全文获取类型
收费全文 | 860篇 |
免费 | 41篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 4篇 |
大气科学 | 78篇 |
地球物理 | 178篇 |
地质学 | 384篇 |
海洋学 | 87篇 |
天文学 | 89篇 |
综合类 | 1篇 |
自然地理 | 84篇 |
出版年
2022年 | 2篇 |
2021年 | 14篇 |
2020年 | 18篇 |
2019年 | 18篇 |
2018年 | 20篇 |
2017年 | 26篇 |
2016年 | 32篇 |
2015年 | 33篇 |
2014年 | 29篇 |
2013年 | 55篇 |
2012年 | 41篇 |
2011年 | 62篇 |
2010年 | 46篇 |
2009年 | 42篇 |
2008年 | 48篇 |
2007年 | 51篇 |
2006年 | 50篇 |
2005年 | 41篇 |
2004年 | 23篇 |
2003年 | 22篇 |
2002年 | 26篇 |
2001年 | 15篇 |
2000年 | 18篇 |
1999年 | 16篇 |
1998年 | 10篇 |
1997年 | 11篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 9篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 13篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1969年 | 1篇 |
1968年 | 2篇 |
排序方式: 共有905条查询结果,搜索用时 46 毫秒
861.
Alla Shogenova Anne Kleesment Ann Hirt Enn Pirrus Toivo Kallaste Kazbulat Shogenov Rein Vaher 《Studia Geophysica et Geodaetica》2009,53(1):111-131
The formation of hematite and goethite concretions in different sedimentary rocks including sandstones is an important diagenetic
process in the geologic history of the Earth. Its interpretation can also contribute to understanding the diagenetic history
of Martian iron hydroxide concretions.
A case study of iron-rich concretions from Estonian Middle Devonian sandstones exposed in ancient river valleys in southeastern
Estonia was carried out based on the results of mineralogical, petrographical, geochemical, petrophysical and magnetic analyses.
It was found that the high Fe2O3(total) content (25.0–39.5%), high magnetic susceptibility, bulk and grain density, very low porosity, corrosion and fracturing
of the quartz grains of the platy iron concretions are in contrast with properties of the Devonian host sandstones. However
the ferrous iron content (measured as FeO) of iron-rich concretions was as low as in the other Devonian rocks, suggesting
an oxidizing environment and arid climate during the cementation by iron-hydroxides. The fracturing of quartz grains cemented
by iron hydroxides could take place at near-surface conditions including vadose and phreatic zones in arid climate with high
evaporation rates. Such climatic conditions have been reported for the Baltic region during Devonian, Upper Permian and Triassic
times.
We have found that goethite is prevalent in the cement, replacing clay and carbonate minerals. We assume that this iron-rich
cement is originated from the mobilization of iron in host sandstones by groundwater, associated with tectonic activity at
the end of the Middle Devonian, evidenced by fracturing in Devonian outcrops and caves. Although this mobilization could occur
under reducing conditions, precipitation of goethite and hematite for the cementation could take place in oxidizing environment
along bedding planes close to the surface during short sedimentation breaks. Another possible time for the formation of iron
concretions could be Permian, under the condition of both arid climate and tectonic activity. 相似文献
862.
Lagrangian approaches are well suited to transport in contrasted media but have been considered irrelevant when inversion is envisioned. The randomness of results for the same transport scenario adds to the rough evaluation by perturbation of the sensitivities, yielding an inaccurate search of parameters. It is shown here how a Time Domain Random Walk (TDRW) method can be inverted by deriving the sensitivities analytically. The calculations are very rapid and provide a precise evaluation of the descent directions followed by a Gauss–Newton optimizer. The method handles advection–dispersion + retention by matrix diffusion or sorption with first-order kinetics and proves its worth in all cases. Since analytical sensitivities are available, calculations are rigorous and allow discussing the inversion feasibility, the accuracy of the sought parameters, according to the predominant mechanism involved in the transport scenario. 相似文献
863.
A caudal vertebra collected from conglomerates of the Al-Khod Formation (Late Cretaceous) in the Al-Khod area, Sultanate of Oman, is assigned to a medium-sized theropod dinosaur. The Al-Khod discovery represents one of the very few dinosaur records from the Middle East. 相似文献
864.
We present new palaeomagnetic and isotopic data from the southern Victoria Land region of the Transantarctic Mountains in East Antarctica that constrain the palaeogeographic position of this region during the Late Cambrian and Early Ordovician. A new pole has been determined from a dioritic intrusion at Killer Ridge (40 Ar/39 Ar biotite age of 499 ± 3 Ma) and hornblende diorite dykes at Mt. Loke (21°E, 7°S, A 95 = 8°, N = 6 VGPs). The new Killer Ridge/Mt. Loke pole is indistinguishable from Gondwana Late Cambrian and Early Ordovician poles. Previously reported palaeomagnetic poles from southern Victoria Land have new isotopic age constraints that place them in the Late Cambrian rather than the Early Ordovician. Based upon the new palaeomagnetic and isotopic data, new Gondwana Late Cambrian and Early Ordovician mean poles have been calculated. 相似文献
865.
Trine Marianne Holm Karin A. Koinig Tom Andersen Espen Donali Anne Hormes Dag Klaveness Roland Psenner 《Aquatic Sciences - Research Across Boundaries》2012,74(3):385-395
High Arctic lakes are among the most sensitive ecosystems and climate change strongly affects their physical properties, especially water temperature, and mixing processes. To study the effect of recent climate change on such a lake in the Arctic environment, we measured water chemistry and temperature from 2005 to 2010 in Kongressvatn, a crenogenic meromictic lake in Spitsbergen (Svalbard). In addition, we monitored water column temperatures during two consecutive years and compared them to regional air temperature data and physicochemical lake data from 1962 and 1968, two relatively cold years. Summer surface water temperature was highly correlated to air temperature, and both have increased by approximately 2°C since 1962. Temperature monitoring during 2?years showed that the warm summer of 2007 resulted in increased water temperatures even in the stratified, denser hypolimnion. Our water chemistry measurements showed that the chemocline position in 2005?C2010 was ca 12?m deeper than in 1962?C1968, and a second, weaker, chemocline appeared at metalimnetic depths of 7?C15?m. During the study period, the water level decreased by 4?m, and this change accelerated between 2008 and 2010. Our data support the hypothesis that water temperatures and stratification patterns are changing rapidly with air temperature, but changes in the catchment, such as glacial retreat and permafrost melting, may have an even stronger impact on lake properties. 相似文献
866.
Kroon FJ Kuhnert PM Henderson BL Wilkinson SN Kinsey-Henderson A Abbott B Brodie JE Turner RD 《Marine pollution bulletin》2012,65(4-9):167-181
Degradation of coastal ecosystems in the Great Barrier Reef (GBR) lagoon, Australia, has been linked with increased land-based runoff of suspended solids, nutrients and pesticides since European settlement. This study estimated the increase in river loads for all 35 GBR basins, using the best available estimates of pre-European and current loads derived from catchment modelling and monitoring. The mean-annual load to the GBR lagoon for (i) total suspended solids has increased by 5.5 times to 17,000ktonnes/year, (ii) total nitrogen by 5.7 times to 80,000tonnes/year, (iii) total phosphorus by 8.9 times to 16,000tonnes/year, and (iv) PSII herbicides is 30,000kg/year. The increases in river loads differ across the 10 pollutants and 35 basins examined, reflecting differences in surface runoff, urbanisation, deforestation, agricultural practices, mining and retention by reservoirs. These estimates will facilitate target setting for water quality and desired ecosystem states, and enable prioritisation of critical sources for management. 相似文献
867.
Collignon A Hecq JH Glagani F Voisin P Collard F Goffart A 《Marine pollution bulletin》2012,64(4):861-864
Neustonic microplastic and zooplankton abundance was determined in the North Western Mediterranean Sea during a summer cruise between July 9th and August 6th 2010, with a break between July 22 th and 25th due to a strong wind event. Ninety percent of the 40 stations contained microplastic particles (size 0.3-5mm) of various compositions: e.g., filaments, polystyrene, thin plastic films. An average concentration of 0.116 particles/m(2) was observed. The highest abundances (>0.36 particles/m(2)) were observed in shelf stations. The neustonic plastic particles concentrations were 5 times higher before than after the strong wind event which increased the mixing and the vertical repartition of plastic particles in the upper layers of the water column. The values rise in the same order of magnitude than in the North Pacific Gyre. The average ratio between microplastics and mesozooplankton weights was 0.5 for the whole survey and might induce a potential confusion for zooplankton feeders. 相似文献
868.
Global Dynamics of the MLT 总被引:3,自引:0,他引:3
Anne K. Smith 《Surveys in Geophysics》2012,33(6):1177-1230
The transition between the middle atmosphere and the thermosphere is known as the MLT region (for mesosphere and lower thermosphere). This area has some characteristics that set it apart from other regions of the atmosphere. Most notably, it is the altitude region with the lowest overall temperature and has the unique characteristic that the temperature is much lower in summer than in winter. The summer-to-winter-temperature gradient is the result of adiabatic cooling and warming associated with a vigorous circulation driven primarily by gravity waves. Tides and planetary waves also contribute to the circulation and to the large dynamical variability in the MLT. The past decade has seen much progress in describing and understanding the dynamics of the MLT and the interactions of dynamics with chemistry and radiation. This review describes recent observations and numerical modeling as they relate to understanding the dynamical processes that control the MLT and its variability. Results from the Whole Atmosphere Community Climate Model (WACCM), which is a comprehensive high-top general circulation model with interactive chemistry, are used to illustrate the dynamical processes. Selected observations from the Sounding the Atmosphere with Broadband Emission Radiometry (SABER) instrument are shown for comparison. WACCM simulations of MLT dynamics have some differences with observations. These differences and other questions and discrepancies described in recent papers point to a number of ongoing uncertainties about the MLT dynamical system. 相似文献
869.
Phytolith indices as proxies of grass subfamilies on East African tropical mountains 总被引:2,自引:0,他引:2
Laurent Bremond Anne Alexandre Matthew J. Wooller Christelle Hly David Williamson Peter A. Schfer Amos Majule Joël Guiot 《Global and Planetary Change》2008,61(3-4):209-224
The main objective of this paper is to provide researchers that investigate fossil phytolith assemblages and model/data comparisons a new tool for estimating C3/C4 grass composition over time. We tested the reliability of modern soil phytolith assemblages and phytolith indices for tracing the dominance of different grass subfamilies and tree cover density. We analyzed modern soil phytolith assemblages from sites over elevation gradients on Mount Kenya (Kenya), Mount Rungwe and around Lake Masoko (southern Tanzania). These data were compared with available botanical data. A phytolith index named Ic, proved to be an effective proxy of the proportions of Pooideae, Arundinoideae and Bambusoideae grasses (mainly C3 grasses) versus Panicoideae grasses (mainly C4 grasses), increasing with elevation in East-Africa. When tropical mountains are covered by open habitats (e.g. grasses and shrublands), Ic should be a reliable proxy of the C3/C4 grass composition. These results highlight the value of the phytolith index Ic, when interpreting paleo-environmental records from tropical mountains, to: 1) better understand past local and regional C3/C4 grass distributions and associated climatic changes and 2) increase the set of C3/C4 data available for model/data comparisons. 相似文献
870.
Global atmosphere-ocean general circulation models are the tool by which projections for climate changes due to radiative forcing scenarios have been produced. Further, regional atmospheric downscaling of the global models may be applied in order to evaluate the details in, e.g., temperature and precipitation patterns. Similarly, detailed regional information is needed in order to assess the implications of future climate change for the marine ecosystems. However, regional results for climate change in the ocean are sparse. We present the results for the circulation and hydrography of the Barents Sea from the ocean component of two global models and from a corresponding pair of regional model configurations. The global models used are the GISS AOM and the NCAR CCSM3. The ROMS ocean model is used for the regional downscaling of these results (ROMS-G and ROMS-N configurations, respectively). This investigation was undertaken in order to shed light on two questions that are essential in the context of regional ocean projections: (1) How should a regional model be set up in order to take advantage of the results from global projections; (2) What limits to quality in the results of regional models are imposed by the quality of global models? We approached the first question by initializing the ocean model in the control simulation by a realistic ocean analysis and specifying air-sea fluxes according to the results from the global models. For the projection simulation, the global models’ oceanic anomalies from their control simulation results were added upon initialization. Regarding the second question, the present set of simulations includes regional downscalings of the present-day climate as well as projected climate change. Thus, we study separately how downscaling changes the results in the control climate case, and how scenario results are changed. For the present-day climate, we find that downscaling reduces the differences in the Barents Sea between the original global models. Furthermore, the downscaled results are closer to observations. On the other hand, the downscaled results from the scenario simulations are significantly different: while the heat transport into the Barents Sea and the salinity distribution change modestly from control to scenario with ROMS-G, in ROMS-N the heat transport is much larger in the scenario simulation, and the water masses become much less saline. The lack of robustness in the results from the scenario simulations leads us to conclude that the results for the regional oceanic response to changes in the radiative forcing depend on the choice of AOGCM and is not settled. Consequently, the effect of climate change on the marine ecosystem of the Barents Sea is anything but certain. 相似文献