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   663篇
  免费   14篇
  国内免费   5篇
测绘学   12篇
大气科学   28篇
地球物理   130篇
地质学   273篇
海洋学   42篇
天文学   149篇
综合类   2篇
自然地理   46篇
  2023年   3篇
  2021年   4篇
  2020年   5篇
  2019年   11篇
  2018年   14篇
  2017年   4篇
  2016年   22篇
  2015年   15篇
  2014年   17篇
  2013年   39篇
  2012年   25篇
  2011年   33篇
  2010年   37篇
  2009年   47篇
  2008年   40篇
  2007年   29篇
  2006年   28篇
  2005年   22篇
  2004年   25篇
  2003年   32篇
  2002年   26篇
  2001年   17篇
  2000年   16篇
  1999年   17篇
  1998年   8篇
  1997年   9篇
  1996年   8篇
  1995年   4篇
  1994年   7篇
  1993年   10篇
  1992年   5篇
  1991年   8篇
  1990年   10篇
  1989年   5篇
  1988年   7篇
  1986年   10篇
  1985年   5篇
  1984年   6篇
  1983年   6篇
  1982年   7篇
  1981年   6篇
  1980年   8篇
  1979年   2篇
  1978年   3篇
  1977年   2篇
  1976年   2篇
  1975年   2篇
  1974年   5篇
  1973年   3篇
  1969年   2篇
排序方式: 共有682条查询结果,搜索用时 78 毫秒
1.
Kiacatoo Man, a large, rugged Aboriginal adult buried in the Lachlan riverine plains of southeastern Australia, was discovered in 2011. Laser‐ablation uranium series analysis on bone yielded a minimum age for the burial of 27.4 ± 0.4 ka (2σ). Single‐grain, optically stimulated luminescence ages on quartz sediment in which the grave had been dug gave a weighted mean age of 26.4 ± 1.5 ka (1σ). Luminescence samples from the grave infill and from sediment beneath the grave exhibit overdispersed dose distributions consistent with bioturbation or other disturbance, which has obscured the burial signal. The overlap between the minimum (U‐series) and maximum (luminescence) ages places the burial between 27.0 and 29.4 ka (2σ). Luminescence ages obtained from the channel belt of between 28 ± 2 and 25 ± 3 ka indicate that fluvial sedimentation was occurring before the Last Glacial Maximum, which is consistent with the broader geomorphic setting. Together, these results are internally and regionally consistent, and indicate that Kiacatoo Man was one of the more ancient individuals so far identified in Australia. His remains are important to our understanding of patterns of biological variation and other processes that have shaped people in the Murray‐Darling Basin through time. Copyright © 2019 John Wiley & Sons, Ltd.  相似文献   
2.
3.
Summary ?The rare earth, major and trace element geochemistry of Jurassic deep marine manganese shales allow insight into their environment of deposition. We present data of 24 samples from the Northern Calcareous Alps (Eastern Alps), collected from Late Jurassic strata of the Tennengebirge and from Early Jurassic strata of the Karwendel Mts. Whereas major and trace element geochemistry is controlled by the detrital input, rare earth element data and the relationship between total organic carbon and (pyritic) sulfur can be used to constrain the origin of the manganese accumulation. The data of this study are compatible with a hydrogenous precipitation of manganese by strongly varying redox-conditions at the sediment–water interface of a slope basin. Received December 21, 2001; revised version accepted February 18, 2002  相似文献   
4.
Abstract— The low temperature fine‐grained material in unequilibrated chondrites, which occurs as matrix, rims, and dark inclusions, carries information about the solar nebula and the earliest stages of planetesimal accretion. The microdistribution of primordial noble gases among these components helps to reveal their accretionary and alteration histories. We measured the Ne and Ar isotopic ratios and concentrations of small samples of matrix, rims, and dark inclusions from the unequilibrated carbonaceous chondrites Allende (CV3), Leoville (CV3), and Renazzo (CR2) and from the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1) to decipher their genetic relationships. The primordial noble gas concentrations of Semarkona, and—with certain restrictions—also of Leoville, Bishunpur, and Allende decrease from rims to matrices. This indicates a progressive accretion of nebular dust from regions with decreasing noble gas contents and cannot be explained by a formation of the rims on parent bodies. The decrease is probably due to dilution of the noble‐gas‐carrying phases with noble‐gas‐poor material in the nebula. Krymka and Renazzo both show an increase of primordial noble gas concentrations from rims to matrices. In the case of Krymka, this indicates the admixture of noble gas‐rich dust to the nebular region from which first rims and then matrix accreted. This also explains the increase of the primordial elemental ratio 36Ar/ 20Ne from rims to matrix. Larger clasts of the noble‐gas‐rich dust form macroscopic dark inclusions in this meteorite, which seem to represent unusually pristine material. The interpretation of the Renazzo data is ambiguous. Rims could have formed by aqueous alteration of matrix or—as in the case of Krymka—by progressive admixture of noble gas‐rich dust to the reservoir from which the Renazzo constituents accreted. The Leoville and Krymka dark inclusions, as well as one dark inclusion of Allende, show noble gas signatures different from those of the respective host meteorites. The Allende dark inclusion probably accreted from the same region as Allende rims and matrix but suffered a higher degree of alteration. The Leoville and Krymka dark inclusions must have accreted from regions different from those of their respective rims and matrices and were later incorporated into their host meteorites. The noble gas data imply a heterogeneous reservoir with respect to its primordial noble gas content in the accretion region of the studied meteorites. Further studies will have to decide whether these differences are primary or evolved from an originally uniform reservoir.  相似文献   
5.
Models of aggradation versus progradation in the Himalayan Foreland   总被引:1,自引:0,他引:1  
A frequent goal of decompaction analysis is to reconstruct histories of basin subsidence and tectonic loading. In marine environments, eustatic and paleobathymetric uncertainties limit the resolution of these reconstructions. Whereas in the terrestrial basins, these ambiguities are absent, it is still necessary to account for depositional slopes between localities in order to analyze three-dimensional patterns of subsidence. We define two end-members for depositional surfaces: aggradation and progradation. The relative importance of either end-member is a function of the interplay between the rate of net sediment accumulation and the rate of basin subsidence. The models predict the patterns of major drainages (transverse versus longitudinal) and the way in which provenance should be reflected within different portions of a basin. Consequently, paleocurrent and provenance data from the ancient stratigraphic record can be used to distinguish between these endmembers. The subhorizontal depositional surfaces that dominate during times of aggradation provide a well defined reference frame for regional analysis of decompacted stratigraphies and related subsidence. Depositional slopes during progradation can not be as precisely specified, and consequently yield greater uncertainties in reconstructions of subsidence. These models are applied to the Mio-Pliocene foreland basin of the northwestern Himalaya, where sequences of isochronous strata have been analyzed throughout the basin. These time-controlled data delineate a distinctive evolution from largely aggradational to largely progradational depositional geometries as deformation progressively encroaches on the foreland. Such a reconstruction of past depositional surfaces provides a well constrained reference frame for subsequent integration of subsidence histories from throughout the foreland.
Zusammenfassung Ein häufiges Ziel der Dekompaktionsanalyse ist es die Beckenabsenkung und die tektonische Belastung zu rekonstruieren. In marinen Ablagerungsräumen limitieren eustatische und paläobathymetrische Unsicherheiten die Auflösung der Rekonstruktion. Bei terrestrischen Becken fehlen diese Zweideutigkeiten; es ist aber trotzdem notwendig, Rechenschaft über den Ablagerungshang zwischen verschiedenen Lokalitäten abzulegen, um dreidimensionale Subsidenzmuster zu analysieren. Wir definieren zwei Endglieder von Ablagerangsflächen: Aggradation und Progradation. Die relative Wichtigkeit des jeweiligen Endglieds ist eine Funktion des Zusammenspiels zwischen der Nettorate der Sedimentakkumulation und der Beckensubsidenz. Die Modelle sagen die Hauptentwässerungsmuster (quer- oder längsverlaufend) vorher, sowie den Weg in dem die Sedimentherkunft innerhalb verschiedener Bereiche des Beckens berücksichtigt werden sollte. Folglich können Paläoströmungs- und Herkunftsdaten alter stratigraphischer Überlieferungen benutzt werden, um zwischen den Endgliedern zu unterscheiden. Die subhorizontale Ablagerungsfläche welche zur Zeit der Aggradation dominant ist, liefert einen gut definierten Referenzrahmen für die regionale Analyse von dekomprimierten Formationen und der damit verknüpften Subsidenz. Ablagerangshänge während Progradation können nicht präzise spezifiziert werden und beinhalten daher größere Unsicherheiten bei der Rekonstruktion der Subsidenz. Diese Modelle wurden übertragen auf das miozäne bis pliozäne Vorgebirgsbecken des nordwestlichen Himalayas, wo Sequenzen von isochronen Schichten durch das gesamte Becken analysiert werden konnten. Diese zeitkontrollierten Daten schildern eine ganz bestimmte Entwicklung, die von einer hauptsächlich aggradierenden zu einer progradierenden Ablagerangsgeometrie verlief, während der die Deformation schrittweise in Richtung Vorland übergriff. Diese Rekonstruktion von ehemaligen Ablagerangsflächen liefert einen guten Referenzrahmen für die folgende Integration der Subsidenzgeschichte des gesamten Vorlands.

Résumé L'analyse de décompaction a souvent pour but de reconstituer l'histoire de la subsidence d'un bassin et de la charge tectonique. Dans les milieux marins, de telles reconstitutions sont limitées par des incertitudes de caractère eustatique et paléobathymétrique. Par contre, ces ambiguïtés ne se présentent pas dans le cas des bassins continentaux, où il convient néanmoins de tenir compte de la pente de la surface de dépôt entre les divers points considérés pour établir un schéma tridimensionnel de la subsidence. Nous définissons deux situations extrêmes pour les surfaces de dépôt: l'aggradation et la progradation. L'importance relative de ces deux extrêmes est fonction de l'interaction entre le taux d'accumulation net des sédiments et le taux de subsidence du bassin. Les modèles prévoient la répartition des drainages principaux (transverse ou longitudinal) et la manière dont l'origine des sédiments peut se répercuter dans les diverses parties d'un bassin. Il en résulte que des informations fournies par les relevés stratigraphiques à propos des paléocourants et de la source des sédiments peuvent être utilisées pour faire la distinction entre les deux cas extrêmes. Les surfaces de dépôt subhorizontales, qui prédominent pendant les périodes d'aggradation, fournissent un bon cadre de référence pour les analyses régionales de formations décompactées et de la subsidence qui leur est associée. Les surfaces de dépôt inclinées qui se présentent au cours des progradations ne peuvent pas être définies de manière aussi précise et engendrent par conséquent plus d'incertitude dans la reconstitution de la subsidence. Les auteurs appliquent ces modèles au bassin mio-pliocène d'avant-pays de l'Himalaya nord-occidental, dans lequel des séquences de couches isochrones ont été suivies à travers tout le bassin. Ces données, chronologiquement définies, fournissent l'image d'une évolution nette, depuis des géométries typiques d'aggradation jusqu' à des géométries typiques de progradation, au fur et à mesure de l'emprise progressive de la déformation sur l'avant-pays. Une telle reconstitution des surfaces de dépôt anciennes fournit un bon cadre de référence en vue de l'intégration ultérieure de l'histoire de la subsidence dans l'ensemble de l'avant-pays.

. . ; , , . « » « ». . , , . , , , . , , . , . , . , , . .
  相似文献   
6.
Trace metals (Mn, Fe, Mo, U, Cr, V) were studied in pore waters of an intertidal flat located in the German Wadden Sea. The study system is an example of a permeable tidal flat system where pore water exchange is affected by tidal driven pressure gradients besides diffusion. Permanently installed in situ samplers were used to extract pore waters down to 5 m depth throughout one year. The samplers were either located close to the tidal flat margin or in central parts of the tidal flat. Despite dynamic sedimentological and hydrological conditions, the general trends with depth in deep tidal flat pore waters are remarkably similar to those observed in deep sea environments. Rates of trace metal cycling must be comparably large in order to maintain the observed pore water profiles. Trace metals further show similar general trends with depth close to the margin and in central parts of the tidal flat. Seasonal sampling revealed that V and Cr vary concurrent with seasonal changes in dissolved organic carbon (DOC) concentration. This effect is most notable close to the tidal flat margin where sulphate, DOC, and nutrients vary with season down to some metres depth. Seasonal variations of Mn, Fe, Mo, and U are by contrast limited to the upper decimetres of the sediment. Their seasonal patterns depend on organic matter supply, redox stratification, and particulate matter deposited on sediment surfaces. Pore water sampling within one tidal cycle provides evidence for pore water advection in margin sediments. During low tide pore water flow towards the creekbank is generated by a hydraulic gradient suggesting that deep pore waters may be seeping out of creekbank sediments. Owing to the enrichment of specific elements like Mn in pore water compared to sea water, seeping pore waters may have an impact on the chemistry of the open water column. Mass balance calculations reveal that the impact of deep pore waters on the Mn budget in the open water column is below 4%. Mn deep pore water discharge of the whole Wadden Sea is estimated to be about 9% of the total dissolved riverine Mn input into the Southern North Sea.  相似文献   
7.
8.
In autumn 2002 a time-series station was installed in the tidal inlet between the Islands of Langeoog and Spiekeroog (Southern North Sea, NW Germany) to continuously measure physical, chemical, and meteorological parameters, even during extreme weather conditions (gale-force storms, drifting ice). Inside the pole of the station sensor tubes are installed in direction of the prevailing tidal currents. The tubes are equipped with hydrographic sensors (pressure, temperature, conductivity) and allow retrieval of water for nutrient analysis by automated instruments located inside the pole. Dissolved methane and the nutrients ammonia, nitrite, nitrate, phosphate, and silicate are measured at the station.  相似文献   
9.
A new extended Gibbs thermodynamic potential of seawater   总被引:1,自引:0,他引:1  
A new and extended Gibbs thermodynamic potential function of seawater is proposed to overcome generally known weaknesses of the International Equation of State of Seawater 1980 and its associated formulas (EOS80). It is valid for applied pressures up to 100 MPa (10,000 dbar), temperatures from −2-40°C, and practical salinities up to 42. At ambient pressure, it is applicable in heat capacity and density up to salinity 50. It includes the triple point of water for reference and is, over its range of validity, fully consistent with the current 1995 international scientific pure water standard, IAPWS95. In conjunction with an improved Gibbs potential of ice, it provides freezing temperatures of seawater for pressures up to 50 MPa (5000 dbar). It is compiled from an extensive set of experimental seawater data, rather than being derived from EOS80 equations. Older seawater data were specifically recalibrated for compatibility with the recent pure water standard. By this procedure, experimental high-pressure densities proved consistent with sound speeds confirmed by deep-sea travel time measurements. Temperatures of maximum density are described within their experimental uncertainty. For very low salinities, Debye-Hückel limiting laws are recompiled using latest physical and chemical constants. The potential function is expressed in the 1990 International Temperature Scale ITS-90.  相似文献   
10.
This excursion guide results form a field trip to the Glarus nappe complex organized by the Swiss Tectonic Studies Group in 2006. The aim of the excursion was to discuss old and recent concepts related to the evolution of the Glarus thrust. The major aspects were (i) the interplay between deformation, fluid flow and geochemical alteration, (ii) episodic versus continuous deformation and fluid flow, and (iii) the link between large-scale structures, microstructures, and geochemical aspects. Despite 150 years of research in the Glarus nappe complex and the new results discussed during the excursion, there exist controversies that still are unsolved.  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