全文获取类型
| 收费全文 | 2000篇 |
| 免费 | 60篇 |
| 国内免费 | 23篇 |
专业分类
| 测绘学 | 59篇 |
| 大气科学 | 161篇 |
| 地球物理 | 474篇 |
| 地质学 | 712篇 |
| 海洋学 | 164篇 |
| 天文学 | 368篇 |
| 综合类 | 12篇 |
| 自然地理 | 133篇 |
出版年
| 2021年 | 24篇 |
| 2020年 | 22篇 |
| 2019年 | 24篇 |
| 2018年 | 55篇 |
| 2017年 | 36篇 |
| 2016年 | 58篇 |
| 2015年 | 36篇 |
| 2014年 | 53篇 |
| 2013年 | 102篇 |
| 2012年 | 65篇 |
| 2011年 | 110篇 |
| 2010年 | 80篇 |
| 2009年 | 90篇 |
| 2008年 | 89篇 |
| 2007年 | 76篇 |
| 2006年 | 59篇 |
| 2005年 | 65篇 |
| 2004年 | 45篇 |
| 2003年 | 40篇 |
| 2002年 | 58篇 |
| 2001年 | 36篇 |
| 2000年 | 52篇 |
| 1999年 | 55篇 |
| 1998年 | 32篇 |
| 1997年 | 49篇 |
| 1996年 | 43篇 |
| 1995年 | 34篇 |
| 1994年 | 31篇 |
| 1993年 | 29篇 |
| 1992年 | 21篇 |
| 1991年 | 22篇 |
| 1990年 | 22篇 |
| 1989年 | 14篇 |
| 1988年 | 13篇 |
| 1987年 | 34篇 |
| 1986年 | 18篇 |
| 1985年 | 24篇 |
| 1984年 | 29篇 |
| 1983年 | 27篇 |
| 1982年 | 35篇 |
| 1981年 | 21篇 |
| 1980年 | 29篇 |
| 1979年 | 20篇 |
| 1978年 | 19篇 |
| 1977年 | 21篇 |
| 1976年 | 21篇 |
| 1975年 | 25篇 |
| 1974年 | 18篇 |
| 1973年 | 18篇 |
| 1971年 | 12篇 |
排序方式： 共有2083条查询结果，搜索用时 31 毫秒
81.
敦煌造山带长山子地区变质演化及年代学研究 总被引：1，自引：1，他引：0
长山子地区位于敦煌造山带东北部,瓜州南部约100km处。该区主要出露一套中-高级变质表壳岩,主要岩石类型有长英质片麻岩、变泥质麻粒岩、高压基性麻粒岩、斜长角闪片麻岩。高压基性麻粒岩岩块、斜长角闪片麻岩岩块以构造透镜体或布丁(长度为0. 5~15m)的形式,被夹持于长英质片麻岩、变泥质麻粒岩组成的基质之中,呈现典型"基质夹岩块"的混杂带特征。高压基性麻粒岩、斜长角闪片麻岩、泥质麻粒岩中,普遍保留了二至三个阶段的变质矿物组合。进变质阶段矿物组合(M1)为石榴子石变斑晶中的细小矿物包裹体,变质高峰期矿物组合(M2)为石榴子石变斑晶和基质矿物,退变质阶段矿物组合(M3)主要为围绕石榴子石变斑晶边部发育的"白眼圈"状后成合晶。本区各类变质岩石均记录了顺时针型变质作用P-T轨迹,系典型俯冲-碰撞造山带变质作用特征。退变质阶段P-T轨迹属于西阿尔卑斯型,说明变质岩折返速率较快。变质高峰期(M2)属于中压变质相系,P-T条件分别为790~870℃/1.29~1.37GPa(高压基性麻粒岩)、680~685℃/0.89~0.97GPa(斜长角闪片麻岩)、860~880℃/0.90~1.14GPa(变泥质麻粒岩),它们之间存在大的差异。这说明,它们是形成于同一俯冲隧道内不同深度的变质岩石,在构造折返阶段才混杂在一起形成构造混杂岩。二次离子质谱(SIMS)锆石U-Pb定年表明,长山子地区变质杂岩记录了早泥盆世的俯冲事件(419~417Ma)。 相似文献
82.
Aude?ValadeEmail authorView author&#;s OrcID profile Sebastiaan?Luyssaert Patrick?Vallet Sylvestre?Njakou Djomo Ingride?Jesus Van Der Kellen Valentin?Bellassen 《Carbon balance and management》2018,13(1):26
Background
Concern about climate change has motivated France to reduce its reliance on fossil fuel by setting targets for increased biomass-based renewable energy production. This study quantifies the carbon costs and benefits for the French forestry sector in meeting these targets. A forest growth and harvest simulator was developed for French forests using recent forest inventory data, and the wood-use chain was reconstructed from national wood product statistics. We then projected wood production, bioenergy production, and carbon balance for three realistic intensification scenarios and a business-as-usual scenario. These intensification scenarios targeted either overstocked, harvest-delayed or currently actively managed stands.Results
All three intensification strategies produced 11.6–12.4 million tonnes of oil equivalent per year of wood-based energy by 2026, which corresponds to the target assigned to French wood-energy to meet the EU 2020 renewable energy target. Sustaining this level past 2026 will be challenging, let alone further increasing it. Although energy production targets can be reached, the management intensification required will degrade the near-term carbon balance of the forestry sector, compared to continuing present-day management. Even for the best-performing intensification strategy, i.e., reducing the harvest diameter of actively managed stands, the carbon benefits would only become apparent after 2040. The carbon balance of a strategy putting abandoned forests back into production would only break even by 2055; the carbon balance from increasing thinning in managed but untended stands would not break even within the studied time periods, i.e. 2015–2045 and 2046–2100. Owing to the temporal dynamics in the components of the carbon balance, i.e., the biomass stock in the forest, the carbon stock in wood products, and substitution benefits, the merit order of the examined strategies varies over time.Conclusions
No single solution was found to improve the carbon balance of the forestry sector by 2040 in a way that also met energy targets. We therefore searched for the intensification scenario that produces energy at the lowest carbon cost. Reducing rotation time of actively managed stands is slightly more efficient than targeting harvest-delayed stands, but in both cases, each unit of energy produced has a carbon cost that only turns into a benefit between 2060 and 2080.83.
Luu Chinh Bui Quynh Duy Costache Romulus Nguyen Luan Thanh Nguyen Thu Thuy Van Phong Tran Van Le Hiep Pham Binh Thai 《Natural Hazards》2021,108(3):3229-3251
Natural Hazards - Vietnam’s central coastal region is the most vulnerable and always at flood risk, severely affecting people’s livelihoods and socio-economic development. In... 相似文献
84.
85.
Caurant F Aubail A Lahaye V Van Canneyt O Rogan E López A Addink M Churlaud C Robert M Bustamante P 《Marine environmental research》2006,62(2):131-148
Lead concentrations and isotopic composition have been measured in bone and teeth of small cetaceans belonging to three species (Delphinus delphis, Phocoena phocoena and Stenella coeruleoalba), to evaluate the toxicological risk and to determine sources of lead in the European waters. Lead concentrations, far lower than threshold value inducing toxic effects in human, were higher in teeth than in bones, but highly correlated between the two tissues (r=0.92, p<0.001). Large variations of 206Pb/207Pb values in bone tissue showed that cetaceans must be submitted to various atmospheric influences. No geographical differences appeared which is consistent with studies on their distribution indicating seasonal movements between Brittany waters and the Bay of Biscay. The negative correlation between 206Pb/207Pb ratios and age of the individuals reflected the decrease in the production of alkyl lead in Europe, i.e., the increasing use of unleaded gasoline. 相似文献
86.
Houda El Kerni Hasnaa Chennaoui Aoudjehane David Baratoux Mohammed Aoudjehane Andr Charrire Hassan Ibouh Pierre Rochette Yoann Quesnel Minoru Uehara Thomas Kenkmann Gerwin Wulf Michael Poelchau Van Binh Nguyen Maria Aboulahris Samira Makhoukhi Georges Aumaître Didier Bourls Karim Keddadouche 《Meteoritics & planetary science》2019,54(10):2483-2509
Since the discovery of shatter cones (SCs) near the village of Agoudal (Morocco, Central High Atlas Mountains) in 2013, the absence of one or several associated circular structures led to speculation about the age of the impact event, the number, and the size of the impact crater or craters. Additional constraints on the crater size, age, and erosion rates are obtained here from geological, structural, and geophysical mapping and from cosmogenic nuclide data. Our geological maps of the Agoudal impact site at the scales of 1:30,000 (6 km2) and 1:15,000 (2.25 km2) include all known occurrences of SCs in target rocks, breccias, and vertical to overturned strata. Considering that strata surrounding the impact site are subhorizontal, we argue that disturbed strata are related to the impact event. Three types of breccias have been observed. Two of them (br1‐2 and br2) could be produced by erosion–sedimentation–consolidation processes, with no evidence for impact breccias, while breccia (br1) might be impact related. The most probable center of the structure is estimated at 31°59′13.73?N, 5°30′55.14?W using the concentric deviation method applied to the orientation of strata over the disturbed area. Despite the absence of a morphological expression, the ground magnetic and electromagnetic surveys reveal anomalies spatially associated with disturbed strata and SC occurrences. The geophysical data, the structural observations, and the area of occurrence of SCs in target rocks are all consistent with an original size of 1.4–4.2 km in diameter. Cosmogenic nuclide data (36Cl) constrain the local erosion rates between 220 ± 22 m Ma?1 and 430 ± 43 m Ma?1. These erosion rates may remove the topographic expression of such a crater and its ejecta in a time period of about 0.3–1.9 Ma. This age is older than the Agoudal iron meteorite age (105 ± 40 kyr). This new age constraint excludes the possibility of a genetic relationship between the Agoudal iron meteorite fall and the formation of the Agoudal impact site. A chronolgy chart including the Atlas orogeny, the alternation of sedimentation and erosion periods, and the meteoritic impacts is presented based on all obtained and combined data. 相似文献
87.
Sidney Van Den Bergh 《Astronomische Nachrichten》1979,300(5):225-228
It is shown that there is no significant dependence of the frequency with which barred spirals occur on environment. Therefore, the difference between normal and barred spirals must be largely due to initial conditions. The excess of early type galaxie (in particular ellipticals and SO's) among the statistically selected binary samples is probably caused by a contamination by cluster members and will therefore result in a systematic overestimate of the masses of binary galaxies. The large fraction of multiple first-ranked galaxies found in rich clusters of BAUTZ -MORGAN type I is surprising in view of tidal effects and dynamical friction, which should lead to the destruction of such massive binary systems on a time scale much shorter than the age of the Universe. 相似文献
88.
G. De Backer M. Vantorre C. Beels J. De Pré S. Victor J. De Rouck C. Blommaert W. Van Paepegem 《Applied Ocean Research》2009,31(3):143-156
The results of an elaborate experimental investigation on bottom slamming of axisymmetric objects are presented. Drop tests have been performed on a hemisphere and two conical shapes with different deadrise angles. The test setup is designed so as to prevent small rotations of the test objects which cause scatter in the measurement data. The pressure distribution and evolution as well as the body motion parameters are measured during impact. By means of a high speed camera the water uprise is visualized and the wetting factor is determined for the cones. The results are compared with a three-dimensional asymptotic theory for axisymmetric rigid bodies with constant entry velocity. The ratio between the registered peak pressures and the asymptotic theory are in accordance with comparable experiments in the literature. The asymptotic theory, however, is found to be quite conservative, since the measured peak pressure levels appear to be approximately 50% to 75% of the theoretical levels. 相似文献
89.
90.
Els Verfaillie Steven Degraer Kristien Schelfaut Wouter Willems Vera Van Lancker 《Estuarine, Coastal and Shelf Science》2009
Mapping ecologically relevant zones in the marine environment has become increasingly important. Biological data are however often scarce and alternatives are being sought in optimal classifications of abiotic variables. The concept of ‘marine landscapes’ is based on a hierarchical classification of geological, hydrographic and other physical data. This approach is however subject to many assumptions and subjective decisions. 相似文献