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   297篇
  免费   16篇
  国内免费   2篇
测绘学   6篇
大气科学   17篇
地球物理   48篇
地质学   98篇
海洋学   47篇
天文学   57篇
自然地理   42篇
  2021年   2篇
  2019年   5篇
  2018年   2篇
  2017年   7篇
  2016年   10篇
  2015年   11篇
  2014年   6篇
  2013年   19篇
  2012年   15篇
  2011年   9篇
  2010年   14篇
  2009年   13篇
  2008年   11篇
  2007年   12篇
  2006年   11篇
  2005年   7篇
  2004年   10篇
  2003年   7篇
  2002年   10篇
  2001年   4篇
  2000年   4篇
  1999年   2篇
  1998年   2篇
  1997年   7篇
  1996年   6篇
  1993年   4篇
  1992年   3篇
  1991年   4篇
  1990年   4篇
  1989年   4篇
  1988年   3篇
  1987年   3篇
  1986年   3篇
  1985年   5篇
  1984年   4篇
  1983年   9篇
  1982年   9篇
  1981年   4篇
  1979年   7篇
  1978年   4篇
  1977年   4篇
  1975年   3篇
  1974年   3篇
  1972年   6篇
  1971年   8篇
  1970年   2篇
  1967年   2篇
  1959年   1篇
  1954年   1篇
  1940年   1篇
排序方式: 共有315条查询结果,搜索用时 31 毫秒
81.
Cellular exposure to particulate matter with concomitant formation of reactive oxygen species (ROS) and oxidization of biomolecules may lead to negative health outcomes. Evaluating the particle-induced formation of ROS and the oxidation products from reaction of ROS with biomolecules is useful for gaining a mechanistic understanding of particle-induced oxidative stress. Aqueous suspensions of pyrite particles have been shown to form hydroxyl radicals and degrade nucleic acids. Reactions between pyrite-induced hydroxyl radicals and nucleic acid bases, however, remain to be determined. Here, we compared the oxidation of adenine by Fenton-generated (i.e., ferrous iron and hydrogen peroxide) hydroxyl radicals to adenine oxidation by hydroxyl radicals generated in pyrite aqueous suspensions. Results show that adenine oxidizes in the presence of pyrite (without the addition of hydrogen peroxide) and that the rate of oxidation is dependent on the pyrite loading. Adenine oxidation was prevented by addition of either catalase or ethanol to the pyrite/adenine suspensions, which implies that hydrogen peroxide and hydroxyl radicals are causing the adenine oxidation. The adenine oxidation products, 8-oxoadenine and 2-hydroxyadenine, were the same whether hydroxyl radicals were generated by Fenton or pyrite-initiated reactions. Although nucleic acid bases are unlikely to be directly exposed to pyrite particles, the formation of ROS in the vicinity of cells may lead to oxidative stress.  相似文献   
82.
The sensitivity of ice sheets to climate change influences the return of meltwater to the oceans. Here we track the Laurentide Ice Sheet along a ~400 km long transect spanning about 6000 yr of retreat during the major climate oscillations of the lateglacial. Thunder Bay, Ontario is near a major topographic drainage divide, thus terrestrial ablation processes are the primary forcers of ice margin recession in the study area. During deglaciation three major moraine sets were produced, and have been assigned minimum ages of 13.9 ± 0.2, 12.3 ± 0.2–12.1 ± 0.1, and 11.2 ± 0.2 cal ka BP from south to north. These define a slow retreat (~10–50 m/a) prior to major climate oscillations which was then followed by a factor of ~2 increase during the Bölling–Alleröd, and an additional increase during the early Holocene. When compared to retreat rates in other terrestrial settings of the ice sheet, nearly identical patterns emerge. However this becomes problematic because a key control on retreat rates is the surface slope of the ice sheet and this should vary considerably over areas of so-called hard and soft beds. Further these ice margin reconstructions would not allow meltwater sourced in the Hudson Basin to drain into the Atlantic basin until after Younger Dryas time.  相似文献   
83.
84.
Carbon-14 production rate variations that are not explainable by geomagnetic changes are thought to be in antiphase with solar activity and as such should be in antiphase with paleotemperature records or proxy temperature histories such as those obtainable from oxygen isotope analyses of ice cores. Oxygen isotope records from Camp Century, Greenland and Devon Island Ice Cap are in phase with each other over thousands of years and in antiphase to the 14C production rate residuals.  相似文献   
85.
Uranium contents of 67 oceanic basalts have been measured by fission track analysis. Average value for ridge and intraplate basalts is ~ 78 ppb, for the Easter Hot Line it is ~1600 ppb. Estimates of mantle concentrations derived from the ridge and intraplate samples are insufficient to account for the observed surface heat flow. The whole-Earth concentration of U is > 8 ppb. and < 33 ppb if all heat generated within the Earth reaches the surface.  相似文献   
86.
Oxygen-isotope profiles for the Devon Island ice cap and Camp Century Greenland are affected by a number of variables, some of which must have been the same for both sites. The two δ(18O) records spanning about 120,000 years are brought into relative alignment by comparison of major δ features, and subsequent verification that the insoluble particulate concentration records were also in phase for this alignment. The difference between the δ profiles is shown to be mainly a function of the altitude of the accumulation area for Camp Century. This altitude seems to have been higher than present for the last 100,000 years, suggesting the present flow line through the site has never been shorter. The maximum altitude for the Camp Century accumulation area is 1500 m above the present site and is almost synchronous with the maximum in particulate concentration that occurs at 16,000 yr B.P. The synchronism is likely due to the maximum sea-level lowering that exposed vast areas of continental shelf to wind erosion.  相似文献   
87.
Olivine clasts, which have mantles formed by reaction of the olivine with the breccia matrix, are present in the high-grade thermally metamorphosed Apollo 14 breccias. The mantled olivine clasts are most abundant in 14311, but they are also present in 14304 and 14319. Typically the mantles consist of two zones: an inner corona containing pyroxene, ilmenite and commonly plagioclase, and an outer light-colored halo where the matrix is depleted in ilmenite. The growth of the coronas involved matrix-to-corona diffusion of TiO2 and corona-to-matrix diffusion of MgO and FeO. These diffusive fluxes can be attributed to chemical potential gradients developed between mineral assemblages in local equilibrium at the olivine-corona boundary and the matrix.  相似文献   
88.
89.
U was measured by fission track analysis in 115 samples of hypersthene, bronzite, amphoterite and carbonaceous chondrites. On a weight basis the average values for the Cl carbonaceous and bronzite chondrites are similar to the “classic” value of 11 ppb, but the hypersthenes and amphoterites are ~50 per cent higher. Each class shows a well-determined peak in the U abundance distribution, allowing the calculation of radiogenic ages and comparison with other elements of interest.  相似文献   
90.
U/He ages of deep sea basalts which show unequivocal evidence of excess Ar are generally higher than the corresponding K/Ar ages, suggesting that this result is diagnostic of the presence of excess rare gases. Concordant U/He and K/Ar ages are indicative of the validity of such ages. U/He ages lower than the corresponding K/Ar ages are not informative.  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