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991.
The Olmedo bauxite deposit occurs in the Nurra district of northwest Sardinia. It forms a stratiform horizon in Cretaceous
limestone and marl. Uplift in mid-Cretaceous had exposed recently deposited limestone to karst weathering, and a layer of
argillaceous debris accumulated on its surface and was partly converted to bauxite. Intermediate products were desiccated
marl, bauxitic argillite and argillaceous bauxite. Subsidence followed, and the bauxite was preserved by the deposition of
late Cretaceous limestone and other sediments. Uplift in Oligocene-Miocene time, with ensuing erosion, exposed the bauxite
horizon to its present configuration. Concentrations of normative minerals illustrate chemical processes and the build-up
of Al in the bauxite horizon. Plots of chemical data and correlation coefficients show that Al, Ti, Zr, Nb, Th, Cr and V were
immobile during the bauxitization process. Mass changes point to large net removal of Si, Mg and K from the system, although
some of this material and slightly mobile Al were reprecipitated in the underlying argillite and altered marl. Immobile element
ratios trace the source of the bauxite to the underlying argillaceous limestone. Al in the bauxite was accumulated from the
degradation of 25 to 50 m of the argillaceous limestone.
Received: 10 June 1996 / Accepted: 15 April 1997 相似文献
992.
Estimation of groundwater evaporation and salt flux from Owens Lake, California, USA 总被引:3,自引:0,他引:3
S. W. Tyler S. Kranz M. B. Parlange J. Albertson G. G. Katul G. F. Cochran B. A. Lyles G. Holder 《Journal of Hydrology》1997,200(1-4):110-135
Groundwater evaporation and subsequent precipitation of soluble salts at Owens Lake in eastern California have created one of the single largest sources of airborne dust in the USA, yet the evaporation and salt flux have not been fully quantified. In this study, we compare eddy correlation, microlysimeters and solute profiling methods to determine their validity and sensitivity in playa environments. These techniques are often used to estimate evaporative losses, yet have not been critically compared at one field site to judge their relative effectiveness and accuracy. Results suggest that eddy correlation methods are the most widely applicable for the variety of conditions found on large playa lakes. Chloride profiling is shown to be highly sensitive to thermal and density-driven fluxes in the near surface and, as a result, appears to underestimate yearly groundwater evaporation. Yearly mean groundwater evaporation from the playa surface estimated from the three study areas was found to range from 88 to 104 mm year−1, whereas mean evaporation from the brine-covered areas was 872 mm year−1. Uncertainties on these mean rates were estimated to be ±25%, based on comparisons between eddy correlation and lysimeter estimates. On a yearly basis, evaporation accounts for approximately 47 × 106 m3 of water loss from the playa surface and open-water areas of the lake. Over the playa area, as much as 7.5 × 108 kg (7.5 × 105 t) of salt are annually concentrated by evaporation at or near the playa surface, much of which appears to be lost during dust storms in area. 相似文献
993.
994.
Measurement and research method of metallogenic depths of Jiaojia orefield and Linglong orefield Shandong Province, China 总被引:1,自引:0,他引:1
Guxian Lü 《中国科学D辑(英文版)》1997,40(5):449-455
Taking the contribution of the tectonic force to the total hydrostatic pressure into account, the author puts forward a new
method on the calculation of the depth of petrogenesis and metallogenesis which is summarized as follows: first the tectonic
added hydrostatic pressured Ps is subtracted from the total hydrostatic pressure P, then using their difference Pr, according to the general method the depth of petrogenesis and metallogenesis can be determined in consideration of lateral
constraint.
By the new method the following data on the depths of the metallogenesis are obtained: 2 243.6 m (No. I and No. II veins with
metallogenic epoch of 105 Ma) and 1 632. 38 m (No. III vein with 105 Ma) for Jiaojia orefield, and 3 454.97 m (NE-trending
zone with 213.2 Ma), 1 902.79 III (ENE-trending zone with 100.28 Ma), 1 090.97 m (NE-trending zone with 80.67 Ma) and 720.55
m (NNE-trending zone with 71.86 Ma) for Linglong orefield.
Project supported by the Foundation of the State Planning Commission, China and the Foundation of the State Science and Technology
Commission, China. 相似文献
995.
Jürg W. Schlatter Alfred Wüest Dieter M. Imboden 《Aquatic Sciences - Research Across Boundaries》1997,59(3):225-242
The artificial tracer sulphur hexafluoride (SF6) has been used to study the density-driven deep water exchange between two sill-separated basins of Lake Lucerne, Gersauersee and Urnersee. The sources of the density gradients between the two basins are (1) salinity differences between the major inlets due to the different geology of their drainage areas, and (2) temperature differences due to spatial variation of wind forcing. Wind speeds are generally larger in Urnersee, especially in spring during the so-called Föhn events, when winds blow from the south. In contrast, Gersauersee is protected form these winds. In spring 1989, a total of 630 g of SF6 was released at 80 to 120 m depth in the small Treib Basin located between Urnersee and Gersauersee. During about 100 days the distribution of SF6 in the lake was determined by gaschromatography. Two models are used to quantify the exchange flow, (1) a one-box mass balance model for SF6 in the deep part of Treib Basin, and (2) a one-dimensional diffusion/advection model describing the temporal and vertical temperature variation in Urnersee. According to the first model, the flow into the deep hypolimnion of Urnersee, decreases from 21·106 m3·d?1 at the end of March to about 8·106 m3·d?1 in late April. The second model yields similar flow rates. The decrease of the flow rate during spring, confirmed by both approaches, is consistent (1) with the decreasing strength of the density gradient above the sill during spring and early summer, and (2) with hydrographic information collected in Lake Lucerne during other years. 相似文献
996.
HanBin Xue René Gächter Laura Sigg 《Aquatic Sciences - Research Across Boundaries》1997,59(2):176-189
Data on the cycling of Cu and Zn in two eutrophic lakes are presented: Lake Greifen that becomes seasonally anoxic in its
hypolimnion and Lake Sempach that is aerated during winter and oxygenated during summer. They suggest that hypolimnetic oxygenation
1) enhances the release of copper from the sediment but 2) also accelerates the entrapment and deposition of Cu and Zn by
freshly formed Mn- and Fe-oxides. 相似文献
997.
M. W. Kemblowski C-M. Chang I. Kamil 《Stochastic Environmental Research and Risk Assessment (SERRA)》1997,11(3):255-266
It has been observed that the field biodegradation rates for soluble hydrocarbon plumes are significantly smaller than the
aerobic rates observed in the laboratory. It is believed that this difference is related to the fact that in the field oxygen
and hydrocarbon must be mixed before the biodegradation reaction can occur, and that the effective degradation rate is controlled
by the actual, not mean, concentrations of oxygen and hydrocarbon. In this work, we present a conceptual model of oxygen-mixing
limited biodegradation, which indicates that the effective degradation rate should depend on the cross correlation between
the oxygen and hydrocarbon concentration fluctuations. This is followed by a development of a rigorous, field-scale model. 相似文献
998.
Testing an earthquake prediction algorithm 总被引:1,自引:0,他引:1
Vladimir G. Kossobokov John H. Healy James W. Dewey 《Pure and Applied Geophysics》1997,149(1):219-232
A test to evaluate earthquake prediction algorithms is being applied to a Russian algorithm known asM8 TheM8 algorithm makes intermediate term predictions for earthquakes to occur in a large circle, based on integral counts of transient seismicity in the circle. In a retroactive prediction for the period January 1, 1985 to July 1, 1991 the algorithm as configured for the forward test would have predicted eight of ten strong earthquakes in the test area. A null hypothesis, based on random assignment of predictions, predicts eight earthquakes in 2.87% of the trials. The forward test began July 1, 1991 and will run through December 31, 1997. As of July 1, 1995, the algorithm had forward predicted five out of nine earthquakes in the test area, which success ratio would have been achieved in 53% of random trials with the null hypothesis. 相似文献
999.
W. K. Hocking 《Annales Geophysicae》1997,15(9):1111-1122
Radars have been used successfully for many years to measure atmospheric motions over a wide range of altitudes, from ground level up to heights of several hundred kilometres into the ionosphere. In this paper we particularly wish to concentrate on the accuracy of these measurements for winds in the middle atmosphere (i.e. 10–100–km altitude). We begin by briefly reviewing the literature relating to comparisons between radar methods and other techniques. We demonstrate where the radar data are most and least reliable and then, in parallel with a discussion about the basic principles of the method, discuss why these different regimes have the different accuracies and precisions they do. This discussion is used to highlight the strengths and weaknesses of radar methods. Issues like radar volume, aspect sensitivity, gravity wave effects and scatterer intermittency in producing wind biases, and the degree by which the intermittent generation of scatterers at quasi-random points in space could skew the radar measurements, are all considered. We also investigate the possibility that MF radar techniques can be contaminated by E-region scatter to heights as low as 92–95–km altitude (i.e. up to 8–10 km below the ionospheric peak echo). Within all these comments, however, we also recognize that radar methods still represent powerful techniques which have an important future at all levels of the atmosphere. 相似文献
1000.
Effects of convection instability due to incompatibility between ocean dynamics and surface forcings
The study demonstrates that an incompatibility between a surface temperature climatology and a given ocean model, into which the climatology is assimilated via Haney restoration, can cause model ocean climate drift and interdecadal oscillations when the ocean is switched to a weaker restoration. This is made using an idealized Atlantic Ocean model driven by thermal and wind forcing only. Initially, the temperature climatology is forcefully assimilated into the model, and an implied heat flux field is diagnosed. During this stage any incompatibility is suppressed. The restoring boundary condition is then switched to a new forcing consisting of a part of the diagnosed flux and a part of the restoring forcing in such a way that at the moment of the switching the heat flux is identical to that prior to the switching. Under this new forcing condition, the incompatibility becomes manifest, causing changes in convection patterns, and producing drift and interdecadal oscillations. The mechanisms are described. 相似文献