全文获取类型
收费全文 | 8267篇 |
免费 | 292篇 |
国内免费 | 96篇 |
专业分类
测绘学 | 200篇 |
大气科学 | 568篇 |
地球物理 | 1932篇 |
地质学 | 2983篇 |
海洋学 | 752篇 |
天文学 | 1210篇 |
综合类 | 19篇 |
自然地理 | 991篇 |
出版年
2021年 | 89篇 |
2020年 | 113篇 |
2019年 | 114篇 |
2018年 | 168篇 |
2017年 | 165篇 |
2016年 | 226篇 |
2015年 | 194篇 |
2014年 | 198篇 |
2013年 | 470篇 |
2012年 | 249篇 |
2011年 | 309篇 |
2010年 | 282篇 |
2009年 | 339篇 |
2008年 | 321篇 |
2007年 | 281篇 |
2006年 | 315篇 |
2005年 | 238篇 |
2004年 | 291篇 |
2003年 | 263篇 |
2002年 | 266篇 |
2001年 | 173篇 |
2000年 | 172篇 |
1999年 | 141篇 |
1998年 | 141篇 |
1997年 | 104篇 |
1996年 | 114篇 |
1995年 | 114篇 |
1994年 | 128篇 |
1993年 | 113篇 |
1992年 | 114篇 |
1991年 | 109篇 |
1990年 | 101篇 |
1989年 | 85篇 |
1988年 | 87篇 |
1987年 | 123篇 |
1986年 | 102篇 |
1985年 | 164篇 |
1984年 | 187篇 |
1983年 | 144篇 |
1982年 | 126篇 |
1981年 | 131篇 |
1980年 | 108篇 |
1979年 | 125篇 |
1978年 | 118篇 |
1977年 | 103篇 |
1976年 | 96篇 |
1975年 | 91篇 |
1974年 | 67篇 |
1973年 | 83篇 |
1972年 | 49篇 |
排序方式: 共有8655条查询结果,搜索用时 15 毫秒
101.
Francine McCarthy Sarah Tiffin Adam Sarvis John McAndrews Stephan Blasco 《Journal of Paleolimnology》2012,47(3):429-445
Microfossils have been critical in unravelling the complex postglacial history of Georgian Bay. Thecamoebians (testate amoebae/rhizopods)
record paleolimnological conditions, and pollen stratigraphy allows correlation across the basin, where sedimentation has
been spatially and temporally discontinuous. Because parts of Georgian Bay have been non-depositional or erosional since the
end of the Nipissing transgression (~5,000 (5,800 cal) BP), early Holocene features are exposed on the lakebed. Among these
are shoreline features, such as submerged beaches and relict channels, associated with low-level Lake Hough that was driven
far below the level of basin overflow. Cores taken throughout Georgian Bay record the existence of closed basin conditions
that persisted several centuries around 7,500 (8,300 cal) BP, corresponding to the late Lake Hough lowstand. Evidence for
hydrologic closure includes a low-diversity centropyxid-dominated thecamoebian fauna around the boundary between pollen subzones
2a and 2b in the Flowerpot Beach core, Flowerpot and Killarney basins, and in Severn Sound. This low-diversity centropyxid-dominated
fauna is interpreted as recording the development of slightly brackish conditions as a result of a hydrologic deficit associated
with relatively arid conditions in the Great Lakes basin during the early Holocene pine zone (~8,800–7,200 (9,900–8,050 cal)
BP). The rest of the Holocene record in Georgian Bay (where it is preserved) is more diverse and dominated by difflugiid thecamoebians:
predominantly Difflugia oblonga prior to human settlement, and Cucurbitella tricuspis since high-density human occupation and agriculture (and resulting eutrophication) began with the Wendat First Nations people
around Severn Sound about 750 years ago. The implication that water budget fluctuations leading to discernible variations
in lake level and water chemistry occurred in the relatively recent geologic past is significant to studies of global climate
change and resource management in the Great Lakes, one of the world’s largest freshwater resources. 相似文献
102.
现代地层学研究中一个关键的问题是陆相和浅海相地层的对比和整和,尽管层序地层学在两种地层中均有很大发展,可是很少有成功的两者对比研究,本文的目的就是用陆地有机相和海相地层水进水退来产生一种新的层序地层演化的整和模式。 相似文献
103.
John A. Knox 《Mathematical Geology》2000,32(2):203-215
The Fahrenheit-to-Celsius temperature-conversion equation is a basic component of many introductory earth science courses. Despite its simplicity, it presents a challenge to students and instructors alike because residents of the United States are unfamiliar with the Celsius scale. By solving for the point at which these two temperature scales are equal, it is possible to use the equations for temperature conversion as a springboard to more advanced topics. It is demonstrated that temperature-conversion equations and chaotic equations can be solved using identical numerical and graphical techniques. As a result, the fundamental concepts of chaos theory and numerical methods can be introduced to students in the context of the simplest equations in the earth sciences. These solution methods are applied to the quantitative theory of the extratropical cyclone as an example of the utility and broad scope of this educational approach. 相似文献
104.
105.
Johanna Mattila Glen Chaplin Michele R. Eilers Kenneth L. Heck Jr. Jonathan P. O'Neal John F. Valentine 《Journal of Sea Research》1999,41(4):209
Fish, epibenthos and macroinfauna were collected in a Zostera marina bed and nearby unvegetated sediments in the estuary of the Damariscotta River, on the mid-coast of Maine. Samples of epibenthic fauna and fish were collected at low tides both during day and night, and samples of infauna at low tides during the day. The mean density of Zostera shoots in the study area was 335 m−2. Abundance and species number of fish were greater at night than during the day and greater in eelgrass beds (Z. marina) than in unvegetated habitats. Daytime fish collections were dominated by Atlantic silversides (Medinia medinia), while juvenile winter flounder (Pseudopleuronectes americanus) dominated night collections. Also Zostera-associated epifaunal abundances and number of species were significantly higher at night than during the day. Mysis stenolepis, Idotea balthica and Littorina obtusata were dominant species in the epifauna samples. Of the total of 37 invertebrate species encountered, only five occurred both in the infaunal and epifaunal samples. Nineteen different taxa were collected from the benthic core samples. The most abundant invertebrate infaunal taxa were sipunculids, the polychaete Nereis virens, and oligochaetes. Infaunal invertebrate abundances and species diversity were significantly higher in eelgrass beds than in unvegetated sediments. The abundance and number of species of benthic invertebrates were also positively correlated to seagrass biomass. Community diversity values (H′) were relatively low but fit well in the general pattern of decreasing diversity towards northern latitudes. 相似文献
106.
Christian Betzler Jörn Fürstenau Thomas Lüdmann Christian Hübscher Sebastian Lindhorst Andreas Paul John J. G. Reijmer André W. Droxler 《Basin Research》2013,25(2):172-196
Multichannel high‐resolution seismic and multibeam data were acquired from the Maldives‐isolated carbonate platform in the Indian Ocean for a detailed characterization of the Neogene bank architecture of this edifice. The goal of the research is to decipher the controlling factors of platform evolution, with a special emphasis on sea‐level changes and changes of the oceanic currents. The stacking pattern of Lower to Middle Miocene depositional sequences, with an evolution of a ramp geometry to a flat‐topped platform, reflects variations of accommodation, which here are proposed to be primarily governed by fluctuations of relative sea level. Easterly currents during this stage of bank growth controlled an asymmetric east‐directed progradation of the bank edge. During the late middle Miocene, this system was replaced by a twofold configuration of bank development. Bank growth continued synchronously with partial bank demise and associated sediment‐drift deposition. This turnover is attributed to the onset and/or intensification of the Indian monsoon and related upwelling and occurrence of currents, locally changing environmental conditions and impinging upon the carbonate system. Mega spill over lobes, shaped by reversing currents, formed as large‐scale prograding complexes, which have previously been interpreted as deposits formed during a forced regression. On a regional scale, a complex carbonate‐platform growth can occur, with a coexistence of bank‐margin progradation and aggradation, as well as partial drowning. It is further shown that a downward shift of clinoforms and offlapping geometries in carbonate platforms are not necessarily indicative for a sea‐level driven forced regression. Findings are expected to be applicable to other examples of Cenozoic platforms in the Indo‐Pacific region. 相似文献
107.
A possible suture between an old craton in West Africa and the Arabian—Nubian shield is marked by a persistent zone of Phanerozoic weakness. This zone extends between the Nile and Niger deltas and passes through the Qattara embayment, along the Bahariya fault, through the Kufra basin, between the Tibesti and Jabal al Uwaynat massifs, through the Bahr el Ghazal and Lake Chad areas, and along the Benoue trough. 相似文献
108.
109.
110.
Reliable land cover land use (LCLU) information, and change over time, is important for Green House Gas (GHG) reporting for climate change documentation. Four different organizations have independently created LCLU maps from 2010 satellite imagery for Malawi for GHG reporting. This analysis compares the procedures and results for those four activities. Four different classification methods were employed; traditional visual interpretation, segmentation and visual labelling, digital clustering with visual identification and supervised signature extraction with application of a decision rule followed by analyst editing. One effort did not report classification accuracy and the other three had very similar and excellent overall thematic accuracies ranging from 85 to 89%. However, despite these high thematic accuracies there were very significant differences in results. National percentages for forest ranged from 18.2 to 28.7% and cropland from 40.5 to 53.7%. These significant differences are concerns for both remote-sensing scientists and decision-makers in Malawi. 相似文献