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311.
Saioa Suárez Hazel M. Prichard Francisco Velasco Peter C. Fisher Iain McDonald 《Mineralium Deposita》2010,45(4):331-350
The distribution, mineralogy and mobility of the platinum-group elements (PGE) in the surface environment are poorly understood.
This study of the lower, less altered and upper, more altered gossan, overlying the Aguablanca Ni–Cu-(PGE) magmatic deposit
(Spain), has shown that the platinum-group minerals (PGM) are progressively oxidised and dispersed into iron oxides that form
the gossan. A combination of the characterization of PGE in host PGM, using a scanning electron microscope, and measurement
of PGE at lower concentrations in host iron oxides, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS),
has for the first time allowed the total distribution of the PGE within a gossan to be documented. This study has revealed
a complete in situ alteration and dispersion sequence of the PGM including (1) breakdown of both the more stable Pt-arsenides,
Pt/Pd-tellurides and the less stable bismuthotellurides, (2) formation of partially oxidised PGM, (3) development of a wide
range of oxidised Pt- and Pd-bearing phases, (4) subsequent formation of Fe–PGE-oxides and PGE-hydroxides, (5) incorporation
of PGE into ferruginous supergene products and lastly (6) concentration of PGE at the edges of veins and iron oxides. Dispersion
of Pd is greater than for the other PGE with Pd being widely distributed throughout the iron oxides. This oxidising environment
produced PGE-oxides rather than PGE-alloys, also commonly found in the surface environment, especially in placers. These results
provide critical evidence for the stages of mineralogical change from PGE host mineralogy in magmatic ores to surface weathering
producing PGE-oxides. 相似文献
312.
Michelle Elliott Christopher T. Fisher Roberto S. Molina Garza Deborah M. Pearsall 《Quaternary Research》2010,74(1):26-35
Scholars attribute the growth and decline of Classic period (AD 200-900) settlements in the semi-arid northern frontier zone of Mesoamerica to rainfall cycles that controlled the extent of arable land. However, there is little empirical evidence to support this claim. We present phytolith, organic carbon, and magnetic susceptibility analyses of a 4000-yr alluvial record of climate and human land use from the Malpaso Valley, the site of one such Classic frontier community. The earliest farming occupation is detected around 500 BC and appears related to a slight increase of aridity, similar to the level of the modern day valley. By AD 500, the valley's Classic period Mesoamerican settlements were founded under these same dry conditions, which continued into the Postclassic period. This indicates that the La Quemada occupation did not develop during a period of increased rainfall, but rather an arid phase. The most dramatic changes detected in the valley resulted from the erosion associated with Spanish Colonial grazing and deforestation that began in the 16th century. The landscape of the modern Malpaso Valley is thus primarily the product of a series of intense and rapid transformations that were concentrated within the last 400 yr. 相似文献
313.
One strand of research relates the magnitude of severe weather disasters to climatic and human development factors; another highlights dramatic growth in catastrophe losses. However, there have been few attempts to put the two strands together. Here we use an explicit modeling framework to determine the contribution of climate variability relative to human factors in reported catastrophe losses. We then examine how future climate change can be expected to affect losses from natural disasters. Simultaneous regression models are constructed from three equations in which the dependent variables are U.S. flood loss, U.S. hurricane loss and U.S. catastrophe loss. Then two kinds of simulation under two climate change scenarios explore how climate change would affect losses. The climate change scenarios respectively project 13.5% and 21.5% increases in annual precipitation. The first simulation increases only the mean value of annual precipitation; the second simulation assumes that the mean and standard deviation of annual precipitation change in the same proportion. Results show that the growth in reported losses from weather-related natural disasters is due mainly to three socioeconomic factors: inflation, population growth and growth in per capita real wealth. However, weather variables such as precipitation and the number of hurricanes per period also clearly affect losses. The three stage least squares (3SLS) simultaneous equation model shows that a 1% increase in annual precipitation would enlarge catastrophe loss by as much as 2.8%. These findings are suggestive as planning signals to decision makers. 相似文献
314.
A. Bregman F. Arnold V. Bürger H. Fisher J. Lelieveld B. A. Scheeren J. Schneider P. C. Siegmund J. Ström A. Waibel W. M. F. Wauben 《Journal of Atmospheric Chemistry》1997,26(3):275-310
In situ aircraft measurements of O3, CO,HNO3, and aerosol particles are presented,performed over the North Sea region in the summerlower stratosphere during the STREAM II campaign(Stratosphere Troposphere Experiments by AircraftMeasurements) in July 1994. Occasionally, high COconcentrations of 200-300 pbbv were measured in thelowermost stratosphere, together with relatively highHNO3 concentrations up to 1.6 ppbv. The particlenumber concentration (at standard pressure andtemperature) between 0.018-1 m decreased acrossthe tropopause, from >1000 cm-3 in the uppertroposphere to <500 cm-3 in the lowermoststratosphere. Since the CO sources are found in thetroposphere, the elevated CO mixing ratios areattributed to mixing of polluted tropospheric air intothe lowermost extratropical stratosphere. Further wehave used a chemical model to illustrate that nitrogenoxide reservoir species (mainly HNO3) determinethe availability of NOx (=NO + NO2) andtherefore largely control the total net O3production in the lower kilometers of thestratosphere. Model simulations, applying additionalNOx perturbations from aircraft, show that theO3 production efficiency of NOx is smallerthan previously assumed, under conditions withrelatively high HNO3 mixing ratios, as observedduring STREAM II. The model simulations furthersuggest a relatively high O3 productionefficiency from CO oxidation, as a result of therelatively high ambient HNO3 and NOxconcentrations, implying that upward transport of COrich air enhances O3 production in the lowermoststratosphere. Analysis of the measurements and themodel calculations suggest that the lowermoststratosphere is a transition region in which thechemistry deviates from both the upper troposphere andlower stratosphere. 相似文献
315.
Igneous zircon: trace element composition as an indicator of source rock type 总被引:225,自引:2,他引:225