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731.
Fine quartz silt (1–10μm dia, important in aerosol dust) isolated from a large number of soils, as well as Phanerozoic shales, sandstones and recent sands of the southern African stratigraphie column (Carboniferous, Permian and Triassic) exhibits a mean δ18O of 12.2 ± 2.1%0. These values are similar to those reported for South Pacific pelagic sediments and adjacent land areas of similar latitude but are lower than those reported for this size fraction of 30 Mesozoic and Paleozoic shales (20.7 ± 1.8%.) and soils from mid-continental U.S.A., for aerosol quartz in the North Pacific pelagic sediments, and for eolian caps in high-elevation soils of Hawaii. The lower oxygen isotopic ratios in fine quartz silt of southern Africa reinforces the earlier indication that Southern Hemisphere detritai sedimentary reservoirs contain a higher proportion of igneous and metamorphic quartz (lower δ18O) and less lowtemperature authigenic quartz (higherδ18O) than Northern Hemisphere detritai sediments. The difference reflects climates, as continents drifted in latitude. 相似文献
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733.
I.J. Jackson 《Singapore journal of tropical geography》1995,15(2):128-144
Using a range of rainfall variables, rainy seasons in Hong Kong in each of 99 years have been characterised and classified into 11 groups. Many of the correlations between variables are low, indicating the need for a multivariate approach to classification. The main characteristics of each group of seasons, their frequencies of occurrence and a small number of sequences of types which are unlikely to have occurred by chance are presented. Characteristics of the rainfall variables and aspects of their temporal variations are also presented. 相似文献
734.
A glacial outwash aquifer underlying the Gloucester Landfill near Ottawa, Canada, has become polluted with various organic chemicals following the disposal of laboratory solvents in shallow trenches immediately above the aquifer. Several remedial alternatives have been considered by the government of Canada. Impermeable barrier walls were rejected as being unsuitable given the permeable nature of the underlying bedrock. It appears improbable that pools of liquid organic chemicals (DNAPLs) exist within the aquifer, although ganglia are likely present. Therefore, much of the contaminant plume can be removed hydraulically over a period of five years by the operation of four purge wells discharging to an on-site treatment plant from which the purified water is returned to the aquifer by recharge wells. The residual contamination is anticipated to be cleaned up by in situ biorestoration techniques currently under development. 相似文献
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736.
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740.
The crystal structure of ScAlO3 has been refined at temperatures up to 1100° C on the basis of x-ray powder diffraction data. The thermal expansion is adequately
described by a Grüneisen-Debye model with the elastic Debye temperature and an effective Grüneisen parameter of 1.6. The volumetric
thermal expansion of 3.0% between 10 and 1100° C, corresponding to a mean thermal expansion coefficient of 2.7 × 10−5 K−1, is entirely attributable to the expansion of the AlO6 octahedra. The interoctahedral angles, though not fixed by symmetry, do not vary significantly with temperature —indicating
that the expansivities of the constituent AlO6 and distorted ScO8 polyhedra are well matched. Similar considerations of polyhedral expansivity suggest thermal expansion coefficients of ∼2
× 10−5K−1 for cubic CaSiO3 perovskite and a value between 2 × 10−5 K−1 and 4 × 10−5 K−1 for MgSiO3 perovskite. The lower value is consistent with the reconnaissance measurements for Mg0.9Fe0.1SiO3 (Knittle et al. 1986) below 350° C, with low-temperature measurements of single-crystal MgSiO3 (Ross and Hazen 1989), and with the results of some recent calculations. The markedly greater expansivity ∼4 × 10−5 K−1 measured at higher temperatures (350–570° C) by Knittle et al. is inconsistent with the simple Grüneisen-Debye quasiharmonic
model and may reflect the marginal metastability of the orthorhombic perovskite phase. Under these circumstances, extrapolation
of the measured expansivity is hazardous and may result in the under-estimation of lower mantle densities and the drawing
of inappropriate inferences concerning the need for chemical stratification of the Earth's mantle. 相似文献