全文获取类型
收费全文 | 2931篇 |
免费 | 137篇 |
国内免费 | 32篇 |
专业分类
测绘学 | 68篇 |
大气科学 | 227篇 |
地球物理 | 729篇 |
地质学 | 986篇 |
海洋学 | 262篇 |
天文学 | 442篇 |
综合类 | 14篇 |
自然地理 | 372篇 |
出版年
2024年 | 5篇 |
2023年 | 6篇 |
2022年 | 8篇 |
2021年 | 54篇 |
2020年 | 47篇 |
2019年 | 55篇 |
2018年 | 64篇 |
2017年 | 66篇 |
2016年 | 89篇 |
2015年 | 89篇 |
2014年 | 91篇 |
2013年 | 179篇 |
2012年 | 139篇 |
2011年 | 185篇 |
2010年 | 141篇 |
2009年 | 167篇 |
2008年 | 154篇 |
2007年 | 162篇 |
2006年 | 142篇 |
2005年 | 125篇 |
2004年 | 124篇 |
2003年 | 108篇 |
2002年 | 99篇 |
2001年 | 71篇 |
2000年 | 65篇 |
1999年 | 59篇 |
1998年 | 53篇 |
1997年 | 37篇 |
1996年 | 43篇 |
1995年 | 29篇 |
1994年 | 34篇 |
1993年 | 32篇 |
1992年 | 28篇 |
1991年 | 26篇 |
1990年 | 24篇 |
1989年 | 23篇 |
1988年 | 25篇 |
1987年 | 29篇 |
1986年 | 24篇 |
1985年 | 31篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 26篇 |
1981年 | 23篇 |
1980年 | 18篇 |
1979年 | 15篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1974年 | 4篇 |
1971年 | 3篇 |
排序方式: 共有3100条查询结果,搜索用时 31 毫秒
871.
The groundwater system in northern Perry Township in Lake County, Ohio, is a shallow, unconfined aquifer consisting of periglacial
lake beach deposits and less permeable lacustrine plain deposits. Groundwater flow is generally toward Lake Erie from south
to north and is controlled by the top of the Ashtabula Till, but strong, local variations are caused by northward flowing
streams During the study period, water levels in most wells exhibited a seasonal fluctuation of less than 0 3 m from their
mean values. The areal distributions of chloride and nitrate concentrations indicate that road salt runoff easily infiltrates
the aquifer and that nitrate may be sourced from fertilizer application. Ground-water flow and solute transport models indicate
that in excess of 27 years are required to obtain chemical steady-state under hydrologic steady-state conditions. The simulations
also demonstrate that nitrate loading must occur in more than one cultivated field in order to obtain the observed wide-spread
nitrate distribution. 相似文献
872.
Fe-Mn partitioning between garnet and ilmenite: experimental calibration and applications 总被引:2,自引:0,他引:2
Mark I. Pownceby Victor J. Wall Hugh St. C. O'Neill 《Contributions to Mineralogy and Petrology》1987,97(1):116-126
A new mineralogic geothermometer based on the partitioning of Fe and Mn between garnet and ilmenite has been calibrated by reversal experiments in the P-T range 600–900° C, 2 and 5 kbars and for fO2=QFM. The results constitute a sensitive geothermometer applicable over a broad range of composition and conditions. Garnetilmenite thermometry has advantages relative to existing geothermometers because of its accurate calibration, marked temperature sensitivity and the chemical and structural simplicity of the crystalline solutions involved. Application to natural assemblages reveals that the garnet-ilmenite geothermometer yields temperatures that agree well with other estimates. The reactivity of, and relatively rapid Fe-Mn diffusion in ilmenite may lead to retrograde resetting of high temperature partition values, but these factors may be useful for estimating rock cooling rates. Analysis of the experimental data indicates minor positive deviations from ideality for Fe-Mn garnets and ilmenites. Absolute magnitudes of interaction parameters (W
AB) derived from a regression analysis are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the difference between solution parameters. These differences are well constrained with a magnitude of W
FeMn
ilm
–W
FeMn
gar
300 cal mol–1. The accuracy and applicability of garnet-ilmenite thermometry will improve with the availability of better thermodynamic data for garnet crystalline solutions.Abbreviations and symbols used in text
R
universal gas constant (cal/mol/°K)
-
T
absolute temperature (°K or °C)
-
P
pressure (kbars)
- V
0
volume change of reaction (1)
-
H
1, T
0
standard state enthalpy change of reaction (1) at 1 bar and the T of interest, in cal/mole
-
S
T
0
entropy change of reaction (1) at T of interest, in cal/mole/°K
-
G
P,T
0
standard free energy change of reaction (1) at the T and P of interest, in cal/mole
-
distribution coefficient for Fe-Mn partitioning between garnet and ilmenite
-
K
apparent equilibrium coefficient for reaction (1)
-
i
j
activity of component i in phase j
-
W
A-B
binary A-B interaction (Margules) parameter
-
gar
garnet
-
ilm
ilmenite
-
biot
biotite
-
ol
olivine
-
opx
orthopyroxene 相似文献
873.
Xenoliths in primitive olivine tholeiite lavas from Merelava Volcano, Vanuatu, include recrystallized wehrlites and harzburgites characterized by extremely fine grain size (0.02–2 mm) and equigranular textures. The harzburgites display mineral segregations, have highly variable ratios of ol: opx, minor clinopyroxene and accessory Cr-spinel, and are interpreted as the residues of high degrees of melting of upper mantle peridotite. Annealed Cr-spinel aggregates in harzburgite sample # 31564B enclose numerous small inclusions of sodic sanidine and minor plagioclase, attributed to infiltration of the harzburgite by a residual melt derived from an earlier period of island arc magmatism. The recrystallized wehrlites have high ol/cpx ratios and depleted REE patterns compatible with a cumulus origin. The refractory nature of the phases in both groups of recrystallized xenoliths compares closely with phases in Alpine-type peridotites and primitive arc lavas, and is incompatible with compositions of abyssal peridotites. The recrystallized wehrlites give equilibration temperatures of 1070–1130° C and are interpreted as cumulates derived from an earlier period of Vanuatu Arc magmatism. The range of composition displayed by phases in the harzburgites is greater than phase variation in the wehrlites, and reflects a more complex thermal history. Textural, mineralogical, and geothermometric considerations indicate the harzburgites underwent cooling to 800°/900° C before being re-heated to 1000–1100° C by the current magmatic regime. A shallow crustal origin for these xenoliths is indicated by gravity data and tectonic considerations which strongly imply the presence of an ophiolite body beneath Merelava, representing the northward extension of the Pentecost Ophiolite. These interpretations are compatible with a published model for generation of the host basalts by partial melting at the crust/mantle boundary (ca. 17 km). Sr isotopic data show that the harzburgites are incompatible with residues of ocean-floor magmatism, or with residues of Merelava and Central Chain magmatism, but suggest an affinity with Vitiaz Arc magmatism of Eocene-lower Miocene age. Both groups of xenoliths were apparently entrained from wall rocks during ascent of the host magmas. 相似文献
874.
Analysis of three-dimensional wind profiles recorded by an acoustic sounder near Cape Town has indicated that extreme subsidence (-35 cm s-1) is a mean feature throughout the atmospheric boundary layer (50–1000m) during summertime southerly winds. Over the SW Cape coast, the atmospheric subsidence translates into a N-S gradient of the mean summer water deficit (-20 to -32 cm month-1). The rapid drying out of the air mass along a northward trajectory is linked to a number of factors including synoptic-scale divergence of the surface wind and the effects of the local orography which produce a hydraulic jump of the southerly wind. The along-coast reduction in sea surface temperature provides a major constraint on the height of the moist marine layer. As the depth of the marine air mass shrinks, its potential for inland penetration becomes limited. In addition, dry air is entrained towards the surface as evidenced by aerial survey data. A model is formulated which indicates the importance of the surface heat fluxes in reducing the depth of the Agulhas air mass as it passes northward over the SW tip of Africa during summer. 相似文献
875.
Mark Speech 《Geoforum》1989,20(4)
Research conducted by the Western Sudan Agricultural Research Project allows examination of the impact of the recent African drought upon agricultural market performance and structure. Price volatility, as well as high prices, was a major destabilizing factor in agricultural commodity markets. Small farmers and petty traders are largely risk avoiders rather than profit maximizers, and tend not to move product when faced with price uncertainty. At the same time, responsibility for marketing functions shifts to these small operators and producers. These are the channel members least able to finance increased functional responsibility. Again, the result is that products may be held off of the market. Policy makers must recognize these marketing factors when devising strategies appropriate to semi-arid regions such as South Kordofan. Policy recommendations can be offered which can facilitate the movement of product, and thus greatly ease the marketing constraints associated with drought. 相似文献
876.
877.
878.
879.
880.
E.Bruce Watson T.Mark Harrison F.James Ryerson 《Geochimica et cosmochimica acta》1985,49(8):1813-1823
Diffusion coefficients for Sm, Sr, and Pb in fluorapatite at 900°–1250°C were obtained by measuring experimentally-induced diffusional uptake profiles of these elements in the margins of gem-quality apatite crystals. The crystals were immersed in synthetic melts enriched in the trace elements of interest and presaturated in apatite, and the resulting diffusion gradients were characterized by electron microprobe analysis. Except in the case of Pb, the diffusivities define good Arrhenius lines for the respective elements: DSm = 2.3 × 10?6exp(?52,200/RT) DSr = 412 exp(?100,000/RT). (Diffusion perpendicular to and parallel to c is measurably different in the case of Sr; the Arrhenius equation given above is an average for the two directions). Results on Pb were erratic, probably because extremely Pb-rich melts were used for some of the experiments. Data believed to be reliable define the following Arrhenius line: DPb = 0.035 exp(?70,000/RT). Constraints based on closure of natural apatites with respect to Pb suggest that the experimental data can be extrapolated, with sizeable uncertainty, to temperatures as low as 550°C.When applied to the question of isotopic and trace-element equilibration of residual or entrained apatites with crustal melts, the measured diffusivities indicate that 0.05-cm crystals will rarely preserve the original Pb-isotope characteristics of the source; the same is not true, however, of Sr (and, under some conditions, the REE), which may be unaffected at crystal cores during typical melting events. 相似文献