全文获取类型
收费全文 | 21305篇 |
免费 | 2682篇 |
国内免费 | 2838篇 |
专业分类
测绘学 | 2978篇 |
大气科学 | 3876篇 |
地球物理 | 3910篇 |
地质学 | 6234篇 |
海洋学 | 2370篇 |
天文学 | 169篇 |
综合类 | 1485篇 |
自然地理 | 5803篇 |
出版年
2024年 | 125篇 |
2023年 | 365篇 |
2022年 | 840篇 |
2021年 | 1029篇 |
2020年 | 972篇 |
2019年 | 1087篇 |
2018年 | 792篇 |
2017年 | 1008篇 |
2016年 | 979篇 |
2015年 | 978篇 |
2014年 | 1216篇 |
2013年 | 1664篇 |
2012年 | 1221篇 |
2011年 | 1217篇 |
2010年 | 1028篇 |
2009年 | 1216篇 |
2008年 | 1254篇 |
2007年 | 1316篇 |
2006年 | 1224篇 |
2005年 | 1073篇 |
2004年 | 915篇 |
2003年 | 810篇 |
2002年 | 733篇 |
2001年 | 664篇 |
2000年 | 520篇 |
1999年 | 437篇 |
1998年 | 451篇 |
1997年 | 380篇 |
1996年 | 256篇 |
1995年 | 248篇 |
1994年 | 218篇 |
1993年 | 150篇 |
1992年 | 119篇 |
1991年 | 76篇 |
1990年 | 56篇 |
1989年 | 47篇 |
1988年 | 44篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 17篇 |
1984年 | 16篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Sediment proxy records from a continuous, 1.5 million year long deep‐sea sediment core from a site in the western Norwegian Sea were used to obtain new insights into the nature of palaeoceanographic change in the northern North Atlantic (Nordic seas) during the climatic shift of the Mid‐Pleistocene Revolution (MPR). Red‐green sediment colour and magnetic susceptibility records both reveal significant differences in their mean values when comparing the intervals older than 700 000 yr (700 ka) with those from the past 500 kyr. The timing and duration of these changes indicates that the MPR in the Nordic seas is characterised by a gradual transition lasting about 200 kyr. Together with further sedimentological evidence this suggests that the mid‐Pleistocene climate shift was accompanied by a general change in ice‐drift pattern. It is further proposed that prior to the onset of the major late Pleistocene glaciations in the Northern Hemisphere a significant proportion of the ice in the eastern Nordic seas originated from a southern provenance, whereas later it dominantly came from the surrounding landmasses. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
2.
Advanced material constitutive models are used to describe complex soil behaviour. These models are often used in the solution of boundary value problems under general loading conditions. Users and developers of constitutive models need to methodically investigate the represented soil response under a wide range of loading conditions. This paper presents a systematic procedure for probing constitutive models. A general incremental strain probe, 6D hyperspherical strain probe (HSP), is introduced to examine rate‐independent model response under all possible strain loading conditions. Two special cases of HSP, the true triaxial strain probe (TTSP) and the plane‐strain strain probe (PSSP), are used to generate 3‐D objects that represent model stress response to probing. The TTSP, PSSP and general HSP procedures are demonstrated using elasto‐plastic models. The objects resulting from the probing procedure readily highlight important model characteristics including anisotropy, yielding, hardening, softening and failure. The PSSP procedure is applied to a Neural Network (NN) based constitutive model. It shows that this probing is especially useful in understanding NN constitutive models, which do not contain explicit functions for yield surface, hardening, or anisotropy. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
3.
Kenai, located on the west coast of the Kenai Peninsula, Alaska, subsided during the great earthquake of AD 1964. Regional land subsidence is recorded within the estuarine stratigraphy as peat overlain by tidal silt and clay. Reconstructions using quantitative diatom transfer functions estimate co‐seismic subsidence (relative sea‐level rise) between 0.28±0.28 m and 0.70±0.28 m followed by rapid post‐seismic recovery. Stratigraphy records an earlier co‐seismic event as a second peat‐silt couplet, dated to ~1500–1400 cal. yr BP with 1.14±0.28 m subsidence. Two decimetre‐scale relative sea‐level rises are more likely the result of glacio‐isostatic responses to late Holocene and Little Ice Age glacier expansions rather than to co‐seismic subsidence during great earthquakes. Comparison with other sites around Cook Inlet, at Girdwood and Ocean View, helps in constructing regional patterns of land‐level change associated with three great earthquakes, AD 1964, ~950–850 cal. yr BP and ~1500–1400 cal. yr BP. Each earthquake has a different spatial pattern of co‐seismic subsidence which indicates that assessment of seismic hazard in southern Alaska requires an understanding of multiple great earthquakes, not only the most recent. All three earthquakes show a pre‐seismic phase of gradual land subsidence that marked the end of relative land uplift caused by inter‐seismic strain accumulation. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
4.
A growing body of evidence implies that the concept of 'treeless tundra' in eastern and northern Europe fails to explain the rapidity of Lateglacial and postglacial tree population dynamics of the region, yet the knowledge of the geographic locations and shifting of tree populations is fragmentary. Pollen, stomata and plant macrofossil stratigraphies from Lake Kurjanovas in the poorly studied eastern Baltic region provide improved knowledge of ranges of north‐eastern European trees during the Lateglacial and subsequent plant population responses to the abrupt climatic changes of the Lateglacial/Holocene transition. The results prove the Lateglacial presence of tree populations (Betula, Pinus and Picea) in the eastern Baltic region. Particularly relevant is the stomatal and plant macrofossil evidence showing the local presence of reproductive Picea populations during the Younger Dryas stadial at 12 900–11 700 cal. a BP, occurring along with Dryas octopetala and arctic herbs, indicating semi‐open vegetation. The spread of Pinus–Betula forest at ca. 14 400 cal. a BP, the rise of Picea at ca. 12 800 cal. a BP and the re‐establishment of Pinus–Betula forest at ca. 11 700 cal. a BP within a span of centuries further suggest strikingly rapid, climate‐driven ecosystem changes rather than gradual plant succession on a newly deglaciated land. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
5.
A new analysis of all 346 published 14C dated Holocene alluvial units in Britain offers a unique insight into the regional impacts of global change and shows how surprisingly sensitive British rivers have been to relatively modest but repeated changes in climate. Fourteen major but probably brief periods of flooding are identified bracketed within the periods 400–1070, 1940–3940, 7520–8100 and at ca. 10 420 cal. yr BP. There is a strong correspondence between climatic deteriorations inferred from mire wet shifts and major periods of flooding, especially at ca. 8000 cal. yr BP and since ca. 4000 cal. yr BP. The unusually long and complete British record also demonstrates that alterations in land cover have resulted in a step change in river basin sensitivity to variations in climate. This has very important implications for assessing and mitigating the impact of increasing severe flooding. In small and medium‐sized river basins land use is likely to play a key role in either moderating or amplifying the climatic signal. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
6.
This paper describes the application of environmental isotopes and injected tracer techniques in estimating the contribution of storms as well as annual precipitation to groundwater recharge and its circulation, in the semi‐arid region of Bagepalli, Kolar district, Karnataka. Environmental isotopes 2H, 18O and 3H were used to study the effect of storms on the hydrological system, and an isotope balance was used to compute the contribution of a storm component to the groundwater. Some of the groundwater samples collected during the post‐storm periods were highly depleted in stable isotope content with higher deuterium excess relative to groundwater from the pre‐storm periods. Significant variation in deuterium excess in groundwater from the same area, collected in two different periods, indicates the different origin of air masses. The estimated recharge component of a storm event of 600 mm to the groundwater was found to be in the range of 117–165 mm. There was no significant variation in environmental tritium content of post‐storm and pre‐storm groundwater, indicating the fast circulation of groundwater in the system. After completion of the environmental isotope work, an injected radiotracer 3H technique was applied to estimate the direct recharge of total precipitation to the groundwater. The estimated recharge to the groundwater is 33 mm of the 550 mm annual precipitation during 1992. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
7.
High‐resolution pollen, plant macrofossil and sedimentary analyses from early Holocene lacustrine sediments on the Faroe Islands have detected a significant vegetation perturbation suggesting a rapid change in climate between ca. 10 380 cal. yr BP and the Saksunarvatn ash (10 240±60 cal. yr BP). This episode may be synchronous with the decline in δ18O values in the Greenland ice‐cores. It also correlates with a short, cold event detected in marine cores from the North Atlantic that has been ascribed to a weakening of thermohaline circulation associated with the sudden drainage of Lake Agassiz into the northwest Atlantic, or, alternatively, a period with distinctly decreased solar forcing. The vegetation sequence begins at ca. 10 500 cal. yr BP with a succession from tundra to shrub‐tundra and increasing lake productivity. Rapid population increases of aquatic plants suggest high summer temperatures between 10 450 and 10 380 cal. yr BP. High pollen percentages, concentrations and influx of Betula, Juniperus and Salix together with macrofossil leaves indicate shrub growth around the site during the initial phases of vegetation colonisation. Unstable conditions followed ca. 10 380 cal. yr BP that changed both the upland vegetation and the aquatic plant communities. A decrease in percentage values of shrub pollen is recorded, with replacement of both aquatics and herbaceous plants by pioneer plant communities. An increase in total pollen accumulation rates not seen in the concentration data suggests increased sediment delivery. The catchment changes are consistent with less seasonal, moister conditions. Subsequent climatic amelioration reinitiated a warmth‐driven succession and catchment stabilisation, but retained high precipitation levels influencing the composition of the post‐event communities. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
8.
C. S. M. Turney K. Van Den Burg S. Wastegrd S. M. Davies N. J. Whitehouse J. R. Pilcher C. Callaghan 《第四纪科学杂志》2006,21(4):335-345
High‐precision correlation of palaeoclimatic and palaeoenvironmental records is crucial for testing hypotheses of synchronous change. Although radiocarbon is the traditional method for dating late Quaternary sedimentary sequences, particularly during the last glacial–interglacial transition (LGIT; 15–9 ka), there are inherent problems with the method, particularly during periods of climate change which are often accompanied by major perturbations in atmospheric radiocarbon content. An alternative method is the use of tephras that act as time‐parallel marker horizons. Within Europe, numerous volcanic centres are known to have erupted during the LGIT, providing considerable potential for high‐precision correlation independent of past radiocarbon fluctuations. Here we report the first identification of the Vedde Ash and Askja Tephra in Ireland, significantly extending the known provenance of these events. We have also identified two new horizons (the Roddans Port Tephras A and B) and tentatively recognise an additional horizon from Vallensgård Mose (Denmark) that provide crucial additional chronological control for the LGIT. Two phases of the Laacher See Tephra (LST) are reported, the lower Laacher See Tephra (LLST) and probably the C2 phase of the Middle Laacher See Tephra (MLST‐C2) indicating a more northeasterly distribution of this fan than reported previously. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
9.
J. van der Plicht B. van Geel S. J. P. Bohncke J. A. A. Bos M. Blaauw A. O. M. Speranza R. Muscheler S. Bjrck 《第四纪科学杂志》2004,19(3):263-269
Accurate chronologies are essential for linking palaeoclimate archives. Carbon‐14 wiggle‐match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas–Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric 14C; (ii) a temporary decline of atmospheric CO2; and (iii) an increase in the GRIP 10Be flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
10.
LabVIEW设计中压力传感器的RBF神经网络温度补偿 总被引:5,自引:0,他引:5
在石油平台注水压力监测系统设计中 ,采用LabVIEW虚拟仪器平台 ,嵌入逼近能力强和收敛速度快的RBF神经网络 ,以人工环境实验数据为样本进行训练 ,实现了压力传感器的智能网络温度补偿。结果显示 ,此方法能够在压力、温度变化较大的恶劣环境下 ,获得很高的补偿精度。 相似文献