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691.
Jules M. Blais Jacob Kalff R. Jack Cornett R. Douglas Evans 《Journal of Paleolimnology》1995,13(2):169-178
We used sediment chronology data from fourteen published studies of lake cores across much of North America and Scandinavia in order to make a large scale comparison of the different dating techniques. The uncertainty of210Pb derived dates was determined using common sediment event markers: the stable Pb rise, the137Cs rise, and theAmbrosia pollen rise. For all data combined, the 95% confidence intervals for the stable Pb rise and theAmbrosia rise, were approximately 30 years. These 95% confidence intervals are slightly higher than those derived by First-Order Error analysis performed by others on210Pb derived dates. When comparing the concordance of two210Pb models (CRS and CIC) against markers of known history, we found that the CRS model dates (constant rate of supply) had consistently better agreement than the CIC model dates (constant initial concentration). Major discrepancies between137Cs and210Pb were common, but were consistently more severe in sediments of soft water lakes pointing to an inability of sediments with low mineral content to immobilize Cs.This is the ninth of a series of papers to be published by this journal following the 20
th
anniversary of the first application of210Pb dating of lake sediments, Dr. P. G. Appleby is guest editing this series. 相似文献
692.
本文采用酸前法合成工艺,探讨了木薯淀粉羧甲基化反应的一般规律;考察了反就绎产物取代度和粘度的影响,通过正交试验,找到了最佳工艺条件,并对产物的性能与羟甲基纤维素进行了对比测试。 相似文献
693.
694.
太阳常数的微小变化在气候变化中的作用 总被引:3,自引:0,他引:3
将复杂的气候系统抽象为含有云辐射动态反馈过程的高度非线性气候模型,利用分岔理论,分析了该模型的平衡态及其稳定性。计算结果表明,云反照率反馈、地表反照率反馈和水汽反馈是气候系统呈现多平衡态结构的主要因素,是气候变化复杂性根源,而云放射率反馈对系统结构的影响,只有在强烈的水汽放射率反馈条件下才表现明显。较强的地表反照率反馈和水汽放射率反馈,均可在太阳常数仅有微小变化时就能导致全球气候突变。 相似文献
695.
本文作者首次推导均质椭球体的内部和外部引力位的数学表达式 ,由此应用调和延拓的思想引入外大地水准面和外部位函数的概念 ;建立了关于外扰动位的边值问题 ,该问题的界面规则而且具有 O( T2 )的精度 ;作为一个整体 ,讨论了外正高的计算和将地面点的重力测量值在 O( T2 )精度下归算到外大地水准面的方法 相似文献
696.
Gervasio Degrazia Domenico Anfossi Haroldo Fraga De Campos Velho Enrico Ferrero 《Boundary-Layer Meteorology》1998,86(3):525-534
A new method for deriving the Lagrangian decorrelation time scales for inhomogeneous turbulence is described. The expression for the time scales here derived for the convective boundary layer is compared to those estimated by Hanna during the Phoenix experiment. Then the values of C0, the Lagrangian velocity structure function constant, and of Bi, the Lagrangian velocity spectrum constant, were evaluated from the Eulerian velocity spectra and from the Lagrangian time scales derived, under unstable conditions, from Taylor's statistical diffusion theory. The numerical coefficient of the lateral and vertical Lagrangian spectra in the inertial subrange was found equal to 0.21, in good agreement with previous experimental estimates. 相似文献
697.
698.
699.
Kenji Yoshikawa Vladimir Romanovsky Nataria Duxbury Jerry Brown Alexandre Tsapin 《冰川冻土》2004,26(Z1):301-309
Direct current resistivity and ground penetrating radar surveys were employed to obtain the value of the resistivity and dielectric constant in the brine near the Barrow, Alaska. The geophysical surveys were undertaken together with the permafrost drilling program for the measuring of the ground temperature regime and for the core sampling. The sampled cores were measured for their physical and chemical properties in the laboratory under different temperature conditions (-60 to 20 ℃). Laboratory results support field observations and led to the development of a technique for distinguishing freshwater taliks and brine layers in permafrost. These methods were also employed in freshwater taliks near Council,Alaska. The electrical resistivity is a powerful and sensitive parameter for brine detection. However, the resistivity is a less sensitive indicator of the soil type or water content under highly saline conditions.High frequency dielectric constant is an ideal second parameter for the indication of the soil type, liquid water content and other physical properties. The imaginary part of the dielectric constant and resistivity have a significant dependence upon salinity, i.e. upon freezing temperature. The ground temperature regime and the freezing point of the brine layer are important parameters for studying the electric properties of permafrost terrain. 相似文献
700.
An infinite horizontal layer, with vertically stratified temperature and solute concentration, is considered in the case where the viscosity is exponentially dependent on temperature, and the Prandtl number is infinite. Its linear stability is investigated when the destabilizing thermal gradient acts against a stabilizing solute gradient. The analysis is performed using horizontal Fourier and vertical Chebyshev polynomial expansions. For the constant viscosity case, the laws well established in the free boundary configuration are seen to be directly suitable for the rigid one. In the variable viscosity case, characterised by a given viscosity contrast c, the scaling laws with c are settled extrapolating to the double diffusive situation the approach initiated by Stengel et al. (1982). In contrast with the constant viscosity case, the critical wave number is found to be strongly dependent on the solutal Rayleigh number in the marginal oscillatory obtained at large contrast values. 相似文献