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311.
An ecotoxicological assessment is given to the state of small lakes in specially protected natural areas in Northwestern and Central European Russia, which suffer from aerotechnogenic acidification. The toxicity of water and bottom sediments is shown to increase with decreasing water pH. The most sensitive characteristics of aquatic organisms in different trophic groups are chosen, allowing the state of lakes to be monitored. 相似文献
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A criterion previously developed by Heredia-Zavoni and Esteva for selecting optimal sensor locations is used to analyse the optimal instrumentation of structures on soft soils. The stochastic response of a linear structural system on a flexible base is formulated for use of the criterion. The case of MDOF shear systems on flexible base, with uncertain lateral stiffness and subjected to random earthquake ground motions, is studied. The optimal location of accelerometers, the reduction of prior uncertainty on the lateral stiffness, the effects of the base flexibility, the relative influence of translation and rocking of the base, and the influence of recording noise are assessed and discussed. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
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The isotopic composition of lutetium has been measured in a range of terrestrial and meteoritic materials using solid-source mass spectrometric techniques. The meteoritic and terrestrial isotopic abundances are identical within experimental errors. The absolute 175Lu/176Lu ratio as determined in this work is 37.36 ± 0.07 at the 95% confidence level. On the basis of this measurement the atomic weight of lutetium has been calculated to be 174.967 ± 0.002, which is in good agreement with the currently accepted figure of 174.97 ± 0.01.Using the stable isotope dilution technique the abundance of lutetium has been determined in 25 stone, 1 stony-iron and 8 iron meteorites, and in 12 standard rocks, with an accuracy of ±5% at the 95% confidence level. In general, there is good agreement between this work and other published data.The 176Lu-176Hf pair has been proposed as an s process nucleocosmochronometer, because of the long half-life of 176Lu and the unique fact that both are s process isobars. The isotopic and elemental abundances of lutetium as measured in this work have been used with published nuclear data to estimate the mean age of s process nucleosynthesis for this isobaric pair, using the Schramm-Wasserburg formalism. The mean age cannot be accurately determined at the present time because of the lack of 30 keV neutron capture cross-section data for s only process nuclides and an accurate measurement of the branching ratio of 175Lu + n. However, it is possible to place constraints on the nuclear parameters in this mass region using a reasonable s process chronology based on the decay of 176Lu. 相似文献
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D. A. Burney R. V. DeCandido L. P. Burney F. N. Kostel-Hughes T. W. Stafford Jr. H. F. James 《Journal of Paleolimnology》1995,13(3):209-217
A sediment core from a high-elevation bog on Maui in the Hawaiian Islands contains evidence for drier conditions between 9.4–5.8 kyr BP, followed by a wetter interval between 5.8–2.2 kyr BP, and a variable late Holocene. These precipitation changes may be a reflection of vertical displacements of the upper boundary of the mid-Pacific Trade Wind Inversion (TWI) cloud layer. Fires, probably volcanically ignited, occurred in the forests prior to human arrival. Polynesian activity in this high-elevation, remote site was apparently limited, with no pollen, charcoal, or sedimentological evidence for local anthropogenic disturbance. After European contact, grass fires increased and introduced plant species invaded the site. Values for Cd, Cu, Pb, and Zn in sediments throughout the Holocene indicate low trace-metal deposition from atmospheric particulates at the site, even in the twentieth century.This paper is one of a series of papers guest edited by Dr. Mark Brenner on tropical paleolimnology 相似文献