全文获取类型
收费全文 | 24565篇 |
免费 | 176篇 |
国内免费 | 918篇 |
专业分类
测绘学 | 1415篇 |
大气科学 | 1992篇 |
地球物理 | 4559篇 |
地质学 | 11620篇 |
海洋学 | 1031篇 |
天文学 | 1655篇 |
综合类 | 2161篇 |
自然地理 | 1226篇 |
出版年
2023年 | 1篇 |
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 4篇 |
2018年 | 4762篇 |
2017年 | 4038篇 |
2016年 | 2581篇 |
2015年 | 234篇 |
2014年 | 87篇 |
2013年 | 29篇 |
2012年 | 992篇 |
2011年 | 2736篇 |
2010年 | 2021篇 |
2009年 | 2319篇 |
2008年 | 1898篇 |
2007年 | 2371篇 |
2006年 | 70篇 |
2005年 | 202篇 |
2004年 | 412篇 |
2003年 | 411篇 |
2002年 | 256篇 |
2001年 | 52篇 |
2000年 | 55篇 |
1999年 | 23篇 |
1998年 | 21篇 |
1997年 | 4篇 |
1995年 | 1篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1983年 | 2篇 |
1981年 | 24篇 |
1980年 | 20篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 6篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
341.
Przemyslaw Dera John D. Lazarz Vitali B. Prakapenka Madison Barkley Robert T. Downs 《Physics and Chemistry of Minerals》2011,38(7):517-529
Single-crystal X-ray diffraction experiments with SiO2 α-cristobalite reveal that the well-known reversible displacive phase transition to cristobalite-II, which occurs at approximately
1.8 GPa, can be suppressed by rapid pressure increase, leading to an overpressurized metastable state, persisting to pressure
as high as 10 GPa. In another, slow pressure increase experiment, the monoclinic high-pressure phase-II was observed to form
at ~1.8 GPa, in agreement with earlier in situ studies, and its crystal structure has been unambiguously determined. Single-crystal
data have been used to refine the structure models of both phases over the range of pressure up to the threshold of formation
of cristobalite X-I at ~12 GPa, providing important constraints on the feasibility of the two competing silica densification
models proposed in the literature, based on quantum mechanical calculations. Preliminary diffraction data obtained for cristobalite
X-I reveal a monoclinic unit cell that contradicts the currently assumed model. 相似文献
342.
The impact of realistic representation of sea surface temperature (SST) on the numerical simulation of track and intensity
of tropical cyclones formed over the north Indian Ocean is studied using the Weather Research and Forecast (WRF) model. We
have selected two intense tropical cyclones formed over the Bay of Bengal for studying the SST impact. Two different sets
of SSTs were used in this study: one from TRMM Microwave Imager (TMI) satellite and other is the weekly averaged Reynold’s
SST analysis from National Center for Environmental Prediction (NCEP). WRF simulations were conducted using the Reynold’s
and TMI SST as model boundary condition for the two cyclone cases selected. The TMI SST which has a better temporal and spatial
resolution showed sharper gradient when compared to the Reynold’s SST. The use of TMI SST improved the WRF cyclone intensity
prediction when compared to that using Reynold’s SST for both the cases studied. The improvements in intensity were mainly
due to the improved prediction of surface latent and sensible heat fluxes. The use of TMI SST in place of Reynold’s SST improved
cyclone track prediction for Orissa super cyclone but slightly degraded track prediction for cyclone Mala. The present modeling
study supports the well established notion that the horizontal SST gradient is one of the major driving forces for the intensification
and movement of tropical cyclones over the Indian Ocean. 相似文献
343.
This study analyzes the response of glacier to climate change during the past 49 years in Urumqi River source region, the
Tianshan Mountains of China. The temporal and spatial variations of winter mass balance (bn-w) at different time scales were
analyzed to identify their response to climate change during 1988–2006 (The observation of winter mass balance observation
began in 1988) on the Glacier No.1 at the headwaters of the Urumqi River, Tianshan Mountains, China. The winter accumulation
shows a significantly decreasing trend. The results show that the cumulative values on Glacier No.1 is 2,202 mm water equivalent
during 1988–2006 and the mean values is 116 mm a−1. Furthermore, the trend analysis of the winter mass balance indicates a rapid decrease since 1990, and the mean mass balance
is only 79 mm a−1 during 1997–2006. Winter mass balance correlates well negatively with the total evaporation from September to April (r = −0.68, α = 0.01), and positively with the total precipitation from September to April (r = 0.74, α = 0.01). However, winter mass balance shows a weak correlation with mean minimum air temperature during September
to April (r = −0.35), and runoff on September (r = −0.13). 相似文献
344.
Experimental investigations on the influence of cyclical freezing and thawing on physical and mechanical properties of saline soil 总被引:4,自引:1,他引:4
Uniaxial compressive strength experiments were performed on saline loess different water and salt contents, and multiple freeze/thaw
cycles at room temperature. The experiments revealed changes to the stress–strain curve and the failure mode of the saline
loess. The results indicated that soil strength is intensified or weakened during freezing and thawing due to the injection
of sodium sulfate solute. Cyclical freezing and thawing is a process to get a new dynamic equilibrium, and under these experimental
conditions, six freeze–thaw cycles are need for loess with sodium sulfate to reach a new dynamic equilibrium. 相似文献
345.
Daniel Meshesha Ryuichi Shinjo Risa Matsumura Takele Chekol 《Contributions to Mineralogy and Petrology》2011,162(5):889-907
Mantle xenoliths entrained in Quaternary alkaline basalts from the Turkana Depression in southern Ethiopia (the East Africa
Rift) were studied for their geochemical and Sr–Nd–Pb isotopic compositions to constrain the evolution of the lithosphere.
The investigated mantle xenoliths are spinel lherzolites in composition with a protogranular texture. They can be classified
into two types: anhydrous and hydrous spinel lherzolites; the latter group characterized by the occurrences of pargasite and
phlogopite. The compositions of whole-rock basaltic component (CaO = 3.8–5.6 wt%, Al2O3 = 2.5–4.1 wt%, and MgO = 34.7–38.1 wt%), spinel (Cr# = 0.062–0.117, Al2O3 = 59.0–64.4 wt%) and clinopyroxene (Mg# = 88.4–91.7, Al2O3 = 5.2–6.7 wt%) indicate that the lherzolites are fertile and have not experienced significant partial melting. Both types
are characterized by depleted 87Sr/86Sr (0.70180–0.70295) and high 143Nd/144Nd (0.51299–0.51348) with wide ranges of 206Pb/204Pb (17.86–19.68) isotopic compositions. The variations of geochemical and isotopic compositions can be explained by silicate
metasomatism induced by different degree of magma infiltrations from ascending mantle plume. The thermobarometric estimations
suggest that the spinel lherzolites were derived from depths of 50–70 km (15.6–22.2 kb) and entrained in the alkaline magma
at 847–1,052°C. Most of the spinel lherzolites from this study record an elevated geotherm (60–90 mW/m2) that is related to the presence of rising mantle plume in an active tectonic setting. Sm–Nd isotopic systematic gives a
mean TDM model age of 0.95 Ga, interpreted as the minimum depletion age of the subcontinental lithosphere beneath the region. 相似文献
346.
Threat of land subsidence in and around Kolkata City and East Kolkata Wetlands,West Bengal,India 总被引:4,自引:0,他引:4
This paper attempts to estimate the possible rate of land subsidence of Kolkata City including Salt Lake City and the adjoining
East Kolkata Wetlands located at the lower part of the deltaic alluvial plain of South Bengal basin. Demand of groundwater
for drinking, agricultural and industrial purposes has increased due to rapid urbanization. The subsurface geology consists
of Quaternary sediments comprising a succession of clay, silty clay and sand of various grades. Groundwater occurs mostly
under confined condition except in those places where the top aquitard has been obliterated due to the scouring action of
past channels. Currently, the piezometric head shows a falling trend and it may be accelerated due to further over-withdrawal
of groundwater resulting in land subsidence. The estimated mean land subsidence rate is 13.53 mm/year and for 1 m drop in
the piezometric head, the mean subsidence is 3.28 cm. The surface expression of the estimated land subsidence is however,
cryptic because of a time lag between the settlement of the thick low-permeable aquitard at the top and its surface expression.
Therefore, groundwater of the cities and wetland areas should be developed cautiously based on the groundwater potential to
minimize the threat of land subsidence. 相似文献
347.
Yan-Jie Tang Hong-Fu Zhang Eizo Nakamura Ji-Feng Ying 《Contributions to Mineralogy and Petrology》2011,161(6):845-861
Elemental and Li–Sr–Nd isotopic data of minerals in spinel peridotites hosted by Cenozoic basalts allow us to refine the existing
models for Li isotopic fractionation in mantle peridotites and constrain the melt/fluid-peridotite interaction in the lithospheric
mantle beneath the North China Craton. Highly elevated Li concentrations in cpx (up to 24 ppm) relative to coexisting opx
and olivine (<4 ppm) indicate that the peridotites experienced metasomatism by mafic silicate melts and/or fluids. The mineral
δ7Li vary greatly, with olivine (+0.7 to +5.4‰) being isotopically heavier than coexisting opx (−4.4 to −25.9‰) and cpx (−3.3
to −21.4‰) in most samples. The δ7Li in pyroxenes are considerably lower than the normal mantle values and show negative correlation with their Li abundances,
likely due to recent Li ingress attended by diffusive fractionation of Li isotopes. Two exceptional samples have olivine δ7Li of −3.0 and −7.9‰, indicating the existence of low δ7Li domains in the mantle, which could be transient and generated by meter-scale diffusion of Li during melt/fluid-peridotite
interaction. The 143Nd/144Nd (0.5123–0.5139) and 87Sr/86Sr (0.7018–0.7062) in the pyroxenes also show a large variation, in which the cpx are apparently lower in 87Sr/86Sr and slightly higher in 143Nd/144Nd than coexisting opx, implying an intermineral Sr–Nd isotopic disequilibrium. This is observed more apparently in peridotites
having low 87Sr/86Sr and high 143Nd/144Nd ratios than in those with high 87Sr/86Sr and low 143Nd/144Nd, suggesting that a relatively recent interaction existed between an ancient metasomatized lithospheric mantle and asthenospheric
melt, which transformed the refractory peridotites with highly radiogenic Sr and unradiogenic Nd isotopic compositions to
the fertile lherzolites with unradiogenic Sr and radiogenic Nd isotopic compositions. Therefore, we argue that the lithospheric
mantle represented by the peridotites has been heterogeneously refertilized by multistage melt/fluid-peridotite interactions. 相似文献
348.
Lihe Yin Guangcai Hou Yan Dou Zhengping Tao Ying Li 《Environmental Earth Sciences》2011,64(6):1575-1584
Hydrogeochemistry and isotopes were used to understand the origin and geochemical evolution in the Habor Lake Basin, northwestern
China. Groundwater samples were taken, and the isotopic compositions δD, δ18O and major ions were analyzed. The groundwater can be divided into three types: the Quaternary groundwater, the shallow Cretaceous
groundwater and the deep Cretaceous groundwater. The groundwater chemistry is mainly controlled by the feldspar weathering
and dolomite weathering, the dissolution of Glauber’s salt, and cation exchange. Chemistry of lake water is mainly controlled
by evaporation and precipitation. The stable isotopes of oxygen and hydrogen in groundwater cluster along the local meteoric
water line, indicating that groundwater is of meteoric origin. Comparing with shallow groundwater, deep groundwater is depleted
in heavy isotopes indicating that deep groundwater was recharged during late Pleistocene and Holocene, during which the climate
was more wetter and colder than today. 相似文献
349.
Maren Brehme Traugott Scheytt Mehmet Çelik Uğur Erdem Dokuz 《Environmental Earth Sciences》2011,63(6):1395-1408
The degradation of ground water quality due to human activities is a widespread environmental problem. Furthermore, coastal aquifers are threatened by sea water intrusion as a result of increasing water exploitation. The investigation area near Dörtyol is located at the Mediterranean coast, southern Turkey, and dominated by agricultural land use. Drinking, domestic and agricultural purposes require different amounts of uncontaminated water. In this study the hydrogeological characteristics of the coastal plain were investigated. Discharge and ground water level measurements as well as geological investigations were undertaken in field in addition to anion and cation analyses of ground and surface water. The results show geological and anthropogenic influence on ground and surface water composition. In spite of a large number of wells in the coastal area sea water intrusion was not detected. With the goal of devising sustainable water use regulations, more emphasis and research needs to be directed to the long-term observation of ground and surface water quality as well as the detailed investigation of hydraulic characteristics of the local aquifer. 相似文献
350.
Linglin Chu Andreas Enggist Robert W. Luth 《Contributions to Mineralogy and Petrology》2011,162(3):565-571
To examine the effect of KCl-bearing fluids on the melting behavior of the Earth’s mantle, we conducted experiments in the
Mg2SiO4–MgSiO3–H2O and Mg2SiO4–MgSiO3–KCl–H2O systems at 5 GPa. In the Mg2SiO4–MgSiO3–H2O system, the temperature of the fluid-saturated solidus is bracketed between 1,200–1,250°C, and both forsterite and enstatite
coexist with the liquid under supersolidus conditions. In the Mg2SiO4–MgSiO3–KCl–H2O systems with molar Cl/(Cl + H2O) ratios of 0.2, 0.4, and 0.6, the temperatures of the fluid-saturated solidus are bracketed between 1,400–1,450°C, 1,550–1,600°C,
and 1,600–1,650°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. This increase in
the temperature of the solidus demonstrates the significant effect of KCl on reducing the activity of H2O in the fluid in the Mg2SiO4–MgSiO3–H2O system. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich
melt) could extend to pressures above 5 GPa in KCl-bearing systems, in contrast to the behavior in the KCl-free system. 相似文献