首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   279篇
  免费   8篇
  国内免费   4篇
测绘学   6篇
大气科学   20篇
地球物理   55篇
地质学   98篇
海洋学   37篇
天文学   37篇
自然地理   38篇
  2023年   1篇
  2021年   3篇
  2020年   4篇
  2019年   6篇
  2018年   8篇
  2017年   10篇
  2016年   8篇
  2015年   4篇
  2014年   7篇
  2013年   18篇
  2012年   6篇
  2011年   17篇
  2010年   7篇
  2009年   18篇
  2008年   20篇
  2007年   12篇
  2006年   9篇
  2005年   7篇
  2004年   19篇
  2003年   13篇
  2002年   6篇
  2001年   7篇
  2000年   8篇
  1999年   8篇
  1998年   5篇
  1997年   3篇
  1996年   3篇
  1995年   4篇
  1994年   8篇
  1993年   2篇
  1992年   4篇
  1990年   5篇
  1989年   7篇
  1988年   3篇
  1987年   2篇
  1986年   1篇
  1985年   2篇
  1984年   1篇
  1983年   1篇
  1982年   2篇
  1981年   3篇
  1980年   2篇
  1978年   4篇
  1974年   2篇
  1970年   1篇
排序方式: 共有291条查询结果,搜索用时 15 毫秒
251.
The importance of the El Niño-Southern Oscillation (ENSO) on regional-scale climate variability is well recognized although the associated effects on local weather patterns are poorly understood. Little work has addressed the ancillary impacts of climate variability at the community level, which require analysis at a local scale. In coastal communities water quality and public health effects are of particular interest. Here we describe the historical influence of ENSO events on coastal water quality in Tampa Bay, Florida (USA) as a test case. Using approximate randomized statistics, we show significant ENSO influences on water quality particularly during winter months, with significantly greater fecal pollution levels during strong El Niño winters and significantly lower levels during strong La Niña winters as compared to neutral conditions. Similar significant patterns were also noted for El Niño and La Niña fall periods. The success of the analysis demonstrates the feasibility of assessing local effects associated with large-scale climate variability. It also highlights the possibility of using ENSO forecasts to predict periods of poor coastal water quality in urban region which local agencies may use to make appropriate prepations.  相似文献   
252.
Photometry obtained in 1973 on the uvby system yields high-precision rotational light curves for Io, Europa, and Ganymede at a mean phase angle of ~6°. By combining our observations with photometry obtained by others over a broader range of phase angle, we alsi derive improved values for the phase coefficients and opposition surges of the four Galilean satellites. The values of V(1, 0) obtained by linear extrapolation to zero phase are accurate to ±0.03 magnitudes. We also derive the colors of the sun of the uvby system and use these to obtain albedos of the satellites in four colors.  相似文献   
253.
SeaMARC II side-scan sonar data reveal that a large area of seafloor north and west of Easter Island has been disrupted by recent submarine volcanism. A large volcanic area begins approximately 60 km WNW of the island and extends for over 130 km to the west. The volcanic field is characterized by high backscatter intensity in the side-scan sonar records and is elevated 400–1000 m above the N-S seafloor fabric that surrounds it. This field, the Abu Volcanic Field, covers at least 2500 km2 and appears to consist of recent lava flows and small volcanoes. Backscatter intensity of the Abu Volcanic Field is similar to that of the adjacent ridge flank which is less than 0.4 Ma, suggesting a similar age for its formation. Two additional areas of high backscatter immediately north of Easter Island cover a combined area of over 300 km2. The sidescan sonar records show that these features are clearly of volcanic origin and are not debris flows from the nearby island. The flows are nearly 300 m thick and are morphologically similar to subaerial pahoehoe lava shields. Their high backscatter indicates that they are also the products of relatively recent submarine volcanic activity. The presence of these large areas of recent volcanism in the vicinity of Easter Island has important implications for the various models that have been proposed to explain the origin of the Easter Seamount Chain. In addition, the similar ages of Easter Island and the Easter Microplate suggest that the presence of a hotspot near or beneath this fast-spreading portion of the East Pacific Rise about 4.5 m.y. ago may have initiated the large-scale rift propagation that created the microplate.  相似文献   
254.
This work is the beginning of a coastal water quality monitoring program to establish the baseline for the implementation of an integrated coastal management of the Yucatán Peninsula tropical ecosystem. Coastal water quality is affected by the increasing economic development. This area has no rivers because of its karst geomorphology, and the coastal freshwater comes from springs or seeps. Coastal water quality was studied in four towns from January to December 2000. Statistically significant differences among water quality variables and processes are discussed. Along with groundwater discharge, domestic and shrimp farming sewage are the main sources of nutrients, predominantly of nitrogen and silica. Salinity dilution is used to estimate the groundwater fraction that influences each area in northwestern Yucatán.  相似文献   
255.
A high-pressure single-crystal x-ray diffraction study of perovskite-type MgSiO3 has been completed to 12.6 GPa. The compressibility of MgSiO3 perovskite is anisotropic with b approximately 23% less compressible than a or c which have similar compressibilities. The observed unit cell compression gives a bulk modulus of 254 GPa using a Birch-Murnaghan equation of state with K set equal to 4 and V/V 0 at room pressure equal to one. Between room pressure and 5 GPa, the primary response of the structure to pressure is compression of the Mg-O and Si-O bonds. Above 5 GPa, the SiO6 octahedra tilt, particularly in the [bc]-plane. The distortion of the MgO12 site increases under compression. The variation of the O(2)-O(2)-O(2) angles and bondlength distortion of the MgO12 site with pressure in MgSiO3 perovskite follow trends observed in GdFeO3type perovskites with increasing distortion. Such trends might be useful for predicting distortions in GdFeO3-type perovskites as a function of pressure.  相似文献   
256.
Volcanism extending over 11 Ma is represented in the rocks of the Nevados de Payachata region, culminating in the formation of two large composite stratocones within the last 500 000 years. Chemically distinct mafic magmas are erupted at a number of parasitic centers. These cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones. Two distinct trends are defined by the intermediate and evolved rocks; a high LILE (large ion lithophile element), TiO2 and Ce/Yb lineage among the youngest rocks (including the two major stratocones), and a more typical calc-alkaline trend among the older (>1 Ma) rock types. Within individual volcanic centers, differentiation involves fractionation of plagioclase, pyroxene and hornblende, with biotite and K-feldspar in the more-evolved rock types. Isotopic compositions (Sr, Pb, Nd, O) vary little with differentiation from basaltic andesite to rhyolite, or with age. Contamination during differentiation from basalt to rhyolite may occur, but the most mafic rocks erupted in the region are already enriched in incompatible trace elements and therefore may be insensitive to the effects of interaction with the crust. The majority of data are similar to baseline compositions (Cenozoic parental magmas) from other parts of the central Andes and may reflect a relatively homogeneous magma source (or source mixture) throughout this central volcanic zone (CVZ), which is distinct from the southern and northern Andes, and from island-arc volcanic rocks.The detailed study of Nevados de Payachata serves as a useful reference against which to assess magmatism in general in the CVZ. The possibility that central Andean magmas are generated from an enriched subcontinental-lithosphere mantle wedge is rejected on the basis of: (1) thermal considerations (subcontinental mantle lithosphere is probably cold and refractory); (2) lack of consistency between the tectonic history of the region and geochemical variations through time. Instead, parental magmas in the CVZ are thought to be generated by mixing between normal arc magmas originating in the depleted mantle wedge followed by contamination and homogenization with lower crustal melts. In the central Andes, the extent of contamination increased greatly as the crust thickened due to crustal shortening within the last 20 Ma, the thicker crust providing an effective filter to trap and differentiate magma batches repeatedly during ascent.  相似文献   
257.
The mantles of the Earth and Moon are similarly depleted in V, Cr, and Mn relative to the concentrations of these elements in chondritic meteorites. The similar depletions have been used as evidence that the Moon inherited its mantle from the Earth after a giant impact event. We have conducted liquid metal-liquid silicate partitioning experiments for V, Cr, and Mn from 3 to 14 GPa and 1723 to 2573 K to understand the behavior of these elements during planetary core formation. Our experiments have included systematic studies of the effects of temperature, silicate composition, metallic S-content, metallic C-content, and pressure. Temperature has a significant effect on the partitioning of V, Cr, Mn, with all three elements increasing their partitioning into the metallic liquid with increasing temperature. In contrast, pressure is not observed to affect the partitioning behavior. The experimental results show the partitioning of Cr and Mn are hardly dependent on the silicate composition, whereas V partitions more strongly into depolymerized silicate melts. The addition of either S or C to the metallic liquid causes increased metal-silicate partition coefficients for all three elements. Parameterizing and applying the experimental data, we find that the Earth’s mantle depletions of V, Cr, and possibly Mn can be explained by core formation in a high-temperature magma ocean under oxygen fugacity conditions about two log units below the iron-wüstite buffer, though the depletion of Mn may be due entirely to its volatility. However, more oxidizing conditions proposed in recent core formation models for the Earth cannot account for any of the depletions. Additionally, because we observe no pressure effect on the partitioning behavior, the data do not require the mantle of the Moon to be derived from the Earth’s mantle, although this is not ruled out. All that is required to create depletions of V, Cr, and Mn in a mantle is a planetary body that is hot enough and reducing enough during its core formation. Such conditions could have existed on the Moon-forming impactor.  相似文献   
258.
The spatial variability and co-variability of two different types of remote sensing derivatives that portray vegetation and geomorphic patterns are analyzed in the context of estimating regional-scale CO2 flux from land surfaces in the arctic tundra. For a study area encompassing the Kuparuk River watershed of the North Slope of Alaska, we compare satellite-derived maps of the normalized difference vegetation index (NDVI) generated at two different spatial resolutions to a map of vegetation types derived by image classification of data from the Landsat multispectral scanner (MSS). Mean values of NDVI for each cover type stratum are unique (with the exception of moist acidic tundra and shrubland types). Based on analysis of semi-variograms generated for SPOT-NDVI data, most of the vegetation cover and landform features of this arctic tundra landscape have spatial dimensions of less than 1 km. Thaw lakes on the coastal plain and glacial depositional landforms, such as moraines in the foothills, seem to be the largest features, with vegetation units having dimensions no larger than 700 m. Frequency distributions of NDVI and vegetation types extracted for sampling transects flown by an aircraft sensing CO2 flux, relative to distributions for the entire Kuparuk River watershed, suggest a slight sampling bias towards greater cover of mesic wet sedge tundra and thaw lakes and associated lower NDVI values. The regional pattern of NDVI for the North Slope of Alaska corresponds primarily to differences between the two major physiographic provinces of this region.  相似文献   
259.
Mountain rivers can be subject to strong constraints imposed by changes in gradient and grain size supplied by processes such as glaciation and rockfall. Nonetheless, adjustments in the channel geometry and hydraulics of mountain rivers at the reach scale can produce discernible patterns analogous to those in fully alluvial rivers. Mountain rivers can differ in that imposed reach‐scale gradient is an especially important control on reach‐scale channel characteristics, as indicated by examination of North St Vrain Creek in Colorado. North St Vrain Creek drains 250 km2 of the Rocky Mountains. We used 25 study reaches within the basin to examine controls on reach‐scale channel geometry. Variables measured included channel geometry, large woody debris, grain size, and mean velocity. Drainage area at the study reaches ranged from 2·2 to 245 km2, and gradient from 0·013 to 0·147 m m?1. We examined correlations among (1) potential reach‐scale response variables describing channel bankfull dimension and shape, hydraulics, bedform wavelength and amplitude, grain size, ?ow resistance, standard deviation of hydraulic radius, and volume of large woody debris, and (2) potential control variables that change progressively downstream (drainage area, discharge) or that are likely to re?ect a reach‐speci?c control (bed gradient). We tested the hypothesis that response variables correlate most strongly with local bed gradient because of the segmented nature of mountain channels. Results from simple linear regression analyses indicate that most response variables correlate best with gradient, although channel width and width/depth ratio correlate best with discharge. Multiple regression analyses using Mallow's Cp selection criterion and log‐transformation of all variables produced similar results in that most response variables correlate strongly with gradient. These results suggest that the hypothesis is partially supported: channel bed gradient is likely to be a good predictor for many reach‐scale response variables along mountain rivers, but discharge is also an important predictor for some response variables. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
260.
Process-based models used to investigate forest ecosystem response to climate change were not necessarily developed to include the effect of carbon dioxide (CO2) and temperature increases on physiological processes. Simulation of the impacts of climate change with such models may lead to questionable predictions. It is generally believed that significant shifts in the performance of black spruce (Picea mariana [Mill] B.S.P.) will occur under climate change. This species, which accounts for 64% of Ontario's coniferous growing stock and 80% of the annual allowable cut, represents important economic activity throughout the boreal forest region. Forest management planning requires relatively accurate productivity estimates. Thus, it is imperative to ensure that process-based models realistically predict the effect of climate change. In this study, CENTURY and FOREST-BGC models were calibrated for a productive, upland black spruce stand in northwestern Ontario. Even though both models predicted similar relative outcomes after 100 years of climate change, they disagreed on the impacts of temperature in combination with an increase in CO2. Also, absolute amounts of carbon sequestered varied with climate change scenarios. Comparison of both models indicated that the representation of critical processes in these two forest ecosystem models is incomplete. For instance, the interactive effects of CO2 and temperature increases on physiological processes at stand and soil levels are not well documented nor are they easily identifiable in the models. Their incorporation into models is therefore problematic. Practitioners must consequently be wary of assumptions about the inclusion of critical processes in models.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号