全文获取类型
收费全文 | 1537篇 |
免费 | 101篇 |
国内免费 | 20篇 |
专业分类
测绘学 | 64篇 |
大气科学 | 122篇 |
地球物理 | 422篇 |
地质学 | 464篇 |
海洋学 | 115篇 |
天文学 | 321篇 |
综合类 | 4篇 |
自然地理 | 146篇 |
出版年
2022年 | 2篇 |
2021年 | 34篇 |
2020年 | 35篇 |
2019年 | 40篇 |
2018年 | 66篇 |
2017年 | 46篇 |
2016年 | 59篇 |
2015年 | 66篇 |
2014年 | 66篇 |
2013年 | 89篇 |
2012年 | 80篇 |
2011年 | 85篇 |
2010年 | 71篇 |
2009年 | 101篇 |
2008年 | 90篇 |
2007年 | 70篇 |
2006年 | 81篇 |
2005年 | 77篇 |
2004年 | 82篇 |
2003年 | 61篇 |
2002年 | 63篇 |
2001年 | 47篇 |
2000年 | 36篇 |
1999年 | 36篇 |
1998年 | 36篇 |
1997年 | 16篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 10篇 |
1993年 | 13篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有1658条查询结果,搜索用时 46 毫秒
991.
Recent habitat suitability models used to predict the occurrence of vulnerable marine species, particularly framework building cold-water corals, have identified terrain attributes such as slope and bathymetric position index as important predictive parameters. Due to their scale-dependent nature, a realistic representation of terrain attributes is crucial for the development of reliable habitat suitability models. In this paper, three known coral areas and a noncoral control area off the west coast of Ireland were chosen to assess quantitative and distributional differences between terrain attributes derived from bathymetry grids of varying resolution and information content. Correlation analysis identified consistent changes of terrain attributes as grain size was altered. Response characteristics and dimensions depended on terrain attribute types and the dominant morphological length-scales within the study areas. The subsequent effect on habitat suitability maps was demonstrated by preliminary models generated at different grain sizes. This study demonstrates that high resolution habitat suitability models based on terrain parameters derived from multibeam generated bathymetry are required to detect many of the topographical features found in Irish waters that are associated with coral. This has implications for marine spatial planning in the deep sea. Supplemental materials are available for this article. Go to the publisher's online edition of Marine Geodesy to view the free supplemental file. 相似文献
992.
Our Sun and planetary system were born about 4.5 billion years ago. How did this happen, and what is the nature of our heritage from these early times? This review tries to address these questions from an astrochemical point of view. On the one hand, we have some crucial information from meteorites, comets and other small bodies of the Solar System. On the other hand, we have the results of studies on the formation process of Sun-like stars in our Galaxy. These results tell us that Sun-like stars form in dense regions of molecular clouds and that three major steps are involved before the planet-formation period. They are represented by the prestellar core, protostellar envelope and protoplanetary disk phases. Simultaneously with the evolution from one phase to the other, the chemical composition gains increasing complexity. In this review, we first present the information on the chemical composition of meteorites, comets and other small bodies of the Solar System, which is potentially linked to the first phases of the Solar System??s formation. Then we describe the observed chemical composition in the prestellar core, protostellar envelope and protoplanetary-disk phases, including the processes that lead to them. Finally, we draw together pieces from the different objects and phases to understand whether and how much we inherited chemically from the time of the Sun??s birth. 相似文献
993.
Chris Yakymchuk Lyal B. Harris Laurent Godin 《International Journal of Earth Sciences》2012,101(2):463-482
Centrifuge analogue modelling illustrates the progressive development of active folds in multilayers upon a ductile substrate
during layer-parallel shortening. Models simulate folding of a mechanically stratified sedimentary sequence upon migmatitic
gneisses in a large hot orogen, or upon a thick basal evaporite ± shale sequence in deeper levels of fold belts. The absence
of a weak low-viscosity and low-density layer at the interface promotes infolding of the cover sequence and ductile substrate,
whereas a planar upper surface to the basal ductile substrate is preserved when it is present. Whilst fold style, wavelength,
and deformation of the interface with the ductile substrate differ depending on whether a low-viscosity and low-density layer
is present at the base of the cover sequence, there is no marked systematic curvature of fold axes as seen in previous sandbox
models for fault-bend or fault propagation folding during bulk shortening. Bulk shortening of a layered sequence with relatively
thick individual layers above a ductile substrate promotes a regular and upright train of buckle folds, whereas thinner layers
promote a more irregular distribution of buckle folds with variable vergence, style, and amplitude. Buckle folds above a ductile
substrate progressively develop during bulk shortening from open and upright, to angular and tight, and may further develop
into cuspate structures above relatively weak horizons. Relatively thick weak horizons within the layered sequence during
bulk shortening interrupt regular fold patterns up structural section and allow out-of-phase folds to develop above and below
the weak horizon. 相似文献
994.
We developed a food web model of central Puget Sound to provide science-based support for ecosystem-based management and to
refine our understanding of bottom-up and top-down trophic forcing. Phytoplankton accounted for a large fraction of total
biomass, total throughput, and caused considerable bottom-up effects in most functional groups in a dynamic simulation fit
to time series data from 1981 to 2000. Top-down control was most apparent in the case of bald eagles (Haliaeetus leucocephalus), which exhibited keystone tendencies and appeared capable of causing trophic cascades. Increasing top-down control in several
predator–prey relationships improved model fits to time series data from 1981 to 2000, but not as much as introducing non-equilibrium
dynamics (biomass accumulation terms) to several key vertebrates. Fishing had little effect on system dynamics. Our model
appears well-suited for addressing strategic, scenario-based questions of how the community as a whole will respond to management
actions. 相似文献
995.
Travis?J.?TennerEmail author Marc?M.?Hirschmann Anthony?C.?Withers Paola?Ardia 《Contributions to Mineralogy and Petrology》2012,163(2):297-316
The onset of hydrous partial melting in the mantle above the transition zone is dictated by the H2O storage capacity of peridotite, which is defined as the maximum concentration that the solid assemblage can store at P and T without stabilizing a hydrous fluid or melt. H2O storage capacities of minerals in simple systems do not adequately constrain the peridotite water storage capacity because
simpler systems do not account for enhanced hydrous melt stability and reduced H2O activity facilitated by the additional components of multiply saturated peridotite. In this study, we determine peridotite-saturated
olivine and pyroxene water storage capacities at 10–13 GPa and 1,350–1,450°C by employing layered experiments, in which the
bottom ~2/3 of the capsule consists of hydrated KLB-1 oxide analog peridotite and the top ~1/3 of the capsule is a nearly
monomineralic layer of hydrated Mg# 89.6 olivine. This method facilitates the growth of ~200-μm olivine crystals, as well
as accessory low-Ca pyroxenes up to ~50 μm in diameter. The presence of small amounts of hydrous melt ensures that crystalline
phases have maximal H2O contents possible, while in equilibrium with the full peridotite assemblage (melt + ol + pyx + gt). At 12 GPa, olivine and
pyroxene water storage capacities decrease from ~1,000 to 650 ppm, and ~1,400 to 1,100 ppm, respectively, as temperature increases
from 1,350 to 1,450°C. Combining our results with those from a companion study at 5–8 GPa (Ardia et al., in prep.) at 1,450°C,
the olivine water storage capacity increases linearly with increasing pressure and is defined by the relation
C\textH2 \textO\textolivine ( \textppm ) = 57.6( ±16 ) ×P( \textGPa ) - 169( ±18 ). C_{{{\text{H}}_{2} {\text{O}}}}^{\text{olivine}} \left( {\text{ppm}} \right) = 57.6\left( { \pm 16} \right) \times P\left( {\text{GPa}} \right) - 169\left( { \pm 18} \right). Adjustment of this trend for small increases in temperature along the mantle geotherm, combined with experimental determinations
of
D\textH2 \textO\textpyx/olivine D_{{{\text{H}}_{2} {\text{O}}}}^{\text{pyx/olivine}} from this study and estimates of
D\textH2 \textO\textgt/\textolivine D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{gt}}/{\text{olivine}}}} , allows for estimation of peridotite H2O storage capacity, which is 440 ± 200 ppm at 400 km. This suggests that MORB source upper mantle, which contains 50–200 ppm
bulk H2O, is not wet enough to incite a global melt layer above the 410-km discontinuity. However, OIB source mantle and residues
of subducted slabs, which contain 300–1,000 ppm bulk H2O, can exceed the peridotite H2O storage capacity and incite localized hydrous partial melting in the deep upper mantle. Experimentally determined values
of
D\textH2 \textO\textpyx/\textolivine D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{pyx}}/{\text{olivine}}}} at 10–13 GPa have a narrow range of 1.35 ± 0.13, meaning that olivine is probably the most important host of H2O in the deep upper mantle. The increase in hydration of olivine with depth in the upper mantle may have significant influence
on viscosity and other transport properties. 相似文献
996.
997.
Increasing penetration by the market into the governing of agri-environments, and the use of market-oriented approaches in an attempt to produce more sustainable outcomes, is a characteristic feature of what scholars have called the ‘neoliberalisation of nature’. While accepting that neoliberal forms of governing tend to extend market relations into new domains, a number of scholars have argued that they may at the same time create spaces of resistance, open up progressive political possibilities, or incorporate alternative rationalities of governing. This literature has so far focused primarily on the policy and/or programme level with limited connection made to the growing body of research that explores landholder responses to specific market instruments. We address this gap by focusing on a market instrument – Wimmera Habitat Tender – in the State of Victoria, Australia, which aims to provide incentives for farmers in managing native vegetation. This case study explores how a specific tender-based market instrument seeks to construct natural resource managers as neoliberal subjects, as well as the complex ways in which farmers contest or resist the neoliberal governing of their agri-environmental practices. Through our analysis we contend that closer scrutiny of how the techniques underpinning market-based environmental instruments are taken up or resisted contributes to a more robust understanding of the environmental possibilities created by market instruments, as well as the challenges involved in attempts to neoliberalise nature. 相似文献
998.
东南极拉斯曼丘陵泥质麻粒岩变质作用演化 总被引:5,自引:0,他引:5
普里兹湾拉斯曼丘陵代表了东南极一条重要的早古生代的~530Ma泛非期(Pan-African)高级构造活动带。然而,该区早期的晚元古代的~1000Ma格林维尔期(Grenvellian)高级变质作用的演化历史至今仍有争论。该区呈透镜状产出的泥质麻粒岩峰期矿物组合(M1)为石榴石+堇青石+斜方辉石+钾长石+石英,峰期石榴石变斑晶发育堇青石或堇青石+斜方辉石反应边(M2)。利用Thermocalc程序在KFMASH模式体系对该泥质麻粒岩进行的定量模拟表明,其峰期矿物组合是由反应石榴石+黑云母+石英=堇青石+斜方辉石+钾长石+熔体形成的。利用Themocalc平均P-T计算方法获得峰期M1变质P-T条件为~0.9GPa和~900℃,而叠加的M2组合反映了一个减压冷却的过程,其变质P-T条件为~0.7GPa和800~850℃。结合已有的年代学数据,认为该区泥质麻粒岩的峰期M1矿物组合反映晚元古代(~1000Ma)格林维尔期挤压D1构造事件,而叠加的M2矿物组合与M3蠕虫状结构则形成于早古生代泛非期(~530Ma)D2~D3高级扭压剪切构造期间。该扭压事件导致了面状高低应变带的发育以及进步花岗岩和伟晶岩的侵入。 相似文献
999.
News media influence local to global interactions between people, societies, and governments by producing place images. Representations
of Africa in Western news media are heavily imbued with colonialist notions of cultural geography. In particular, Western
news media have represented conflicts in Africa as ‘tribal’, a trope that erases geographic and historical context, and discourages
actions that could prevent or reduce violent conflict. To determine if ‘tribalism’ remains important in coverage of African
conflict, we use framing analysis to evaluate news on Sudan’s Darfur region in The New York Times and The Washington Post during 2003–2009. We find that these newspapers predictably relied on stereotypes related to tribalism to simplify Darfur’s
geography and make the conflict meaningful to intended readers. Tribal portrayal of African war is inherently political, and,
problematically, neither newspaper recognized that their use of the tribal narrative was parallel to the views of both the
Sudanese government and external observers that challenged the actions of the Sudanese government. However, we also found
that stereotypical representations became less prominent over time, apparently because reporters found that the initial, simplistic
framing of the conflict did not match their encounters with geographic reality. We emphasize the decline in stereotypical
tropes, because this suggests behind-the-scenes negotiation about representations in these news organizations. Recognizing
voices that challenge stereotypical portrayals is necessary to developing place images that are geographically more accurate. 相似文献
1000.
D. Lorenzetti S. Antoniucci T. Giannini A. Di Paola A. A. Arkharov V. M. Larionov 《Astrophysics and Space Science》2013,343(2):535-539
We present new near-infrared (IR) spectra (0.80–1.35 μm) of the pre-Main Sequence source PV Cep taken during a monitoring program of eruptive variables we are conducting since some years. Simultaneous photometric and spectroscopic observations are systematically carried out during outburst and quiescence periods. By correlating extinction-free parameters, such as HI recombination lines and underlying continuum, it is possible to infer on the mechanism(s) responsible for their origin. Accretion and mass loss processes have a dominant role in determining the PV Cep irregular variability of both continuum and line emission. The potentialities of the observational modality are also discussed. 相似文献