首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   712篇
  免费   26篇
  国内免费   4篇
测绘学   24篇
大气科学   73篇
地球物理   162篇
地质学   262篇
海洋学   69篇
天文学   74篇
综合类   2篇
自然地理   76篇
  2023年   3篇
  2022年   3篇
  2021年   2篇
  2020年   13篇
  2019年   10篇
  2018年   27篇
  2017年   20篇
  2016年   25篇
  2015年   22篇
  2014年   23篇
  2013年   27篇
  2012年   25篇
  2011年   34篇
  2010年   41篇
  2009年   52篇
  2008年   43篇
  2007年   39篇
  2006年   22篇
  2005年   37篇
  2004年   25篇
  2003年   24篇
  2002年   22篇
  2001年   15篇
  2000年   7篇
  1999年   10篇
  1998年   17篇
  1997年   4篇
  1996年   8篇
  1995年   7篇
  1994年   8篇
  1993年   3篇
  1992年   4篇
  1991年   5篇
  1990年   4篇
  1989年   3篇
  1988年   4篇
  1987年   6篇
  1986年   7篇
  1985年   7篇
  1984年   16篇
  1983年   7篇
  1982年   11篇
  1981年   9篇
  1980年   7篇
  1979年   7篇
  1978年   8篇
  1977年   4篇
  1976年   3篇
  1975年   2篇
  1973年   4篇
排序方式: 共有742条查询结果,搜索用时 15 毫秒
1.
2.
3.
The representation of women on geography faculties did not increase appreciably between 1971 and 1977, but women in other academic disciplines have fared somewhat better. The latter have improved their status primarily as a result of actions taken by professional associations representing their fields. When a professional association ignores its female constituency or refuses to commit itself to improving their circumstances, federal agencies now offer alternative channels for obtaining redress.  相似文献   
4.
Following Early Cretaceous nappe stacking, the Eastern Alps were affected by late-orogenic extension during the Late Cretaceous. In the eastern segment of this range, a Late Cretaceous detachment separates a very low- to low-grade metamorphic cover (Graz Paleozoic Nappe Complex, GPNC) above a low- to high-grade metamorphic basement. Synchronously, the Kainach Gosau Basin (KGB) collapsed and subsided on top of the section.Metamorphism of organic material within this section has been investigated using vitrinite reflectance data and Raman spectra of extracted carbonaceous material. In the southern part of the GPNC, vitrinite reflectance indicates a decrease in organic maturity towards the stratigraphic youngest unit. The remaining part of the GPNC is characterized by an aureole of elevated vitrinite reflectance values and Raman R2 ratios that parallels the margins of the GPNC. Vitrinite reflectance in the KGB shows a steep coalification gradient and increases significantly towards the western basin margin. The observed stratigraphic trend in the southern GPNC is a result of deep Paleozoic to Early Cretaceous burial. This maturity pattern was overprinted along the margins by advective heat and convective fluids during Late Cretaceous to Paleogene exhumation of basement rocks.During shearing, the fault zone was heated up to ca. 500 °C. This overprint is explained by a two-dimensional thermal model with a ramp-flat fault geometry and a slip rate of 1 to 1.5 cm/year during 5 Ma fault movement. The collapse basin above the detachment subsided in a thermal regime which was characterized by relaxing isotherms.  相似文献   
5.
6.
7.
Measured spectra ofθ x,θ y,θ z, the derivatives of temperature in streamwise, lateral and vertical directions, respectively, indicate that the spectral densities ofθ z andθ y are nearly similar but significantly different from the spectral density ofθ x. The high-frequency parts of the three spectra satisfy, in a qualitative sense, local isotropy requirements. In the high-frequency end of the inertial subrange, the relative behaviour of spectra ofθ x,θ y andθ z is also consistent with local isotropy.  相似文献   
8.
An integrated mass balance and modelling approach for analysis of estuarine nutrient fluxes is demonstrated in the Swan River Estuary, a microtidal system with strong hydrological dependence on seasonal river inflows. Mass balance components included estimation of gauged and ungauged inputs to the estuary and losses to the ocean (outflow and tidal exchange). Modelling components included estimation of atmospheric (N fixation, denitrification) and sediment–water column nutrient exchanges. Gross and net denitrification derived using two independent methods were significantly correlated (r2 = 0.49, p < 0.01) with net rates averaging 40% of gross. Annual nitrogen (N) and phosphorus (P) loads from major tributaries were linearly correlated with annual freshwater discharge and were 3-fold higher in wet years than in dry years. Urban drains and groundwater contributed, on average, 26% of N inputs and 19% of P inputs, with higher relative contributions in years of low river discharge. Overall, ungauged inputs accounted for almost 35% of total nitrogen loads. For N, elevated loading in wet years was accompanied by large increases in outflow (7x) and tidal flushing (2x) losses and resulted in overall lower retention efficiency (31%) relative to dry years (70%). For P, tidal flushing losses were similar in wet and dry years, while outflow losses (4-fold higher) were comparable in magnitude to increases in loading. As a result, P retention within the estuary was not substantially affected by inter-annual variation in water and P loading (ca. 50% in all years). Sediment nutrient stores increased in most years (remineralisation efficiency ca. 50%), but sediment nutrient releases were significant and in some circumstances were a net source of nutrients to the water column.  相似文献   
9.
10.
As the world’s largest importer of marine ornamental species for the aquaria, curio, home décor, and jewelry industries, the United States has an opportunity to leverage its considerable market power to promote more sustainable trade and reduce the effects of ornamental trade stress on coral reefs worldwide. Evidence indicates that collection of some coral reef animals for these trades has caused virtual elimination of local populations, major changes in age structure, and promotion of collection practices that destroy reef habitats. Management and enforcement of collection activities in major source countries such as Indonesia and the Philippines remain weak. Strengthening US trade laws and enforcement capabilities combined with increasing consumer and industry demand for responsible conservation can create strong incentives for improving management in source countries. This is particularly important in light of the March 2010 failure of the parties to the Convention on International Trade in Endangered Species (CITES) to take action on key groups of corals.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号