全文获取类型
收费全文 | 401篇 |
免费 | 19篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 37篇 |
地球物理 | 93篇 |
地质学 | 126篇 |
海洋学 | 44篇 |
天文学 | 61篇 |
综合类 | 4篇 |
自然地理 | 44篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 16篇 |
2020年 | 19篇 |
2019年 | 17篇 |
2018年 | 19篇 |
2017年 | 15篇 |
2016年 | 22篇 |
2015年 | 17篇 |
2014年 | 25篇 |
2013年 | 35篇 |
2012年 | 17篇 |
2011年 | 22篇 |
2010年 | 23篇 |
2009年 | 14篇 |
2008年 | 21篇 |
2007年 | 18篇 |
2006年 | 16篇 |
2005年 | 13篇 |
2004年 | 9篇 |
2003年 | 11篇 |
2002年 | 8篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1991年 | 1篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1977年 | 1篇 |
1973年 | 2篇 |
1968年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有421条查询结果,搜索用时 46 毫秒
401.
On early Solar System chronology: Implications of an heterogeneous spatial distribution of Al and Mn
Early Solar System chronology is usually built with the assumption that the distribution of short-lived radionuclides was homogeneous through the solar accretion disk. At present, there is no unambiguous evidence for a homogeneous distribution of short-lived radionuclides in the solar accretion disk, while some data point to a heterogeneous distribution of short-lived radionuclides. In this paper, we explore a possible chronology based on a heterogeneous distribution of 26Al and 53Mn in the accretion disk. Our basic assumption is that the different abundances of extinct short-lived radionuclides in calcium-aluminium-rich inclusions (CAIs) and chondrules are due to spatial rather than temporal differences. We develop a simple model where CAIs and chondrules form contemporaneously, in different spatial locations, and are characterised by distinct initial 26Al and 53Mn abundances. In this model, all evolved bodies are supposed to be originally chondritic, i.e., to be made of a mixture of CAIs, chondrules, and matrix. This mixture determines the initial content in 26Al and 53Mn of a chondritic parent-body as a function of its CAI and chondrule abundance fraction. This approach enables us to calculate coherent 26Al and 53Mn ages from the agglomeration of the parent-body precursors (CAIs and chondrules) until the isotopic closure of 26Al and 53Mn, thereafter called 26Al-53Mn age. We calculate such 26Al-53Mn ages for a diversity of evolved objects, with the constraint that they should be found for realistic chondritic parent-body precursors, i.e., objects having similar or identical petrograpy to the existing chondrite groups. The so defined age of the d’Orbigny angrite is 4.3 ± 1.1 Myr, for the Asuka-881394 eucrite 2.8 ± 1.0 Myr, for the H4 chondrite Sainte Marguerite ∼3 Myr, and for H4 Forest Vale ∼5 Myr. The calculated 26Al-53Mn ages give timescales for the evolution of the respective parent-bodies/meteorites that can be investigated in the light of further petrographic studies. We anchor the calculated relative chronology to an absolute chronology using absolute Pb-Pb ages and relative Hf-W ages of the objects under scrutiny. The precursors of Sainte Marguerite and Forest Vale agglomerated at the same time (∼4565.8 ± 1.2 Ma ago). The precursors of eucrites (Asuka-881394) agglomerated 4564.8 ± 1.2 Ma ago. The precursors of angrites agglomerated late (4561.5 ± 1.8 Ma ago). Our model provides a fully compatible Al-Mg/Mn-Cr/Pb-Pb chronology, and is shown to be robust to reasonable changes in the input parameters. The calculated initial 26Al/27Al ratios are high enough to have 26Al as a possible heat source for differentiation. 相似文献
402.
403.
Vyacheslav M. Zobin J. Francisco Ventura-Ramírez Clarita L. Gutiérrez-Andrade Lidia Hernández Cruz Sara Santibáñez-Ibáñez 《Natural Hazards》2006,38(3):391-410
The Mw 7.4 earthquake of 21 January 2003 occurred within the Mexican subduction zone and produced many damages of masonry
constructions in the towns of Colima state, México. The macroseismic investigation of damages produced by the earthquake in
Colima city was realized for 3,332 constructions within the area of study representing about 20% of the total city area and
covered with the different type of constructions. The 7-grade scale of damage was used to describe the damage distribution.
The damage matrix, constructed for the area, showed that the damage distribution varied from 63% of constructions with relatively
slight damages (grades 1–3) to 29% of constructions that had significative damages (grades 4–5) and 8% of completely destructed
or demolished masonry. The damage matrices, constructed for 12 subzones of the area of study, reflected two tendencies in
the damage distributions: the predominance of slight damages of the recent constructions situated in the northern and eastern
parts of the area and the predominance of significant damages of the older constructions in the southern and western parts
of the area. It was observed a significant dependence of damage index upon the age of constructions and the type of masonry.
The comparison of the observed damage matrix with the damage probability matrix calculated for Colima masonry in 1999 gives
MM intensity VII in Colima. 相似文献
404.
Dominique Similox-Tohon Manuel Sintubin Philippe Muchez Griet Verhaert Kris Vanneste Max Fernandez Sara Vandycke Hannelore Vanhaverbeke Marc Waelkens 《Tectonophysics》2006,420(3-4):371-387
The archaeological site of Sagalassos (SW Turkey) is located in a region characterized by the absence of any significant recent seismic activity, contrary to adjacent regions. However, the assessment of earthquake-related damage at the site suggests that the earthquakes that have been demonstrated to have struck this Pisidian city in ca. AD 500 and in the middle or second half of the 7th century AD are characterized by an MSK intensity of at least VIII and occurred on a fault very close to the city. Different investigation techniques (archaeoseismology, remote sensing and geomorphology, surface geology and structural data, 2D resistivity imaging and palaeoseismological trenching) have been applied at the archaeological site and its direct surroundings in search for the causative fault of these earthquakes. This multidisciplinary approach shows that each of the different approaches independently provides only partial, non-conclusive information with respect to the fault identification. Integration is imperative to give a conclusive answer in the search for the causative fault. This study has, indeed, revealed the existence of a to date unknown active normal fault system passing underneath ancient Sagalassos, i.e. the Sagalassos fault. A historical coseismic surface rupture event on this fault could be identified. This event possibly corresponds to the devastating Sagalassos earthquakes of ca. AD 500 and the middle or second half of the 7th century AD. Finally, this study demonstrates that in the particular geodynamic setting of SW Turkey archaeological sites with extensive earthquake-related damage form an important tool in any attempt to asses the seismic hazard. 相似文献
405.
The Mesozoic ostracod genus Arculicythere Grékoff: further evidence for the southern Gondwana seaway
This paper demonstrates that Arculicythere Grékoff is a widespread taxon in the uppermost Upper Jurassic and Lower Cretaceous of Gondwana. It occurs in Madagascar, India, Israel, South Africa, the Falklands Plateau, Argentina and Australia. The earliest record is from the Upper Tithonian of Madagascar but the most profuse occurrences are in the Albian, when the genus was very widespread around Gondwana. Its distribution seems to be associated with a major southern Gondwana seaway, and the genus provides additional proof of the existence of this corridor. Unlike another endemic gondwanine genus, Majungaella Grékoff, which ranged from earlier in the Jurassic but extended into the Neogene in the Antarctic, or Rostrocytheridea Dingle that survived into the Campanian of the Antarctic, Arculicythere seems to have become extinct in the Albian. 相似文献
406.
The Atlantic horsehoe crab,Limulus polyphemus, is harvested by the biomedical industry for Limulus Amoebocyte Lysate (LAL) a blood compound used to detect endotoxins,
for scientific research on vision, and as bait for the eel and conch fisheries. They are also subject to sources of natural
mortality such as consumption of eggs by shorebirds and stranding. Recently there has been growing concern over possible declines
inLimulus populations along the east coast of the United States. We have used stage-structured matrix modeling to improve our understanding
ofLimulus population dynamics in three Cape Cod estuaries (Stage Harbor, Pleasant Bay, and Barnstable Harbor) with differing harvest
pressures. The results suggest that a low level of harvest is sustainable, particularly if timing and size-specificity of
the harvest are taken into consideration. The model also suggests that efforts should be made to avoid harvesting crabs prior
to sexual maturity, as the population growth rate is more sensitive to changes in survival rate of older juveniles than of
other stages. The information the model provides could assist local and regional fisheries managers in establishing appropriate
regulations for the horseshoe crab fishery. 相似文献
407.
Well-developed filament channels may be present in the solar atmosphere even when there is no trace of filament material inside
them. Such magnetic systems with filament channels without filaments can result in coronal mass ejections that might appear
to have no corresponding solar surface source regions. In this case study, we analyze CMEs on 9 August 2001 and 3 March 2011
and trace their origins to magnetic systems with filament channels containing no obvious filament material on the days around
the eruptions. 相似文献
408.
Rios Sara Kowalska Magdalena Viana da Fonseca António 《Geotechnical and Geological Engineering》2021,39(5):3449-3467
Geotechnical and Geological Engineering - In this paper, the possibility of using fine scrap tyre rubber to improve the mechanical properties of soil subjected to cyclic loading is... 相似文献
409.
Evaluating channel response to an extreme sedimentation event in the context of historical range of variability: Upper Colorado River,USA 总被引:1,自引:0,他引:1
In May 2003, a breach in a large irrigation ditch within Rocky Mountain National Park (RMNP) initiated a debris flow that entered Lulu Creek and the Colorado River, where 36 000 m3 of sediment substantially altered channel forms and processes. We present a proof of concept to understand whether the 2003 disturbance is within the historical range of variability (HRV), and whether the recovery potential of the system is sufficient to adapt to the disturbance. Flow and sediment regimes, and channel morphology and stability were monitored on Lulu Creek and the Colorado River from 2004 to 2011. Dominant channel response following the debris flow within Lulu Creek included step development, bed armoring, and channel widening. Step height‐to‐length ratios (H/L) for three reaches on Lulu Creek are outside the HRV of reference channels, with one reach approaching reference conditions. Erosion of approximately 23% of the debris fan volume occurred as a result of the long duration 2011 peak flow. Sediment within the Lulu Creek fan will persist for ~30–190 years, assuming current maximum and mean removal rates. Planform changes on the Colorado River since the debris flow include an increase in single‐thread geometries, with braided reaches where bar deposition occurred. Bedload transport and grain‐size analysis of bedload indicate translational spreading of a sand wave front with a dispersive component in steeper reaches. Lulu Creek is returning to a condition of natural variability, but the Colorado River is outside the HRV expected for steep‐gradient, pool‐riffle channels. Applying HRV to a situation where management questions require a longer term perspective, and pre‐disturbance baseline data are limited, is a useful approach. The HRV analysis facilitates a better understanding of site variability and delineates the range of possibilities of channel form and process to achieve management goals. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
410.
Marianne Milano Denis Ruelland Sara Fernandez Alain Dezetter Julie Fabre Eric Servat 《Comptes Rendus Geoscience》2012,344(9):432-440
The Mediterranean basin has been identified as one of the world's most vulnerable regions to climatic and anthropogenic changes. A methodology accounting for the basin specific conditions is developed to assess the current and future water stress state of this region. The medium-term evolution of water stress is investigated using climatic scenarios and a water-use scenario based on efficiency improvements following the recommendations of the Mediterranean Strategy for Sustainable Development. Currently, the southern and eastern rims are experiencing high to severe water stress. By the 2050 horizon, a 30–50% decline in freshwater resources is simulated over most of the Mediterranean basin. While total water withdrawals would stabilize, or even decrease (10–40%), in several northern catchments, they would double in southern and eastern catchments. These changes should significantly increase water stress over the Mediterranean basin and exacerbate the disparities between rims. 相似文献