全文获取类型
收费全文 | 4268篇 |
免费 | 176篇 |
国内免费 | 92篇 |
专业分类
测绘学 | 157篇 |
大气科学 | 295篇 |
地球物理 | 1116篇 |
地质学 | 1516篇 |
海洋学 | 376篇 |
天文学 | 694篇 |
综合类 | 21篇 |
自然地理 | 361篇 |
出版年
2022年 | 30篇 |
2021年 | 48篇 |
2020年 | 47篇 |
2019年 | 63篇 |
2018年 | 85篇 |
2017年 | 104篇 |
2016年 | 119篇 |
2015年 | 110篇 |
2014年 | 136篇 |
2013年 | 258篇 |
2012年 | 130篇 |
2011年 | 231篇 |
2010年 | 164篇 |
2009年 | 225篇 |
2008年 | 238篇 |
2007年 | 208篇 |
2006年 | 191篇 |
2005年 | 171篇 |
2004年 | 136篇 |
2003年 | 162篇 |
2002年 | 136篇 |
2001年 | 85篇 |
2000年 | 87篇 |
1999年 | 61篇 |
1998年 | 69篇 |
1997年 | 56篇 |
1996年 | 65篇 |
1995年 | 52篇 |
1994年 | 55篇 |
1993年 | 42篇 |
1992年 | 48篇 |
1991年 | 42篇 |
1990年 | 35篇 |
1989年 | 37篇 |
1988年 | 37篇 |
1987年 | 40篇 |
1986年 | 44篇 |
1985年 | 57篇 |
1984年 | 47篇 |
1983年 | 50篇 |
1982年 | 52篇 |
1981年 | 57篇 |
1980年 | 48篇 |
1979年 | 39篇 |
1978年 | 47篇 |
1977年 | 28篇 |
1976年 | 29篇 |
1975年 | 24篇 |
1974年 | 31篇 |
1973年 | 29篇 |
排序方式: 共有4536条查询结果,搜索用时 15 毫秒
61.
A study of an expanding seagrass bed on the south-west coast of British Columbia, Canada involved documentation and explanation of the pattern of expansion of the vegetation as well as documentation and experimental investigation of the accompanying changes in the distribution of infaunal invertebrates. Expansion followed a major environmental change, improved water clarity initiated in 1969 when a causeway blocked access to the site for silty Fraser River water. The original eelgrass (Zostera marina) bed expanded landward over 30 m year−1, probably causing more and more water to be retained in the bed at low tide and thus improving its own habitat. More rapid expansion occurred from 1979–1983 after Z.japonica colonized at the landward edge of the eelgrass bed and in a separate intertidal area. Expansion ceased around 1983 coincident with, but probably independent of, further construction.Burrowing shrimp (Callianassa californiensis) decreased in abundance between 1977 and 1984 in areas where the two seagrasses colonized. Short-term experiment over one summer and fall showed that removal of all shoots allowed adult shrimp and tube worms to colonize the sediment while addition of shoots of Z. japonica by transplanting caused temporary decreases in shrimp abundance. After a few weeks an established shrimp population destroyed the transplants. Among reasonable alternate hypotheses for the decline in Callianassa, the effect of sediment texture can be eliminated, but either direct negative effects of seagrass, i.e. inhibition of the burrowing of adult shrimp or of settlement of juveniles, or indirect effects, i.e. the harboring of more predators of shrimp in seagrass beds, deserve further study. 相似文献
62.
63.
Watson W. Gregg Paul Ginoux Paul S. Schopf Nancy W. Casey 《Deep Sea Research Part II: Topical Studies in Oceanography》2003,50(22-26):3143
The JGOFS program and NASA ocean-color satellites have provided a wealth of data that can be used to test and validate models of ocean biogeochemistry. A coupled three-dimensional general circulation, biogeochemical, and radiative model of the global oceans was validated using these in situ data sources and satellite data sets. Biogeochemical processes in the model were determined from the influences of circulation and turbulence dynamics, irradiance availability, and the interactions among four phytoplankton functional groups (diatoms, chlorophytes, cyanobacteria, and coccolithophores) and four nutrients (nitrate, ammonium, silica, and dissolved iron).Annual mean log-transformed dissolved iron concentrations in the model were statistically positively correlated on basin scale with observations (P<0.05) over the eight (out of 12) major oceanographic basins where data were available. The model tended to overestimate in situ observations, except in the Antarctic where a large underestimate occurred. Inadequate scavenging and excessive remineralization and/or regeneration were possible reasons for the overestimation.Basin scale model chlorophyll seasonal distributions were positively correlated with SeaWiFS chlorophyll in each of the 12 oceanographic basins (P<0.05). The global mean difference was 3.9% (model higher than SeaWiFS).The four phytoplankton groups were initialized as homogeneous and equal distributions throughout the model domain. After 26 years of simulation, they arrived at reasonable distributions throughout the global oceans: diatoms predominated high latitudes, coastal, and equatorial upwelling areas, cyanobacteria predominated the mid-ocean gyres, and chlorophytes and coccolithophores represented transitional assemblages. Seasonal patterns exhibited a range of relative responses: from a seasonal succession in the North Atlantic with coccolithophores replacing diatoms as the dominant group in mid-summer, to successional patterns with cyanobacteria replacing diatoms in mid-summer in the central North Pacific. Diatoms were associated with regions where nutrient availability was high. Cyanobacteria predominated in quiescent regions with low nutrients.While the overall patterns of phytoplankton functional group distributions exhibited broad qualitative agreement with in situ data, quantitative comparisons were mixed. Three of the four phytoplankton groups exhibited statistically significant correspondence across basins. Diatoms did not. Some basins exhibited excellent correspondence, while most showed moderate agreement, with two functional groups in agreement with data and the other two in disagreement. The results are encouraging for a first attempt at simulating functional groups in a global coupled three-dimensional model but many issues remain. 相似文献
64.
65.
Ocean Waves: Half-a-Century of Discovery 总被引:1,自引:0,他引:1
Paul H. LeBlond 《Journal of Oceanography》2002,58(1):3-9
While the nature of most ocean waves has long been known and their basic physics understood since the nineteenth century,
intense study of ocean waves during the second half of the twentieth century has taken the subject from the realm of mathematical
exercises to that of practical engineering. Modern understanding of the generation, propagation and interactions of ocean
waves with each other and with oceanic features has advanced to a quantitative level offering predictive capacity. This paper
presents a brief qualitative review of advances in knowledge of sound waves, wind waves, tsunamis, tides, internal waves and
long-period vorticity waves. The review is aimed at non-specialists who may benefit from an overview of the current state
of the subject and access to a bibliography of general-interest references.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
66.
67.
This paper describes the collection, reduction, and analysis of 0.4–1.0 m Mars imaging spectroscopy data obtained during the 1988 and 1990 oppositions from Mauna Kea Observatory and provides a general outline for the acquisition and analysis of similar imaging spectroscopy data sets. The U.H. 2.24-m Wide Field Grism CCD Spectrograph was used to collect 13 three-dimensional image cubes covering 90% of the planet south of 50°N in the 0.4–0.8 m region (/=245 at 0.6 m) and covering 55% of the planet south of 50°N in the 0.5–1.0 m region (/=293 at 0.75 m). Spectra extracted from these image cubes reveal the detailed character of the martian near-UV to visible spectrum. Images at red wavelengths reveal the classical albedo markings at 100–500 km spatial resolution while images at blue wavelengths show little surface feature contrast and are dominated by condensate clouds/hazes and polar ice. Many of the data acquisition, reduction, and analysis steps discussed here are new or unique to imaging spectroscopy data sets. These techniques exploit the information contained within the spatial domain of data such as these, thus allowing more traditional point-spectral analysis techniques to be expanded into an imaging format. 相似文献
68.
Guadalupian (Middle Permian) Conodont Faunas at Shangsi Section, Northeast Sichuan Province 总被引:1,自引:0,他引:1
This study recovered the Guadalupian conodont faunas from Shangsi (上寺) Section in Northeast Sichuan (四川). Four genera and nine species were identified, and three conodont zones were recognized and established; they include Jinogondolella nankingensis Zone, J. aserrata Zone, and J. postserrata Zone. The Roadian and Wordian boundary is set in the interval 5 m from the top of Bed 86 by the first appearance of the conodont J. aserrata. The Wordian and Capitanian boundary is set in the interval 2.3 m from the top of the Bed 95 by the first appearance of J. postserrata. Most of the specimens demonstrated low color alteration index (CAI) as 1.5-3, indicating that most part of the Maokou (茅口) Formation may have the suitable thermal conditions for the formation of oil source rocks. 相似文献
69.
Paul A. Crowther Orsola De Marco M. J. Barlow P. J. Storey 《Astrophysics and Space Science》1996,238(1):119-123
We present a detailed, quantitative study of the standard [WC10] Wolf-Rayet central star CPD-56o 8032 based on new high resolution AAT UCLES observations and the Hillier (1990) WR standard model. Our analysis of CPD-56o 8032 gives the wind properties (T
*=34500K, lg (L/L
)=3.8, lg (M/M
a–1)=–5.4,v
=225 km s–1) and chemistry (C/He=0.5, O/He=0.1, by number), the latter suggesting an intimate relationship with the Ovi PN central stars and the PG 1159-035 objects. A comparison between the wind properties of CPD-56o 8032 and Sk-66o 40 (WN 10) indicates that low excitation, low wind velocity WR winds are common to both low mass PN central stars (WC sequence) and high mass post-LBV's (WN sequence). 相似文献
70.
Harry A. Taylor Jr. Leonard Kramer Paul A. Cloutier Shannon S. Walker 《Earth, Moon, and Planets》1995,69(2):173-199
Plasma and field relationships observed across the nightside of Venus evidence a chaotic variety of interactions between the ionosphere and the combined effect of the solar wind and interplanetary magnetic field draped about the planet. Close examination of these data reveal within the chaos a number of repeatable signatures key to understanding fundamental field-plasma interactions. Observed from the Pioneer Venus Orbiter, (PVO), nightside conditions range from extensive, full-up ionospheres with little evidence of dynamic or energetic perturbations, to an almost full depletion, sometimes described as disappearing ionospheres. Between these extremes, the ionospheric structure is often irregular, sometimes exhibiting well-defined density troughs, at other times complex intervals of either abundant or minimal plasma concentration. Consistently, large B-fields (typically exceeding 5–10 nanoteslas) coincide with plasma decreases, whereas stable, abundant plasma distributions are associated with very low-level field. We examine hundreds of nightside orbits, identifying close correlations between regions of elevated magnetic fields featuring polarity reversals, and (a) exclusive low-frequency or distinctive broadband noise, or both, in the electric field data, (b) turbulent, superthermal behavior of the the ions and electrons. We review extensive studies of nightside fields to show that the correlations observed are consistent with theoretical arguments that the presence of strong magnetic fields within normal ionospheric heights indicates the intrusion of magnetosheath fields and plasma within such regions. We find abundant evidence that the ionosphere is frequently disrupted by such events, exhibiting a chaotic, auroral-like complexity appearing over a wide range of altitude and local time. We show that field-plasma disturbances, widely suggested to be similar to conditions in the Earth's auroral regions, are tightly linked to the electric field noise otherwise attributed to lightning. Owing to the coincidence inherent in this relationship, we suggest that natural, predictable plasma instabilities associated with the plasma gradients and current sheets evident within these events produce the E-field noise. The data relationships argue for a more detailed investigation of solar wind induced E-field noise mechanisms as the appropriate scientific procedure for invoking sources for the noise previously attributed to lightning. Consistent with these views, we note that independent analyses have offered alternative explanations of the noise as arising from ionospheric disturbances, that repeated searches for optical evidence of lightning have found no such evidence, and that no accepted theoretical work has yet surfaced to support the inference of lightning at Venus. 相似文献