A numerical scheme for calculating the nonlinear energy transfer among wind waves (RIAM method) was developed on the basis of the rigorous method of Masuda. Then the performance of the RIAM method was examined by applying it to various forms of wind-wave spectra and different situations of wind-wave evolution, in comparison mainly with the WAM method. The computational time of the Masuda method was reduced by a factor of 300 by the RIAM method, which is still 2000 times slower than the WAM method simply because the RIAM method processes thousands of resonance configurations whereas the WAM method does only one. The RIAM method proves to give accurate results even for spectra of narrow band widths or bimodal spectra, whereas the WAM method often calculates an unrealistic magnitude and pattern of nonlinear energy transfer functions. In the duration-limited evolution of wind-wave spectra, the RIAM method yields a unimodal directional distribution on the low-frequency side of the spectral peak, whereas the WAM method produces a spurious bimodal one there. At higher frequencies, however, both methods give a bimodal directional distribution with two oblique maxima. The RIAM method enhances the growth of the total energy and peak period of wind waves in comparison with the WAM method. Nevertheless, Toba's constant of his 3/2-power law approaches almost the same standard value of 0.06 in both methods. For spectra of a narrow band width or for those perturbed by a small hump or depression, the RIAM method tends to recover the monotonic smoother form of spectrum whereas the WAM method often yields unrealistic humps or depressions.     

Interannual variability of North Pacific eastern subtropical mode water formation in the 1990s derived from a 4-dimensional variational ocean data assimilation experiment

The interannual variability of eastern subtropical mode water (ESTMW) formation in the North Pacific is examined using a new ocean dataset constructed by a 4-dimensional variational data assimilation experiment covering the decade of the 1990s. The volume of newly formed ESTMW varies due to interannual variability in the following three physical processes taking place in the surface layer: (1) convergence in the transport of surface saline water induced by Ekman flow in the vicinity of the formation region, (2) thermal stratification in the preconditioning phase in association with the insolation anomaly induced largely by low-level cloud coverage, and (3) wintertime surface cooling in the eastern subtropics. We find that, in addition to the surface forcing, the properties of both the ESTMW and the upper mixed-layer water are broadly controlled by the volume of the new ESTMW component, and that the variations in the upper mixed-layer water affect the properties of ESTMW formed in the following winter. Due to the combined effect of these processes, the ESTMW subducts down to subsurface layers with a wide range of σ_θ values lying between 24.8 and 25.4 and with significant interannual variation in water mass formation.     

达拉布特蛇绿岩带镁铁质岩的地球化学

达拉布特蛇绿岩带的镁铁质岩属亚碱性和碱性系列,可能是弧后盆地火册活动的产物。根据镁铁质岩的层位关系、岩浆来源和微量元素特征,可将该盆地的地质演化概括为扩张阶段和封闭阶段。盆地扩张阶段早、中期喷发的镁铁质岩具有低的稀土和不相容元素丰度,而在盆地封闭阶段晚期喷发的碱性玄武岩,其特点是稀土和大多数不相容元素丰度极大增高,其稀土元素分布模式为轻稀土元素强烈富集型。     

Rare earth element distribution in the Melrose-b howardite: Pre-terrestrial negative Ce anomaly

In order to investigate the genetic nature of the polymict breccia howardite Melrose-b, a whole-rock sample was separated into four fractions after pulverization and the rare earth element (REE) abundances were precisely determined by isotopic dilution in those fractions. Such an investigation shows that the three precipitated fine-particle fractions are depleted in Ce by the same factor (0.62 ～ 0.64) in spite of differences in mineralogy, while the corresponding factor of Ce depletion for the suspended-matter fraction is 0.462; the smaller value indicates a greater depletion in Ce.The weight fraction values of four fractions calculated from the REE abundances are in satisfactory agreement with the actual values. The concentration ratios between plagioclase and mainly pigeonite fractions accord with those observed in similar phases of the Juvinas eucrite [1]. Moreover, the bulk pattern synthesized from the three fine-particle fractions is similar in absolute level to that for the Zmenj howardite [1]. It is concluded that the negative Ce anomaly reflects the effect predating the igneous activity in the parent planetary body.     

Computer modelling of mantled porphyroclasts in Newtonian and non-Newtonian simple shear viscous flows

In computer simulations based on creeping motion equations, the difference in stress sensitivity of strain rate between Newtonian rheology and non-Newtonian power-law rheology does not greatly influence the types of mantled porphyroclast developed in simple shear viscous flows. Porphyroclast type is determined by the radius of an initial spherical mantle. When a thin mantle lies within the separatrix, (δ₂-objects develop at small γ, then complex porphyroclasts appear with increasing γ. When the mantle is thicker and transected by the separatrix, φ-objects form at small γ and δ₂-objects appear with increasing γ during the same deformation. The transition from φ- to δ₂-objects is gradual, the transition strain being dependent on the initial radius of the mantle. Our simulations did not produce θ-, δ₁- or σ-objects.     

Transition of solar cycle length in association with the occurrence of grand solar minima indicated by radiocarbon content in tree-rings

In this paper, we review the variation of the 11-year solar cycle since the 15th century revealed by the measurement of radiocarbon content in single-year tree-rings of Japanese cedar trees. Measurements of radiocarbon content in absolutely dated tree-rings provide a calibration curve for accurate dating of archaeological matters, but at the same time, enable us to examine the variations of solar magnetic activity in the pre-historical period. The Sun holds several long-term quasi-cyclic variations in addition to the fundamental 11-year sunspot activity cycle and the 22-year polarity reversal cycle, and it is speculated that the property of the 11-year and the 22-year solar cycle varies in association with such long-term quasi-cycles. It is essential to reveal the details of solar variations around the transition time of solar dynamo for illuminating the mechanisms of the long-term solar variations. We therefore have investigated the property of the 11-year and 22-year cycles around the two grand solar minima; the Maunder Minimum (1645–1715 AD) and the Spoerer Minimum (1415–1534 AD), the periods of prolonged sunspot minima. As a result, slight stretching of the “11-year” and the “22-year” solar cycles was found during these two grand solar activity minima; continuously during the Maunder Minimum and only intermittently during the Spoerer Minimum. On the contrary, normal or slightly shortened 11-year cycles were detected during the interval period of these two minima. It suggests the inverse correlation between the solar cycle length and solar magnetic activity level, and also the change of meridional flow during the grand solar activity minima. Further measurements for the beginning of the grand solar minima will provide a clue to the occurrence of such prolonged sunspot disappearance. We also discuss the effect of solar variations to radiocarbon dating.     

Microstructural development of fine-grained quartz aggregates by syntectonic recrystallization

Experimental study of syntectonic recrystallization of fine-grained quartz aggregates was carried out in order to simulate the development of some natural microstructures of quartz tectonites and to understand their formation condition. Agate was axially compressed with a constant-strain-rate apparatus. Experiments were conducted at 4 kbar solid confining pressure, 700–1000°C and 10⁻⁴-10⁻⁶ sec⁻¹ to 10%–45% strain. In all runs, deformation has proceeded under wet condition caused by dehydration of pyrophyllite used as pressure medium.Two different types of microstructure were distinguished in the deformed specimens. One is P-type which is characterized by equant, equidimensional, and polygonal grains. The other is S-type which is characterized by the highly oblate grains with the largest dimension perpendicular to the compression axis. The P-type microstructure is developed at higher temperatures and slower strain rates, while the S-type developed at lower temperatures and faster strain rates. The transition between the S- and P-types is found to be very sharp.     

Experimental determination of partition coefficients of ten rare earth elements and barium between clinopyroxene and liquid in the synthetic silicate system at 20 kilobar pressure

Two experiments were carried out at 20 kb pressure for the measurement of partition coefficients of ten lanthanides and barium between diopside clinopyroxene and liquid in the synthetic system diopside-enstatite-silica-H₂O. Starting materials are glass of two different compositions CaMgSi₂O₆ 56, MgSiO₃ 14, SiO₂ 30 weight % and CaMgSi₂O₆ 80, MgSiO₃ 20 weight %.In the experiment on the former composition, the partition coefficient increases regularly with decrease of ionic radii from La to Dy and decreases also regularly from Dy to Lu. In the experiment on the latter composition, stepwise shift of partition coefficient was observed between Dy and Er.The results suggest that the lanthanide cations occupy the site of 8-fold coordination in the clinopyroxene and that the partition coefficients depend on the ionic radii of lanthanide elements and also on the chemical composition of the starting material or heating process during the run.     

Arsenic release from biotite into a Holocene groundwater aquifer in Bangladesh

Continuous core sediments (to a depth of 90.1 m) taken at a transitional area of Holocene and Pleistocene deposits in Sonargaon, Bangladesh were characterized for their mineralogy and chemistry. Among the sediments of the lower part of the Holocene aquifer (depth: 18–29 m), where most domestic wells are installed, As is mostly fixed in biotite and organic phases. A positive correlation of As concentration with those of Al and Fe but not that of total organic C clearly suggests that biotite is a primary source of As. Although microbial reduction–dissolution of As-containing Fe oxyhydroxides is thought to cause As-enriched groundwater in the Ganges–Brahmaputra–Meghna delta plain, the authors conclude that chemical weathering of biotite is the primary formation mechanism and prevailing reducing conditions contribute to the expansion of As-enriched groundwater in the study area.