全文获取类型
收费全文 | 6640篇 |
免费 | 580篇 |
国内免费 | 167篇 |
专业分类
测绘学 | 261篇 |
大气科学 | 736篇 |
地球物理 | 2238篇 |
地质学 | 2637篇 |
海洋学 | 369篇 |
天文学 | 587篇 |
综合类 | 189篇 |
自然地理 | 370篇 |
出版年
2021年 | 24篇 |
2020年 | 19篇 |
2019年 | 22篇 |
2018年 | 450篇 |
2017年 | 398篇 |
2016年 | 278篇 |
2015年 | 169篇 |
2014年 | 144篇 |
2013年 | 189篇 |
2012年 | 675篇 |
2011年 | 477篇 |
2010年 | 159篇 |
2009年 | 207篇 |
2008年 | 161篇 |
2007年 | 149篇 |
2006年 | 173篇 |
2005年 | 865篇 |
2004年 | 919篇 |
2003年 | 702篇 |
2002年 | 208篇 |
2001年 | 106篇 |
2000年 | 75篇 |
1999年 | 55篇 |
1998年 | 38篇 |
1997年 | 40篇 |
1996年 | 35篇 |
1995年 | 27篇 |
1994年 | 24篇 |
1993年 | 27篇 |
1992年 | 26篇 |
1991年 | 26篇 |
1990年 | 26篇 |
1989年 | 25篇 |
1988年 | 15篇 |
1987年 | 22篇 |
1986年 | 13篇 |
1985年 | 18篇 |
1984年 | 33篇 |
1983年 | 20篇 |
1982年 | 20篇 |
1981年 | 31篇 |
1980年 | 28篇 |
1979年 | 22篇 |
1978年 | 28篇 |
1977年 | 19篇 |
1976年 | 19篇 |
1975年 | 29篇 |
1974年 | 12篇 |
1973年 | 13篇 |
1972年 | 13篇 |
排序方式: 共有7387条查询结果,搜索用时 31 毫秒
91.
92.
Flexure of the Indian plate and intraplate earthquakes 总被引:2,自引:0,他引:2
The flexural bulge in central India resulting from India's collision with Tibet has a wavelength of approximately 670 km.
It is manifest topographically and in the free-air gravity anomaly and the geoid. Calculations of the stress distribution
within a flexed Indian plate reveal spatial variations throughout the depth of the plate and also a function of distance from
the Himalaya. The wavelength (and therefore local gradient) of stress variation is a function of the effective elastic thickness
of the plate, estimates of which have been proposed to lie in the range 40–120 km. The imposition of this stress field on
the northward moving Indian plate appears fundamental to explaining the current distribution of intraplate earthquakes and
their mechanisms. The current study highlights an outer trough south of the flexural bulge in central India where surface
stresses are double the contiguous compressional stresses to the north and south. The Bhuj, Latur and Koyna earthquakes and
numerous other recent reverse faulting events occurred in this compressional setting. The N/S spatial gradient of stress exceeds
2 bars/km near the flexural bulge. The overall flexural stress distribution provides a physical basis for earthquake hazard
mapping and suggests that areas of central India where no historic earthquakes are recorded may yet be the locus of future
damaging events. 相似文献
93.
94.
Bitumens extracted from 2.7 to 2.5 billion-year-old (Ga) shales of the Fortescue and Hamersley Groups in the Pilbara Craton, Western Australia, contain traces of molecular fossils. Based on a combination of molecular characteristics typical of many Precambrian bitumens, their consistently and unusually high thermal maturities, and their widespread distribution throughout the Hamersley Basin, the bitumens can be characterized as ‘probably of Archean age’. Accepting this interpretation, the biomarkers open a new window on Archean biodiversity. The presence of hopanes in the Archean rocks confirms the antiquity of the domain Bacteria, and high relative concentrations of 2α-methylhopanes indicate that cyanobacteria were important primary producers. Oxygenic photosynthesis therefore evolved > 2.7 Ga ago, and well before independent evidence suggests significant levels of oxygen accumulated in the atmosphere. Moreover, the abundance of cyanobacterial biomarkers in shales interbedded with oxide-facies banded iron formations (BIF) indicates that although some Archean BIF might have been formed by abiotic photochemical processes or anoxygenic phototrophic bacteria, those in the Hamersley Group formed as a direct consequence of biological oxygen production. Biomarkers of the 3β-methylhopane series suggest that microaerophilic heterotrophic bacteria, probably methanotrophs or methylotrophs, were active in late Archean environments. The presence of steranes in a wide range of structures with relative abundances like those from late Paleoproterozoic to Phanerozoic sediments is convincing evidence for the existence of eukaryotes in the late Archean, 900 Ma before visible fossil evidence indicates that the lineage arose. Sterol biosynthesis in extant eukaryotes requires molecular oxygen. The presence of steranes together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least ∼1% of the present atmospheric level (PAL) and may have been sufficient to support aerobic respiration. 相似文献
95.
Andesitic pillow lavas containing biogenic, solid bitumen (SB) are a constituent of a Neoproterozoic volcanosedimentary sequence (Teplá-Barrandian unit, Bohemian Massif) in the Mítov area of the Czech Republic. A black shale formation that is crosscut by these andesitic basalts is 565 Ma old. Carbon disulfide extracts of two powdered samples of SB contain 0.2 and 0.3 ppm of C60, respectively, as determined by high-pressure liquid chromatography. The peak assignment based on retention time is fully supported by high-resolution electron ionization mass spectrometry (EI-MS). No C70 was detected, nor was C60 found in two other SB samples from this locality. Other investigated carbonaceous samples from Bohemia (coals and anthracites of Upper Paleozoic age and anthraxolite, graphitoids, and graphite of Upper Proterozoic age) did not contain fullerenes at concentrations above the detection limit of 0.01 ppm. The absence of C60 in these samples was confirmed by EI-MS. The proposed mechanism of fullerene formation involves a primary algal phase, generation of a hydrocarbonaceous mixture in the course of thermal evolution of the sedimentary series, and their high-temperature transformation related to the extrusion of basalt. An important feature for fullerene conservation was the enclosure of fullerenes in SB with a structure similar to glasslike carbon, where the fullerene was protected against oxidation. 相似文献
96.
97.
The climatic impacts of land surface change and carbon management, and the implications for climate-change mitigation policy 总被引:4,自引:0,他引:4
Strategies to mitigate anthropogenic climate change recognize that carbon sequestration in the terrestrial biosphere can reduce the build-up of carbon dioxide in the Earth’s atmosphere. However, climate mitigation policies do not generally incorporate the effects of these changes in the land surface on the surface albedo, the fluxes of sensible and latent heat to the atmosphere, and the distribution of energy within the climate system. Changes in these components of the surface energy budget can affect the local, regional, and global climate. Given the goal of mitigating climate change, it is important to consider all of the effects of changes in terrestrial vegetation and to work toward a better understanding of the full climate system. Acknowledging the importance of land surface change as a component of climate change makes it more challenging to create a system of credits and debits wherein emission or sequestration of carbon in the biosphere is equated with emission of carbon from fossil fuels. Recognition of the complexity of human-caused changes in climate does not, however, weaken the importance of actions that would seek to minimize our disturbance of the Earth’s environmental system and that would reduce societal and ecological vulnerability to environmental change and variability. 相似文献
98.
99.
We measured the partial pressure of oxygen (PO2) in the interstitial gas surrounding the sand-swimming Namib moleEremitalpa granti namibensis. At a sand temperature of 26 °C, which produced a nearly maximal rate of oxygen consumption, thePO2near the noses of the animals averaged only 0·9 kPa (6·7 Torr) below the level in the free atmosphere. High oxygen availability was a result of the notably low metabolic rate in the 20 g mammals and the dry, porous and metabolically inactive nature of dune sand. A mathematical model indicated that normal mammals weighing 200 g or more could comfortably exist completely encased in dune sand. We concluded that the moles' small size and low metabolic rate are not adaptations to hypoxia or hypercapnia underground but are probably related to low food availability and the energetic cost of foraging in their desert environment. 相似文献
100.