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921.
A reconnaissance study of oxygen, hydrogen and strontium isotopes in geochemically diverse lakes, Western Nebraska, USA 总被引:1,自引:1,他引:0
David C. Gosselin Peter E. Nabelek Zell E. Peterman Steve Sibray 《Journal of Paleolimnology》1997,17(1):51-65
Reconnaissance 18O,, D, and 87Sr data for fifteen lakes in the Western Lakes Region of the Sand Hills of Nebraska indicate dynamic hydrologic systems. The rather narrow range of 87Sr from lake water (1.1 to 2.1) and groundwater (0.9 to 1.7) indicates that the groundwater is generally unradiogenic. Groundwater residence times and relatively unradiogenic volcanic ash within the dune sediments control the 87Sr values. Based on the mutual variations of 18O and D, the lakes can be divided into three groups. In Group 1, both 18O and D values increase from spring to fall. The 18O and D values in Group 2 decreased from spring to fall. Group 3 are ephemeral lakes that went dry some time during 1992. The data and isotopic modeling show that variations in the ratio of evaporation relative to groundwater inflow, local humidity conditions, and the a has substantial influence on the isotopic composition. In addition, isotopic behavior in ephemeral lakes can be rather unusual because of the changing activities of water and mineral precipitation and redissolution. The annual and interannual isotopic variability of these lakes which is reflected in the paleonvironmental indicators may be the rule rather than the exception in these types of systems. 相似文献
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923.
Infrared spectra of Mars are made up of three separate components, each of which may dominate the spectrum under different Martian meteorological and observational conditions. By means of laboratory examples we show that both the shape and spectral contrast of the spectral curves change dramatically, depending on which component is dominant. Each experimental condition has been experienced during either the Mariner 69 or 71 observations. Comparing the preliminary Mariner 71 radiance data with laboratory transmission spectra, we suggest that the clay mineral montmorillonite could be the major component of the Martian dust cloud. 相似文献
924.
The expression for the exospheric temperature in Jacchia's static diffusion models of the upper atmosphere has a discontinuous gradient at the poles. Therefore it cannot describe the true state of the upper atmosphere in the polar regions. Furthermore, it cannot be used to calculate quantities that depend on the derivative of the exospheric temperature, or the density, like pressure gradients, horizontal forces or horizontal heat fluxes. A modified expression for the exospheric temperature is suggested. This modification yields variables of state of the upper atmosphere that deviate little from Jacchia's values, but it has continuous gradient at the poles and is therefore more suitable for treating dynamical problems like the global wind pattern. 相似文献
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928.
Hans Peter Cornelius 《International Journal of Earth Sciences》1952,40(2):257-261
Ohne Zusammenfassung 相似文献
929.
The records of two large stock and station agents, Loan & Mercantile Agency in Dunedin and Wright Stephenson in Invercargill, allowed us to track the timing, nature, magnitude and rate of landscape change in southern New Zealand between 1896 and 1920. This period extends from the final years of subdivision of large estates, and includes closer settlement, the shift from pastoral farming to intensive agriculture, growth of dairying, and increasing mechanisation of agriculture. These changes are reflected in clients’ annual expenditures on capital items such as fencing and building materials, tools and implements, materials for drains. 相似文献
930.