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51.
Thermal models for the magmatic accretion and subsequent metamorphism of continental crust 总被引:3,自引:0,他引:3
P.R.A. Wells 《Earth and Planetary Science Letters》1980,46(2):253-265
I present one-dimensional conductive relaxation models for the thermal evolution of continental crust which has undergone magmatic thickening in a predominantly recumbent tectonic regime. The sustained addition of magma, and associated heat, profoundly influences the conductive relaxation of the affected sialic crust. The mechanism of accretion, successively beneath (under-accretion), or successively above (over-accretion), previously added material, strongly affects the type of P-T-time history sustained by the rocks in the middle and deep crust. The pressure and temperature conditions which might be recorded by mineral assemblages equilibrating in such tectonothermal regimes may bear very little relation to conductive heat supplies from the upper mantle. P-T values obtained by geobarometry and geothermometry in such complexes must be used with caution to constrain steady state heat flows and geothermal gradients. In a contemporary plate tectonic context I consider the tectonothermal models developed here to apply particularly to island arcs and sites of continental collision associated with extensive plutonic igneous activity. Precambrian orthogneiss complexes, such as the Archaean of southern West Greenland, probably represent exhumed deep crustal elements involved in the intense magmatic thickening processes considered here. 相似文献
52.
Equations are proposed which describe the diffusion of a plume of oil from chemically dispersed oil slicks into the oceanic water column and which yield estimates of oil concentration as a function of the volume of oil dispersed, time-varying plume area, depth, time, and as a function of a single adjustable parameter, vertical diffusivity. A ‘diffusion floor’ at any desired depth may be included. Experimental data were fitted to the equations to yield an estimate of this diffusivity. It is concluded that oil concentrations can be estimated with a mean error of a factor of three, this high value being attributable to the inherently variable nature of the oceanic diffusion processes. Implications for assessing the toxic effects of dispersed slicks are discussed. It may be possible to calculate acute median lethal water depths and volumes for given organisms subjected to given dispersed oil slicks. 相似文献
53.
J.H Vandermeulen D.E Buckley E.M Levy B.F.N Long P McLaren P.G Wells 《Marine pollution bulletin》1979,10(8):222-227
Weathering states of oil ranging from sheen oil to mousse have significant effects during oil stranding on oil-water and oil-sediment interaction. Observations of oiled sandy beaches suggest two mechanisms of beach contamination—general penetration and contamination of beach substrate by films of sheen oil (probably partially emulsified), and secondly the burial of discrete layers of mousse. Depending on timing of oiling with respect to beach dynamics large amounts of stranded oil can be accommodated within beach sediments. These then become available for later long-term release.Acute lethal toxicity of stranded Amoco Cadiz oil is intermediate to Bunker C and Kuwait crude. Simulated weathering studies suggest that there may be some increase in toxicity with weathering. 相似文献
54.
55.
Assessing health of the Bay of Fundy--concepts and framework 总被引:8,自引:0,他引:8
Wells PG 《Marine pollution bulletin》2003,46(9):1059-1077
A discussion of health and ecosystem health (EH) concepts and a conceptual framework for assessing health of the Bay of Fundy are presented. The framework includes:Concepts--What is health? What is EH and marine ecosystem health (MEH)? How does EH relate to other closely related concepts and principles i.e. environmental quality (especially marine or MEQ), ecosystem integrity, and ecosystem sustainability?Importance--Why is EH important, and what are the linkages to people i.e. human health?Approaches and techniques--How do we monitor and measure EH, and in that context, ecosystem or ecological change? i.e. What are the monitoring approaches and tools? What is an appropriate set of EH indicators and indices for the Bay of Fundy and Gulf of Maine? At the present time, can we make unequivocal statements about the status and trends in EH measures of the Bay of Fundy? Do we have adequate guidelines, objectives and standards for assessing environmental quality and EH?Connecting with management needs--Do we have adequate mechanisms in place to address MEH, marine environmental quality (MEQ), and environmental sustainability in the Bay of Fundy? What is the role of periodic marine assessments (state of the marine environment reports) in this activity? What kinds of new directions and new ecosystem science should be given priority? What kinds of other new activities should be pursued?For the Bay of Fundy Coastal Forum at the recent 5th BOFEP Bay of Fundy Science Workshop (May 2002), five core questions flowing from the health framework served to initiate and focus the discussions:Current state--What is the present health or condition of the Bay of Fundy?Changes--Are conditions improving or deteriorating?Indicators--What kinds of indicators do we consider most useful in trying to answer these questions?Existing resources--Are there adequate resources (e.g. institutional, financial, scientific, regulatory) to protect or restore the health of the Bay?Needs--What kinds of new information and approaches do we need to protect the health of the Bay?The background information and ideas of this paper were intended to assist discussion at the Fundy workshop, and to help identify the next steps, both individual and collective, for assessing the health of the Bay of Fundy. 相似文献
56.
This is an exploratory study on the high-pressure (HP) structural evolution of a zeolitic framework (with LEV topology) on the basis of geometric modelling and previously published accurate unit-cell constants measured by means of single-crystal X-ray diffraction. The geometric simulations for 11 P values from 0 to 5 GPa gives more insight into the HP-behaviour of levyne, showing that the anomalous elastic behaviour of this zeolite observed under hydrostatic conditions at low P (P<1 GPa) is due to a double change in the compressional mechanism. Since the geometric simulation is not restricted to using the experimentally determined cell parameters, simulations of uniaxial compression along the [001] direction and of compression in the (001) plane have been performed, shedding more light on the compression mechanisms under non-hydrostatic regimes, which are difficult to access experimentally. The mechanisms associated with compressions along different axes provide insight into the hydrostatic compression mechanisms leading to the anomalous elastic behaviour.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
57.
Numerical and calibrated age determinations of the late Quaternary alluvial fan deposits of the Soda Mountains piedmont in the Mojave Desert provide an opportunity to study the utility of the multiparameter relative-age (RA) method for distinguishing and mapping geomorphic surfaces on a desert piedmont. Most RA parameters could not discriminate between deposits of Holocene age, although pavements have formed over locally significant parts of surfaces as young as middle Holocene. Several parameters, including lithologic composition, particle size, soil development, and varnish cover, permit distinguishing between Holocene surfaces and late Pleistocene surfaces. Statistically significant differences in initial particle size and lithology of the deposits, inferred to be the result of complex interaction among hillslope, alluvial fan, and eolian processes and climatic change, create conditions unfavorable for use of most RA techniques. In contrast, soil-profile development and varnish cover data are successful in discrimination among deposits of Holocene and Pleistocene age. This is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan. 相似文献
58.
The benches and risers at Mormon Point, Death Valley, USA, have long been interpreted as strandlines cut by still-stands of pluvial lakes correlative with oxygen isotope stage (OIS) 5e/6 (120,000–186,000 yr B.P.) and OIS-2 (10,000–35,000 yr B.P.). This study presents geologic mapping and geomorphic analyses (Gilbert's criteria, longitudinal profiles), which indicate that only the highest bench at Mormon Point (90 m above mean sea level (msl)) is a lake strandline. The other prominent benches on the north-descending slope immediately below this strandline are interpreted as fault scarps offsetting a lacustrine abrasion platform. The faults offsetting the abrasion platform most likely join downward into and slip sympathetically with the Mormon Point turtleback fault, implying late Quaternary slip on this low-angle normal fault. Our geomorphic reinterpretation implies that the OIS-5e/6 lake receded rapidly enough not to cut strandlines and was 90 m deep. Consistent with independent core studies of the salt pan, no evidence of OIS-2 lake strandlines was found at Mormon Point, which indicates that the maximum elevation of the OIS-2 lake surface was −30 m msl. Thus, as measured by pluvial lake depth, the OIS-2 effective precipitation was significantly less than during OIS-5e/6, a finding that is more consistent with other studies in the region. The changed geomorphic context indicates that previous surface exposure dates on fault scarps and benches at Mormon Point are uninterpretable with respect to lake history. 相似文献
59.
Excessive soil erosion and deposition is recognised as a significant land degradation issue. Quantifying soil erosion and deposition is a non-trivial task. One of these methods has been the mathematical modelling of soil erosion and deposition patterns and the processes that drive them. Here we examine the capability of a landscape evolution model to predict both soil erosion rate and pattern of erosion and deposition. This numerical model (SIBERIA) uses a Digital Elevation Model (DEM) to represent the landscape and calculates erosion and deposition at each grid point in the DEM. To assess field soil redistribution rates (SRR) and patterns the distribution of the environmental tracer 137Cs has been analysed. Net hill slope SRR predicted by SIBERIA (a soil loss rate of 1.7 to 4.3 t ha-1 yr-1) were found to be in good agreement with 137Cs based estimates (2.1 – 3.4 t ha-1 yr-1) providing confidence in the predictive ability of the model at the hillslope scale. However some differences in predicted erosion/deposition patterns were noted due to historical changes in landscape form (i.e. the addition of a contour bank) and possible causes discussed, as is the finding that soil erosion rates are an order of magnitude higher than likely soil production rates. The finding that SIBERIA can approximate independently quantified erosion and deposition patterns and rates is encouraging, providing confidence in the employment of DEM based models to quantify hillslope erosion rates and demonstrating the potential to upscale for the prediction of whole catchment erosion and deposition. The findings of this study suggest that LEMs can be a reliable alternative to complex and time consuming methods such as that using environmental tracers for the determination of erosion rates. The model and approach demonstrates a new approach to assessing soil erosion that can be employed elsewhere. © 2018 John Wiley & Sons, Ltd. 相似文献
60.
Henrique G. Momm Ronald L. Bingner Randall Emilaire Jurgen Garbrecht Robert R. Wells Roger A. Kuhnle 《水文科学杂志》2017,62(10):1564-1582
A qualitative trial-and-error approach is commonly used to define watershed subdivisions through varying a single topographic threshold value. A methodology has been developed to quantitatively determine spatially variable threshold values using topography and a user-defined landscape reference layer. Optimization and topographic parameterization algorithms were integrated to create solutions that minimize the number of sub-watersheds and maximize the agreement between the discretized watershed and the reference layer. The system was evaluated using different reference datasets such as soil type, land management, and landscape form. Comparison of simulated results indicated that the scenario using land management as the reference layer yielded results closer to the scenario subdivided using a constant topographic threshold but with approximately 10 times more sub-catchments and therefore indicating customization of the watershed subdivision to the user-defined reference layer. The proposed optimization technology could be used in adequately applying watershed modeling technology in developing conservation practice implementation plans. 相似文献