全文获取类型
收费全文 | 424篇 |
免费 | 13篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 11篇 |
大气科学 | 37篇 |
地球物理 | 82篇 |
地质学 | 225篇 |
海洋学 | 24篇 |
天文学 | 25篇 |
综合类 | 4篇 |
自然地理 | 34篇 |
出版年
2022年 | 3篇 |
2020年 | 5篇 |
2019年 | 5篇 |
2018年 | 9篇 |
2017年 | 8篇 |
2016年 | 14篇 |
2015年 | 7篇 |
2014年 | 13篇 |
2013年 | 21篇 |
2012年 | 17篇 |
2011年 | 19篇 |
2010年 | 19篇 |
2009年 | 36篇 |
2008年 | 19篇 |
2007年 | 14篇 |
2006年 | 17篇 |
2005年 | 16篇 |
2004年 | 10篇 |
2003年 | 9篇 |
2002年 | 9篇 |
2001年 | 7篇 |
2000年 | 6篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 5篇 |
1996年 | 8篇 |
1995年 | 3篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1989年 | 3篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1982年 | 4篇 |
1979年 | 3篇 |
1975年 | 3篇 |
1973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 | 5篇 |
1959年 | 3篇 |
1954年 | 4篇 |
1953年 | 4篇 |
1952年 | 3篇 |
1938年 | 3篇 |
1936年 | 2篇 |
排序方式: 共有442条查询结果,搜索用时 46 毫秒
391.
Elmar BUHL Michael H. POELCHAU Georg DRESEN Thomas KENKMANN 《Meteoritics & planetary science》2013,48(1):71-86
Abstract– Hypervelocity impact experiments on dry and water‐saturated targets of fine‐grained quartz sandstone, performed within the MEMIN project, have been investigated to determine the effects of porosity and pore space saturation on deformation mechanisms in the crater’s subsurface. A dry sandstone cube and a 90% water‐saturated sandstone cube (Seeberger Sandstein, 20 cm side length, about 23% porosity) were impacted at the Fraunhofer EMI acceleration facilities by 2.5 mm diameter steel spheres at 4.8 and 5.3 km s?1, respectively. Microstructural postimpact analyses of the bisected craters revealed differences in the subsurface deformation for the dry and the wet target experiments. Enhanced grain comminution and compaction in the dry experiment and a wider extent of localized deformation in the saturated experiment suggest a direct influence of pore water on deformation mechanisms. We suggest that the pore water reduces the shock impedance mismatch between grains and pore space, and thus reduces the peak stresses at grain–grain contacts. This effect inhibits profound grain comminution and effective compaction, but allows for reduced shock wave attenuation and a more effective transport of energy into the target. The reduced shock wave attenuation is supposed to be responsible for the enhanced crater growth and the development of “near surface” fractures in the wet target. 相似文献
392.
20世纪80年代我国鄱阳湖流域实施造林再造林工程,该区域森林面积大幅增加。大规模植物造林可能极大地影响该区域森林碳库与碳收支的变化。因此,气候变化背景下鄱阳湖流域碳平衡对中国碳循环有重要的作用。但是我们对于该地区长时间尺度的碳平衡,特别是在未来气候变化和CO2浓度上升的条件下森林生态系统碳源/汇趋势的了解不多。本研究利用过程模型InTEC模型结合区域气候模式(RIEMS2.0)模拟的未来气候资料估算了鄱阳湖流域1981—2050年碳收支情况。1981—2000年,年NPP的快速增加主要归因于大规模的植树造林;森林土壤有机碳(0-30cm)在植树造林初期每年降低1%。同时该地区森林在过去20年期间从碳源转化为碳汇。2040—2050年森林总碳库相比较2001—2010年增加0.78PgC。基于气候变化和CO2浓度增加(A1B)背景下,鄱阳湖流域NEP趋向于稳定(20—30Tgcy^-1),除了少数年份因为干旱引发了大的碳汇损失。模拟结果同样表明水分是控制该地区NEP年际变化的主要因子而NPP的年际波动主要受到温度的影响。 相似文献
393.
Harald Fritz David R. Dallmeyer Eckart Wallbrecher Jürgen Loizenbauer Georg Hoinkes Peter Neumayr Ali A. Khudeir 《Journal of African Earth Sciences》2002,34(3-4)
Regional cooling in the course of Neoproterozoic core complex exhumation in the Central Eastern Desert of Egypt is constraint by 40Ar/39Ar ages of hornblende and muscovite from Meatiq, Sibai and Hafafit domes. The data reveal highly diachronous cooling with hornblende ages clustering around 580 Ma in the Meatiq and the Hafafit, and 623 and 606 Ma in the Sibai. These 40Ar/39Ar ages are interpreted together with previously published structural and petrological data, radiometric ages obtained from Neoproterozoic plutons, and data on sediment dynamics from the intramontane Kareim molasse basin. Early-stage low velocity exhumation was triggered by magmatism initiated at 650 Ma in the Sibai and caused early deposition of molasses sediments within rim synforms. Rapid late stage exhumation was released by combined effect of strike-slip and normal faulting, exhumed Meatiq and Hafafit domes and continued until 580 Ma. We propose a new model that adopts core complex exhumation in oblique island arc collision-zones and includes transpression combined with lateral extrusion dynamics. In this model, continuous magma generation weakened the crust leading to facilitation of lateral extrusion tectonics. Since horizontal shortening is balanced by extension, no major crustal thickening and no increase of potential energy (gravitational collapse) is necessarily involved in the process of core complex formation. Core complexes were continuously but slowly exhumed without creating a significant mountain topography. 相似文献
394.
Jost Eikenberg Guido Vezzu Irene Zumsteg Sixto Bajo Max Ruethi Georg Wyssling 《Quaternary Science Reviews》2001,20(18):129
In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the 230Th/234U ingrowth system. Additional analyses of further radionuclides within the 238U decay chain, i.e. 226Ra and 210Pb, showed that the Th/U chronometer started with insignificant inherited 230Th over the entire formation period of the travertine setting (i.e. 230Th(0)=0). A contribution from detrital impurities with 230Th/234U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise 230Th/234U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the 238U-decay series, based on decay of excess 226Ra normalized to the initial, i.e.226Raex/226Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the 230Th/234U chronometer. Analytical solutions of the coupled 234U/230Th/226Ra radionuclide system predicted that the 226Raex/226Ra(0) chronometer is independent of the actual 230Th activity build up from decay of 234U, if the systems starts with zero inherited 230Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated 226Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of 226Ra. This is concluded because (i) the 226Raex/226Ra(0) ages agreed well with those derived from 230Th/234U, (ii) all data plot within uncertainty on the 226Raex/226Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, 226Raex/226Ra(0) should be applicable to materials which are suitable for 230Th/234U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support 230Th/234U for samples that have been formed a few thousand years ago. 相似文献
395.
396.
Simulating search behaviour of fish towards bait 总被引:2,自引:0,他引:2
Vabo Rune; Huse Geir; Ferno Anders; Jorgensen Terje; Lokkeborg Svein; Skaret Georg 《ICES Journal of Marine Science》2004,61(7):1224-1232
397.
Three years of mobile barrier operations have been simulated with a hydrodynamic model to check the efficiency of the barriers in defending the city of Venice from flooding. The simulations have been carried out in the actual situation and with a sea-level rise of 30 and 50 cm. Moreover, the interference of the barrier operations with the ship traffic has been studied. It is found that without a security increment for the forecasted water levels, the mobile barriers cannot defend completely Venice from flooding due to the uncertainty in the forecast. With a security increment of 10 cm, the barriers work well in actual conditions but still cannot avoid flooding with a global sea-level rise. The interference with the ship traffic is acceptable under actual conditions but becomes prohibitive with a sea-level rise of 50 cm, when nearly two-thirds of the ship passages are blocked or delayed. 相似文献
398.
399.
Dr. Georg Dresen 《International Journal of Earth Sciences》1985,74(1):95-108
Zusammenfassung Die Kartierung und tektonische Bearbeitung eines Ausschnittes des Westlichen Hohen Atlas südlich von Marrakech ergab detaillierte Vorstellungen über den herrschenden Schollenbau. Ein z. T. spätvariszisch angelegtes Störungsmuster von N 60° E, N 20° E und N 110° E streichenden Verwerfungen kontrollierte sowohl die Subsidenz in der Trias als auch die Heraushebung im Zuge der jungen Atlasgebirgsbildung im östlichen Teil der Axialzone des Gebirges. Die Vertikaltektonik erzeugte ein über den Raum des Arbeitsgebietes hinaus nachweisbares Kippschollenmuster. Das jüngere Deckgebirge wurde z. T. auf salinarem Untergrund abgeschert oder reagierte passiv auf die Bruchtektonik des Sockels. Damit verbundene Seitenverschiebungen werden diskutiert.
Mapping and tectonic analysis of a part of the Western High Atlas near Marrakech reveals a predominating pattern of distinct fault bounded blocks. The consistent frame of N 60° E, N 20° E and N 110° E striking faults is partly herited from late hercynian or even older lineaments. The former controlled sedimentation processes during the Triassic as well as during Tertiary and Quaternary uplift of the central zone (massif ancien). Vertical motions of the rigid basement caused forced folding of the Mesozoic-Tertiary sedimentary layers. Implication of left lateral oblique-slip displacement is being discussed.
Résumé L'étude structurale présentée ici se rapporte à une portion du Haut Atlas marocain située au sud de Marrakech. Elle est fondée sur un levé cartographique et sur l'analyse tectonique de la région qui s'étend entre l'Oued n'Fis et l'Oued Ourika (massif ancien). Cette étude a montré qu'il ne s'y est pas développé une déformation de style homogène mais que la région se compose de blocs isolés, séparés par des failles. Les blocs sont de dimensions diverses et diffèrent aussi bien par leur style de déformation que par les matériaux qui les constituent. Ce cadre structural est défini par des failles inverses de directions caractéristiques N 60 °E, N 20 °E et N 110 °E. Cette distribution se retrouve dans d'autres régions du Haut Atlas. Des changements brusques d'épaisseur aux environs des failles majeures montrent qu'elles ont contrôlé la subsidence pendant le Permo-Trias et en partie déjà pendant le Viséen. L'apparition d'une deformation cassante ou de plis isopaques dépend des matériaux concernés et non du »Stockwerk«. Une schistosité de fracture apparaît sporadiquement. L'intensité de l'orogenèse hercynienne paraît faible comparée à celle de la région située plus à l'Ouest. Des mouvements de décrochement senestres sont rattachés à la dernière activité des failles majeures. Leur signification dans le cadre structural est discutée.Des discordances cartographiques témoignent de la déformation atlasique et du soulèvement de la chaîne du Haut Atlas.Cette inversion est dominée par des mouvements surtout verticaux le long de failles inverses très inclinées ou de type »oblique-slip«.
. , , N 60° , N 20° N 100° , , , . . , . .相似文献
400.
One large (8.5 mm) chondrule in the Bali carbonaceous chondrite is strongly enriched in refractory elements similarily to the white objects in Allende which are widely believed to be “primitive” condensates. However, detailed investigations show the Bali chondrule to have an element distribution opposite to that predicted by the “primitive condensation” models. Refractory elements like Ti and Al are enriched at or near the surface of the chondrule. The contents of some volatile elements, e.g. Na, Si, and Cr, are higher in the central portion. It is suggested that this chondrule - and similar objects in other C-chondrites - originated in one, or more likely several, impact events and that the element distribution is the result of volatilization-condensation. 相似文献