Exsolved garnet-bearing pyroxene megacrysts from some South African kimberlites

Clinopyroxene and orthopyroxene megacrysts containing garnet lamellae up to 1.2 mm thick as an exsolved phase are found rarely in kimberlites from Frank Smith and Bellsbank. Chemically the clinopyroxenes are characteristically subcalcic, being within the range of 100 Ca/Ca + Mg + Fe = 27 to 36, and the orthopyroxenes are characterized by high Al₂O₃ and Cr₂O₃. Immediately after crystallization during very slow cooling, clinopyroxene and orthopyroxene exsolve wide-spaced orthopyroxene and clinopyroxene phases parallel to (100) of the host phases, respectively, then both host and exsolved phases exsolve garnet lamellae. Topotactic relations between pyroxenes and garnet are determined by X-ray for the first time. Partitioning of major and minor elements among the coexisting clinopyroxene, orthopyroxene and garnet in pyroxene megacrysts is the same as that of the granular-type garnet peridotite xenoliths in Lesotho and South African kimberlies. Mineralogy and chemistry indicate that subcalcic clinopyroxene and orthopyroxene megacrysts contain respectively about 10 and 3 mole % of the garnet molecule in solid solution.     

Variations in seawater osmium isotope composition since the last glacial maximum: A case study from the Japan Sea

Concentrations of Re and Os, and the isotopic composition of Os have been measured in the Japan Sea sediments to assess the response of the Japan Sea to glacial–interglacial climate change and associated weathering fluxes. The osmium concentrations in the sediment samples analyzed vary from 59 to 371 pg/g, and ¹⁸⁷Os/¹⁸⁸Os from 0.935 to 1.042. Only ¹⁸⁷Os/¹⁸⁸Os of sediment samples from dark laminations deposited under suboxic to anoxic conditions and having elevated concentrations of Re and Os, and with ≥ 80% hydrogenous Os are explained in terms of seawater composition. Lower ¹⁸⁷Os/¹⁸⁸Os were observed for sediments deposited during the last glacial maximum (LGM) when planktonic foraminifera from the Japan Sea recorded lighter oxygen isotopic composition. Decrease in dissolved Os fluxes from continents and/or change in the composition of the dissolved load to the Japan Sea are suggested as the driving mechanisms for the observed lower LGM ¹⁸⁷Os/¹⁸⁸Os. The results of this study, coupled with lower ¹⁸⁷Os/¹⁸⁸Os during the last glacial observed at other sites from ocean basins with different lithology and contrasting sediment accumulation rates, suggest that this trend is characteristic of the global oceans.

Data from this study show that the Japan Sea recorded higher ¹⁸⁷Os/¹⁸⁸Os during the current interglacial coinciding with excursions of oxygen isotopic compositions of planktonic foraminifera to heavier values. This is explained in terms of preferential release of ¹⁸⁷Os during deglacial weathering and/or higher continental Os flux driven by warm and wet climate. This study demonstrates that Os isotopic composition of reducing margin sediments has immense potential to track variations in the seawater composition. In addition, ¹⁸⁷Os/¹⁸⁸Os of reducing sediments may be used to draw inferences about local paleoceanographic processes in semi-enclosed basins such as the Japan Sea.     

Electron microscopy of clinoenstatite from a boninite and a chondrite

Clinoenstatite crystals from a boninite and the Yamato-74191 chondrite have been studied with an analytical electron microscope. (100) twins and cracks perpendicular and parallel to the c axis are characteristic of their submicroscopic textures. The frequency in appearance along the c axis and widths of the cracks have been explained by the dimensional change of the c axis in the direct transformation of protoenstatite to clinoenstatite and by the cooling rate around the transformation temperature. The cracks in the crystals from the boninite are filled with fibrous crystals of talc, while those from the chondrite are open or filled with glass in which fine crystals of plagioclase are common.     

Axial and equatorial rotations of the Earth's cores associated with the Quaternary ice age

The differential axial and equatorial rotations of both cores associated with the Quaternary glacial cycles were evaluated based on a realistic earth model in density and elastic structures. The rheological model is composed of compressible Maxwell viscoelastic mantle, inviscid outer core and incompressible Maxwell viscoelastic inner core. The present study is, however, preliminary because I assume a rigid rotation for the fluid outer core. In models with no frictional torques at the boundaries of the outer core, the maximum magnitude of the predicted axial rotations of the outer and inner cores amounts to ∼2° year⁻¹ and ∼1° year⁻¹, respectively, but that for the secular equatorial rotations of both cores is ∼0.0001° at most. However, oscillating parts with a period of ∼225 years are predicted in the equatorial rotations for both cores. Then, I evaluated the differential rotations by adopting a time-dependent electromagnetic (EM) torque as a possible coupling mechanism at the core-mantle boundary (CMB) and inner core boundary (ICB). In a realistic radial magnetic field at the CMB estimated from surface magnetic field, the axial and equatorial rotations couple through frictional torques at the CMB, although these rotations decouple for dipole magnetic field model. The differential rotations were evaluated for conductivity models with a conductance of 10⁸ S of the lowermost mantle inferred from studies of nutation and precession of the Earth and decadal variations of length of day (LOD). The secular parts of equatorial rotations are less sensitive to these parameters, but the magnitude for the axial rotations is much smaller than for frictionless model. These models, however, produce oscillating parts in the equatorial rotations of both cores and also in the axial rotations of the whole Earth and outer and inner cores. These oscillations are sensitive to both the magnitude of radial magnetic field at the CMB and the conductivity structure. No sharp isolated spectral peaks are predicted for models with a thin conductive layer (∼200 m) at the bottom of the mantle. In models with a conductive layer of ∼100 km thickness, however, sharp spectral peaks are predicted at periods of ∼225 and ∼25 years for equatorial and axial rotations, respectively, although these depend on the strength of radial magnetic field at the CMB. While the present study is preliminary in modelling the fluid outer core and coupling mechanism at the CMB, the predicted axial rotations of the whole Earth may be important in explaining the observed LOD through interaction between the equatorial and axial rotations.     

Advanced Micro-XRF Method to Separate Sedimentary Rhythms and Event Layers in Sediments: Its Application to Lacustrine Sediment from Lake Suigetsu,Japan

Event-related sedimentary layers, which are deposited occasionally due to volcanic eruptions, earthquakes or heavy rains, are often contained in the rhythmical sequences of lacustrine and marine sediments. We have developed an analytical method for separating the sedimentary rhythms and the event layers identified using the scanning X-ray analytical microscope (SXAM) and obtained sequential profiles of seven elements Al, Si, K, Ca, Ti, Mn, and Fe in the lacustrine sediment from Lake Suigetsu, Japan. Two types of event layers could be detected from the elemental composition of 33 layers of sediment: three known volcanic ash layers and 30 clay layers containing 12 turbidites. The recurrence interval of the latter, which may potentially be initiated and archived by locally important earthquakes, is estimated to be an average of 640 ± 160 years by using Sompi event analysis (SEA) based on an autoregressive (AR) model. After removing those portions that represented event layers from the elemental profiles, we obtained event-removed (ER) temporal profiles based on the tephrochronology of the three volcanic ash layers. The ER temporal profiles of manganese and iron, probably representing the siderite content, showed a millennial-scale variation in the Holocene that corresponded well with ice-rafting events in the North Atlantic.     

The causes of forest cover loss in the hill forests in Bangladesh

The Hill Forests in Bangladesh have been depleted and degraded in volume, area, and quantity, thus requiring urgent forest protection by identifying the causes of forest loss. This study seeks to identify the fundamental causes of the shrinking forest cover in the Hill Forests in Bangladesh. The results indicate four sets of causes: (i) indigenous forest dwellers, having their own types of problems; (ii) migrants, who, because of problems in their places of origin, have decided to move to the forests; (iii) the timber industry, which, legal or not, are cutting too many trees; and (iv) the government through its Forest Department which is not able or willing to implement suitable policies to regulate the cutting trees and to prevent illegal cutting. Because it is a time consuming task to mitigate the first and second sets of factors, we recommend to involve forest dwellers in forestry practices as much as possible and taking necessary steps to alleviate the third and fourth sets and thereby reduce the rate of forest depletion. Accordingly, a number of strategies that should be adopted to halt the loss of remaining forest cover are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of the viscoelastic lithosphere on polar wander speed caused by the Late Pleistocene glacial cycles

Previous studies of the wander of the rotation pole associated with the Late Pleistocene glacial cycles indicate that the predicted polar wander speed is sensitive to the density jump at the 670 km discontinuity, the thickness of the elastic lithosphere, and the lower mantle viscosity. In particular, the M1 mode related to the density jump at 670 km depth has been shown to contribute a dominant portion of predicted polar wander speed for sufficiently small lower mantle viscosities. In this study, we examine the sensitivity of polar wander to variations in the viscosity of the viscoelastic lithosphere using simplified compressible Maxwell viscoelastic earth models. Model calculations for earth models with a viscoelastic lithosphere of finite viscosity indicate that the contribution of the M1 mode is similar to those associated with the density discontinuity at the core–mantle boundary (C0 mode) and the lithosphere (L0 mode). We speculate that this is due to the interaction between the M1 mode and the transient mode associated with the viscoelastic lithosphere, which reduces the magnitude of polar wander rates. Therefore, the M1 mode does not contribute a dominant portion of the predicted polar wander speed for earth models with a viscoelastic lithosphere of finite viscosity. In this case, predictions of polar wander speed as a function of lower mantle viscosity exhibit the qualitative form of an 'inverted parabola', as predicted for the J ˙₂ curve. We caution, however, that these results are obtained for simplified earth models, and the results for seismological earth models such as PREM may be complicated by the interaction between the M1 mode and the large set of transient modes.