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   430篇
  免费   14篇
  国内免费   5篇
测绘学   14篇
大气科学   26篇
地球物理   89篇
地质学   225篇
海洋学   32篇
天文学   25篇
综合类   4篇
自然地理   34篇
  2023年   3篇
  2022年   3篇
  2020年   5篇
  2019年   5篇
  2018年   11篇
  2017年   8篇
  2016年   17篇
  2015年   10篇
  2014年   15篇
  2013年   23篇
  2012年   19篇
  2011年   24篇
  2010年   20篇
  2009年   38篇
  2008年   19篇
  2007年   17篇
  2006年   16篇
  2005年   17篇
  2004年   10篇
  2003年   9篇
  2002年   9篇
  2001年   7篇
  2000年   7篇
  1999年   5篇
  1998年   5篇
  1997年   4篇
  1996年   4篇
  1995年   5篇
  1993年   5篇
  1992年   4篇
  1989年   3篇
  1986年   3篇
  1985年   4篇
  1984年   4篇
  1982年   4篇
  1979年   3篇
  1975年   3篇
  1971年   4篇
  1970年   3篇
  1969年   3篇
  1967年   3篇
  1966年   3篇
  1962年   3篇
  1959年   3篇
  1954年   3篇
  1952年   2篇
  1939年   2篇
  1938年   3篇
  1937年   2篇
  1936年   2篇
排序方式: 共有449条查询结果,搜索用时 62 毫秒
41.
42.
43.
Forest conditions in Europe have been monitored over 20 years jointly by the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) and the European Union (EU). Maps for mean bulk SO4, NO3 and NH4 deposition at around 400 intensive monitoring plots in the years 1999–2001, as well as time trends for the period 1996–2001, are presented. Mean bulk SO4 deposition at 169 plots mostly located in central Europe decreased from 7.4 to 5.8 kgS ha−1 a−1. Mean NH4 bulk deposition decreased from 6.2 to 5.3 kgN ha−1 a−1. Nitrate bulk deposition fluctuated around 5 kgN ha−1 a−1. On average, throughfall deposition was considerably higher than bulk deposition. Time trends for mean tree crown defoliation as an overall indicator for forest condition show a peak in the mid 1990s for most of the monitored main tree species and a recent increase for the years 2003 and 2004. Multivariate linear regression analyses show some significant relations between deposition and defoliation. These relations depend on the tree species and site characteristics. Effects of deposition are moderated by the influence of biotic stress factors such as insects and fungi and by abiotic stress factors, such as weather.  相似文献   
44.
Deep-water acoustical measurements of rainfall are compared to high-resolution ground radar observations for the first time. The measurements of underwater ambient sound were made from a subsurface mooring near Methoni, Greece, in 2004. The acoustical measurements were at 60-, 200-, 1000-, and 2000-m depths. Simultaneous ground-based polarimetric -band radar observations were made over the acoustic mooring. Comparisons show acoustic detection of rain events and storm structure that are in agreement with the radar observations. Results from a comparison between the underwater sound pressure level at different depths and the observed radar reflectivities are presented.  相似文献   
45.
This letter evaluates the consistency of rainfall drop size distribution (DSD) parameters that were estimated from attenuation-corrected X-band dual-polarization (XPOL) radar measurements with estimates from a colocated S-band dual-polarization radar (S-Pol). The DSD retrieval technique uses reflectivity and differential reflectivity, and a constrained relationship to estimate the three parameters of a ldquonormalizedrdquo gamma distribution model. The more definitive S-Pol DSD parameter estimates are used as a reference to assess the performance of the corresponding XPOL estimates for different rain-path attenuation values. Results show that XPOL attenuation-corrected profiles can provide rainfall DSD estimations that are consistent to an S-band dual-polarization radar, even in cases of moderate to high rain-path attenuation.  相似文献   
46.
Zusammenfassung Druck-Lösungserscheinungen sind nicht auf stylolithische Kontakte beschränkt. Die Mehrzahl der Druck-Lösungsflächen ist glatt und nicht als solche erkennbar. Bei Abwesenheit von Stylolithisierung wird Druck-Lösungsrückstand meistens fälschlich als primäres Sediment angesprochen.Druckgelöstes Material wandert von Becken aus. Auf Schwellen führt es zu erhöhtem Stoffangebot für chemische und organische Ablagerung. Druckunlösliches Material konzentriert sich passiv in den Becken.Die Unterschiede zwischen benachbarten lithologischen Horizonten werden in stratigraphischen Abfolgen während der Diagenese über Druck-Lösungstätigkeit verstärkt. Wechsellagerungen können entstehen, indem relativ reine, fast monomineralische Horizonte (Kalkspat, Dolomit, Quarz) von Druck-Lösung kaum betroffen werden, während etwas mehr verunreinigte Schichten, besonders bei Gegenwart von Tonmineralien, bis auf ihren Lösungsrückstand zusammenschrumpfen können.Die während Faltung und Schieferung fortdauernde Druck-Lösungstätigkeit bewirkt weiteren Gesteinsschwund.Hauptsächlich synsedimentäre Lagerstätten können über Druck-Lösungsvorgänge während Diagenese und Orogenese eine weitere Anreicherung erfahren, indem Wertmineralien im Lösungsrückstand zurückbleiben. Verstärkte Anreicherungen sind in Richtung der Becken und Faltenschenkel zu erwarten.
Pressure solution phenomena are not restricted to stylolitic contacts. The majority of pressure solution planes is smooth and not recognizable as such. Pressure solution residue in the absence of stylolitization is often misinterpreted as a primary sediment.Material dissolved under nonuniform pressure migrates from basins to ridges, where it leads to increased precipitation and organic deposition. The material that is insoluble under unilateral pressure thus becomes more concentrated in the basins.The differences between neighbouring lithological units in stratigraphic series are increased during diagenesis by pressure solution activity. Intercalations may result from relatively pure, almost monomineralic layers (e. g. calcite, dolomite, quartz) remaining relatively unaffected by pressure solution processes, while more impure layers, especially those containing clay minerals, may shrink to its solution residue.During folding and cleavage, continuing pressure solution activity results in further loss of rock matter.Mainly synsedimentary ore deposits may be further concentrated by pressure solution processes, both during diagenesis and orogenesis, leaving valuable minerals as part of the solution residue. Elevated concentrations are to be expected towards the basins and limbs of folds.

Résumé Les phénomènes de dissolution par pression inégale ne sont pas limités aux contacts stylolithiques. La plupart des surfaces de dissolution par pression linéaire est lisse et sans caractères particuliers. En l'absence de stylolithisation, les résidus sont considérés souvent à tort comme des sédiments primaires.Le matériel dissous par pression linéaire migre du centre des bassins vers ses bords; cela conduit à un accroissement des dépÔts chimiques et organiques sur les seuils. Le matériel insoluble se concentre passivement au centre des bassins.Les différences entre horizons lithologiques voisins dans les séries stratigraphiques sont accentuées pendant la diagénèse par l'activité de dissolution par pression inégale. Il peut se former des alternances ou des horizons relativement »purs«, presque monominéraux (calcite, dolomite, quartz), qui ne sont pas touchés par la dissolution par pression inégale, tandis que des couches moins »pures« en raison de la présence en particulier de minéraux argileux, peuvent Être réduites à des résidus de dissolution.L'activité persistante de la dissolution par pression linéaire au cours du plissement et de la schistosité entraÎne une nouvelle diminution de matière dans la roche.Ce sont surtout les gisements synsédimentaires qui peuvent subir un enrichissement supplémentaire pendant la diagénèse et l'orogénèse, puisque ce sont les minéraux de valeur qui s'accumulent dans les résidus de dissolution. Ces enrichissements peuvent Être particulièrement renforcés en direction du centre des bassins et des flancs des plis.

; . , ., . . . , , «» - , ., , , , , , «» , , . . , . .
  相似文献   
47.
Structural formulas were calculated for 8 analysed coexisting biotite-muscovitepairs from granitic rocks. Characteristic components of these structural formulas were plotted against the unit cell dimensions of these micas.In accordance with the results of Gower (1957) the substitution of OH by F in trioctahedral micas was found to reduce c 0 · . However, in contrast to his statements, octahedral alumina was found to reduce c 0 · sin also and to an even larger extent than fluorine. This observation is in agreement with the commonly encountered compacting of the mica structure through replacement of 3 large cations (Fe2+, Mg2+, Mn2+) by two smaller ones (Al3+, Fe3+).Concerning b 0 the combined amounts of octahedral Al and tetrahedral Si cause a reduction of this value, which means indirectly that tetrahedral Al alone would increase b 0.Reductions of b 0 and c 0 · sin by Al in synthetic micas were also found by Crowley and Roy (1964) and Seifert (1966) for the substitution Mg + Si by two Al in trioctahedral micas.In an earlier paper the author (Müller, 1966) has already shown with the same mica samples that the substitutions of M2+ by Al3+ and of OH by F also have the strongest effects of all substitutions on the refractive indices, densities, and specific refractive energies of these micas. Therefore it is evident that the latter effects as well as the changes of lattice constants described in this paper are interrelated.

Mein Dank gilt den Herren Dr.W. Harre und Dr. H. Gundlach, Bundesanstalt für Bodenforschung, Hannover, für die Ausführung der chemischen Glimmeranalysen, sowie der Deutschen Forschungsgemeinschaft, Bad Godesberg, für ihre finanzielle Beihilfe. Herrn Dr. F. Seifert, Kiel, danke ich für zahlreiche sachliche Diskussionen und Hinweise und für die Überlassung seines Algol-Rechenprogramms, mit welchem 4 der 16 Strukturformeln elektronisch berechnet wurden. Herrn Prof. Dr. F. Liebau, Kiel, verdanke ich einige freundliche Ratschläge.  相似文献   
48.
Zusammenfassung Ein Versuch mit einer rotierenden Schaumgummi- und Tonkugel wird beschrieben. Strukturelle Analogien zwischen dem beim Experiment auftretenden Schwächeflächen-Netz und gro\tektonischen Lineamenten werden besprochen; die Beziehungen solcher Lineamentmuster zur Lage des gleichalten Pols werden versuchsweise diskutiert.
Summary An experiment with a rotating clay ball is being described. Comparisons between resulting ball-modell-structures and geological lineaments are tried and conclusions are discussed concerning relations between lineament-patterns and former situations of the poles of the Earth.
  相似文献   
49.
The link between spatiotemporal patterns of stream water chemistry and catchment characteristics for the mesoscale Dill catchment (692 km2) in Germany is explored to assess the catchment scale controls on water quality and to characterize water sources. In order to record the spatiotemporal pattern, ‘snapshot sampling’ was applied during low, mean and high flow, including 73 nested sites throughout the catchment. Water samples were analysed for the elements Li, B, Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Sr, Mo, Ba, Pb and U using inductively‐coupled‐plasma mass spectrometry, and for electric conductivity and pH. Principle component analysis and hierarchical cluster analysis were used to find typical element associations and to group water samples according to their hydrochemical fingerprints. This revealed regional hydrochemical patterns of water quality which were subsequently related to catchment attributes to draw conclusions about the controls on stream chemistry. It was found that various lithologic signals and anthropogenic point source inputs controlled the base flow hydrochemistry. During increased flows, stream waters were diluted causing additional hydrochemical variability in response to heterogeneous precipitation inputs and differences in aquifer storage capacities. The hydrochemical patterns further displayed in‐stream mixing of waters. This implied, that stream waters could be apportioned to the identified water sources throughout the catchment. The basin‐wide hydrochemical variability has the potential to outrange the tracer signatures typically inferred in studies at the hillslope scale and is able to strongly influence the complexity of the catchment output. Both have to be considered for further catchment scale tracer and modelling work. Despite the likelihood of non‐conservative behaviour, the minor and trace elements enhanced the rather qualitative discrimination of the various groundwater types, as the major cations were strongly masked by point source inputs. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
50.
Charcoal and fossil wood taken from palaeosols, sediments and artificial structures were analysed in order to evaluate the regional pedoanthracological potential and to obtain information on Holocene environmental changes, particularly on possible past tree occurrences in southern Tibet. This research was initiated by the question to what extent this area is influenced by past human impact. Even recent evaluations have perceived the present treeless desertic environment of southern Tibet as natural, and the previous Holocene palaeoenvironmental changes detected were predominantly interpreted to be climate-determined. The material analysed – comprising a total of 53 botanical spectra and 55 radiocarbon datings from 46 sampling sites (c. 3500–4700 m a.s.l.) – represents the largest systematically obtained data set of charcoal available from Tibet so far. 27 taxa were determined comprising trees, (dwarf-) shrubs and herbs as well as grasses. The predominant tree taxa were Juniperus, Hippophae, Salix and Betula. According to their present-day occurrence in the region, the genera Juniperus and Hippophae can be explicitly attributed to tree species. Further, less frequently detected tree taxa were Populus, Pinus, Quercus, Taxus and Pseudotsuga. Charcoal of Juniperus mainly occurred on southern exposures, whereas Betula was associated with northern exposures. In contrast, the (partly) phreatophytic taxa Hippophae and Salix showed no prevalent orientation. The distribution of radiocarbon ages on charcoal revealed a discontinuous record of burning events cumulating in the Late Holocene (c. 5700–0 cal BP). For southern Tibet, these results indicated a Late Holocene vegetation change from woodlands to the present desertic pastures. As agrarian economies in southern and south-eastern Tibet date back to c. 3700 and 5700 cal BP, respectively, and the present-day climate is suitable for tree growth up to c. 4600 m a.s.l., we concluded that the Late Holocene loss or thinning out of woodlands had been primarily caused by humans.  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