全文获取类型
收费全文 | 4690篇 |
免费 | 205篇 |
国内免费 | 37篇 |
专业分类
测绘学 | 187篇 |
大气科学 | 475篇 |
地球物理 | 1274篇 |
地质学 | 1501篇 |
海洋学 | 454篇 |
天文学 | 673篇 |
综合类 | 13篇 |
自然地理 | 355篇 |
出版年
2022年 | 22篇 |
2021年 | 53篇 |
2020年 | 88篇 |
2019年 | 78篇 |
2018年 | 135篇 |
2017年 | 110篇 |
2016年 | 186篇 |
2015年 | 134篇 |
2014年 | 149篇 |
2013年 | 247篇 |
2012年 | 205篇 |
2011年 | 261篇 |
2010年 | 226篇 |
2009年 | 307篇 |
2008年 | 269篇 |
2007年 | 224篇 |
2006年 | 193篇 |
2005年 | 153篇 |
2004年 | 158篇 |
2003年 | 154篇 |
2002年 | 126篇 |
2001年 | 125篇 |
2000年 | 108篇 |
1999年 | 100篇 |
1998年 | 115篇 |
1997年 | 73篇 |
1996年 | 66篇 |
1995年 | 51篇 |
1994年 | 38篇 |
1993年 | 44篇 |
1992年 | 40篇 |
1991年 | 41篇 |
1990年 | 30篇 |
1989年 | 32篇 |
1988年 | 23篇 |
1987年 | 26篇 |
1986年 | 30篇 |
1985年 | 40篇 |
1984年 | 43篇 |
1983年 | 45篇 |
1982年 | 31篇 |
1981年 | 24篇 |
1980年 | 23篇 |
1979年 | 26篇 |
1978年 | 22篇 |
1977年 | 23篇 |
1976年 | 26篇 |
1975年 | 30篇 |
1973年 | 21篇 |
1971年 | 15篇 |
排序方式: 共有4932条查询结果,搜索用时 46 毫秒
951.
Reconstruction of paleocoastlines for the northwestern South China Sea since the Last Glacial Maximum 总被引:1,自引:0,他引:1
The range of relative sea level rise in the northwestern South China Sea since the Last Glacial Maximum was over 100 m. As
a result, lowland regions including the Northeast Vietnam coast, Beibu Gulf, and South China coast experienced an evolution
from land to sea. Based on the principle of reconstructing paleogeography and using recent digital elevation model, relative
sea level curves, and sediment accumulation data, this paper presents a series of paleogeographic scenarios back to 20 cal.
ka BP for the northwestern South China Sea. The scenarios demonstrate the entire process of coastline changes for the area
of interest. During the late glacial period from 20 to 15 cal. ka BP, coastline slowly retreated, causing a land loss of only
1×104 km2, and thus the land-sea distribution remained nearly unchanged. Later in 15–10 cal. ka BP coastline rapidly retreated and
area of land loss was up to 24×104km2, causing lowlands around Northeast Vietnam and South China soon to be underwater. Coastline retreat continued quite rapidly
during the early Holocene. From 10 to 6 cal. ka BP land area had decreased by 9×104km2, and during that process the Qiongzhou Strait completely opened up. Since the mid Holocene, main controls on coastline change
are from vertical crustal movements and sedimentation. Transgression was surpassed by regression, resulting in a land accretion
of about 10×104km2.
Supported by Key Laboratory of Marginal Sea Geology, Chinese Academy of Sciences (Grant No. MSGL0711), the Guangdong Natural
Science Foundation (Grant No. 04001309) and Open Fund of the Key Laboratory of Marine Geology and Environment, Chinese Academy
of Sciences (Grant No. MGE2007KG04) 相似文献
952.
953.
Paul R. Renne Alan L. Deino Willis E. Hames Matthew T. Heizler Sidney R. Hemming Kip V. Hodges Anthony A.P. Koppers Darren F. Mark Leah E. Morgan David Phillips Brad S. Singer Brent D. Turrin Igor M. Villa Mike Villeneuve Jan R. Wijbrans 《Quaternary Geochronology》2009,4(5):346-352
Data reported in 40Ar/39Ar geochronology studies are commonly insufficient to allow computation of ages. This deficiency renders it difficult to compare ages based on different standards or constants, and often hinders critical evaluation of the results. Herein are presented an enumeration of the data that should be reported in all 40Ar/39Ar studies, including a discussion in support of these requirements. The minimum required data are identified and distinguished from parameters that are useful but may be derived from them by calculation. Finally, recommendations are made for metadata needed to document age calculations (e.g., from age spectrum or isochron analyses). 相似文献
954.
Cassandra R. Fenton Samuel Niedermann Mirjam M. Goethals Björn Schneider Jan Wijbrans 《Quaternary Geochronology》2009,4(6):475-492
Combining cosmogenic 3He and 21Ne (3Hec and 21Nec) measurements on both pyroxene and olivine from the Pleistocene Bar Ten flows (85–107 ka) greatly increases our ability to evaluate the accuracy of 3Hec and 21Nec production rates and, therefore, 3Hec and 21Nec surface exposure ages. Comparison of 3Hec and 21Nec age-pairs yielded by experimentally determined production rates and composition-based model calculations indicates that the former give more accurate surface exposure ages than the latter in this study. However, experimental production rates should be adjusted to the composition of the minerals being analyzed to obtain the best agreement between 3Hec and 21Nec ages for any given sample. 21Nec/3Hec values are 0.400 ± 0.029 and 0.204 ± 0.014 for olivine and pyroxene, respectively, in Bar Ten lava flows, in agreement with previously published values, and indicate that 21Nec/3Hec in olivine and pyroxene is not affected by erosion and remains constant with latitude, elevation, and time (up to 10 Myr). Samples with 21Nec/3Hec that do not agree with these values may indicate the presence of non-cosmogenic helium and/or neon. The neon three-isotope diagram can also indicate whether or not all excess neon in mineral separates comes from cosmogenic sources. An error-weighted regression for olivine defines a spallation line [y = (1.033 ± 0.031)x + (0.09876 ± 0.00033)], which is indistinguishable from that for pyroxene (Schäfer et al., 1999). We have derived a production rate of 25 ± 8 at/g/yr for 21Nec in clinopyroxene (En43–44) based on the 40Ar/39Ar age of the upper Bar Ten flow. Our study indicates that the production rate of 21Nec in olivine may be slightly higher than previously determined. Cosmogenic 3He and 21Ne remain extremely useful, particularly when paired, in determining accurate eruption ages of young olivine- and pyroxene-rich basaltic lava flows. 相似文献
955.
Stphane Kock Jan D. Kramers Frank Preusser Andreas Wetzel 《Quaternary Geochronology》2009,4(5):363-373
Uranium-series dating of pedogenic carbonate crusts from fluvial gravels is tested using Optically Stimulated Luminescence (OSL) ages as references. OSL dating yielded ages of 30–15 ka and 13–11 ka, which correlate with the cold periods of the Last Glacial Maximum and the Younger Dryas. These ages are internally coherent and consistent with the geological background and are thus regarded as reliable. Most of the U/Th results scatter widely in the 230Th/232Th vs. 234U/232Th isochron diagram, making regression unrealistic. Semiquantitative age estimates from the data were found to be mostly older than the OSL ages and the geological context. It is suggested that a heterogeneous initial 230Th input, not relatable to a detrital component, is responsible for the observed discrepancies. This input may be due to bacterial activities and Th transport on organic colloids. It appears necessary to avoid samples where bacteria could have contributed to carbonate precipitation. Further, the relative importance of this problem decreases with sample age, so that U/Th dating of sinters is expected to be more reliable in the >100 ka age range. 相似文献
956.
Yuan-Bao Wu John M. Hanchar Shan Gao Paul J. Sylvester Mike Tubrett Hua-Ning Qiu Jan R. Wijbrans Fraukje M. Brouwer Sai-Hong Yang Qi-Jun Yang Yong-Sheng Liu Hong-lin Yuan 《Earth and Planetary Science Letters》2009,277(3-4):345-354
In situ LA-ICPMS U-Pb, trace element, and Hf isotope data in zircon demonstrate a Carboniferous age for eclogite-facies metamorphism in Siluro-Devonian protoliths in the Huwan shear zone, Dabie Mountains, Central China. This age contrasts with the more prevailing Triassic age for high- to ultrahigh pressure (HP to UHP) metamorphism in the Qinling-Dabie-Sulu orogen. Metamorphic zircon in two eclogite samples from Sujiahe is characterized by low Th/U ratios, small negative Eu anomalies, flat HREE patterns, and low 176Lu/177Hf ratios. These geochemical signatures suggest that the zircon crystallized in the presence of garnet and in the absence of plagioclase feldspar. Furthermore, temperatures of ~ 655 and ~ 638 °C, calculated using the Ti content of zircon, are consistent with their formation during eclogite-facies metamorphism. The weighted mean 206Pb/238U age of 309 ± 4 Ma (2δ) for this zircon improves previous age estimates for eclogite-facies metamorphism in the Huwan shear zone, ranging from 420 to 220 Ma. Metamorphic zircon from one eclogite sample from Hujiawan, most likely formed during prograde metamorphism, yields an equivalent age estimate of 312 ± 11 Ma. Magmatic zircon cores in the three samples yield ages for the magmatic protoliths of the eclogites ranging from 420 ± 7 to 406 ± 5 Ma, and post-dating the middle Paleozoic collision of the North China and the Qinling terrain. The zircon crystals in the three eclogite samples display a large variation of εHf (t) values of ? 4.9 to 21.3. The metamorphic zircon overgrowths show the same range of εHf (t) values as those of the inherited magmatic crystal interiors. This suggests that the metamorphic zircon overgrowths may have formed by dissolution-reprecipitation of pre-existing magmatic zircon thereby preserving their original Hf isotopic composition. The high εHf (t) values suggest that the protoliths were derived from depleted mantle sources, most likely Paleotethyan oceanic crust; while the low εHf (t) values are attributed to crustal contamination. Some eclogites in the Huwan shear zone have a distinctive signature of continental crust most probably derived from the Yangtze Craton. The coexistence of Paleozoic oceanic crust and Neoproterozoic continental crust with similar metamorphic ages in the Huwan shear zone implies that Paleozoic Paleotethyan oceanic crust was produced within a marginal basin of the northern Yangtze Craton. The opening of the Paleo-Tethyan ocean was slightly younger than the collision of the North China Craton and the Qinling terrain during the Late Paleozoic in the Qinling-Dabie-Sulu orogen. Subduction of the Paleo-Tethyan oceanic crust and associated continental basement resulted in the 309 ± 2 Ma (2σ) eclogite-facies metamorphism in the Huwan shear zone. The subsequent Triassic continent-continent collision led to the final coalescence of the Yangtze and Sino-Korean cratons. Amalgamation of the Yangtze and North China cratons was, therefore, a multistage process extending over at least 200 Ma. 相似文献
957.
Weather-induced temporal variations in nitrate concentrations in shallow groundwater 总被引:1,自引:0,他引:1
J.C. Rozemeijer H.P. Broers F.C. van Geer M.F.P. Bierkens 《Journal of Hydrology》2009,378(1-2):119-127
For the evaluation of policy action programs to improve groundwater quality, research institutes and governments intensively monitor nitrate concentrations in shallow or near surface groundwater. However, trend detection is often hampered by the large seasonal and multi-annual temporal variability in nitrate concentrations, especially in shallow groundwater within 0–5 m below the surface in relatively humid regions. This variability is mainly caused by variations in precipitation excess (precipitation minus evapotranspiration) that results in strong variability in groundwater recharge. The objective of this study was to understand and quantify this weather-induced variability in shallow groundwater nitrate concentrations.We present an example of measured weather related variations in shallow groundwater nitrate concentrations from De Marke, an intensively monitored experimental farm in The Netherlands. For the quantification of the weather-induced variability, concentration-indices were calculated using a 1D model for water and solute transport. The results indicate that nitrate concentrations in the upper meter of groundwater at De Marke vary between 55% and 153% of the average concentration due to meteorological variability. The concentration-index quantification method was successfully used to distinguish weather related variability from human-induced trends in the nitrate concentration monitoring data from De Marke. Our model simulations also shows that sampling from fixed monitoring wells produces less short term variability than measuring from open boreholes. In addition, using larger screen depths and longer screens filters out short term temporal variability at the cost of a more delayed detection of trends in groundwater quality. 相似文献
958.
Zs. Kvri F. Vilardell I. Ribas K. Vida L. van Driel‐Gesztelyi C. Jordi K. Olh 《Astronomische Nachrichten》2007,328(9):904-908
We present B and V light curves of a large stellar flare obtained with the Wide Field Camera at the Isaac Newton 2.5‐m telescope (La Palma). The source object is a faint (mV = 21.38) foreground star in the field of the Andromeda galaxy, with its most probable spectral type being dM4. We provide an estimate of the total flare energy in the optical range and find it to be of the order of 1035 erg. The cooling phase of the large flare shows three additional weak flare‐like events, which we interpret as results of a triggering mechanism also observed on the Sun during large coronal mass ejections. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
959.
Coronal holes are regions of dominantly monopolar magnetic field on the Sun where the field is considered to be ‘open’ towards interplanetary space. Magnetic bipoles emerging in proximity to a coronal hole boundary naturally interact with this surrounding open magnetic field. In the case of oppositely aligned polarities between the active region and the coronal hole, we expect interchange reconnection to take place, driven by the coronal expansion of the emerging bipole as well as occasional eruptive events. Using SOHO/EIT and SOHO/MDI data, we present observational evidence of such interchange reconnection by studying AR 10869 which emerged close to a coronal hole. We find closed loops forming between the active region and the coronal hole leading to the retreat of the hole. At the same time, on the far side of the active region, we see dimming of the corona which we interpret as a signature of field line ‘opening’ there, as a consequence of a topological displacement of the ‘open’ field lines of the coronal hole. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
960.
Jan C. M. de Hoog Manfred J. van Bergen 《Contributions to Mineralogy and Petrology》2000,139(4):485-502
Potassium-rich calc-alkaline lavas of Lewotolo volcano, situated in the East Sunda Arc, Indonesia, contain the rare mineral
zirconolite (CaZrTi2O7). Samples in which tiny grains of this mineral (3–25 μm in size) were found span the entire range of lava compositions (47–62 wt%
SiO2). To the best of our knowledge, this is the first record of primary zirconolite in juvenile arc volcanics. The mineral forms
part of a vesicle-filling assemblage consisting of a network of quenched feldspar crystals and an SiO2 phase, probably cristobalite. High contents of Th, U and REE (up to 9.3, 4.3 and 15.6 wt% oxide respectively) and very high
Fe contents (up to 13.5 wt% Fe2O3) distinguish these zirconolites from those of other rock types. The extraction of volatile-rich phases with changing compositions
in successive stages is considered to be responsible for the zirconolite formation. We hypothesise that a fluid capable of
transporting HFSE, REE, Th and U was extracted from the magma and (partly) crystallised within voids which had formed earlier
upon saturation of an aqueous fluid. Assuming that zirconolite compositions largely reflect trace metal contents of the coexisting
fluid phase, significant amounts of `immobile' elements must have been transported on a macroscopic scale. Our findings thus
point to a late-stage transfer of HFSE, REE, Th and U between different domains in a cooling magma body. Such a volatile-induced
redistribution of trace elements at shallow levels of high-K volcanic systems may be significant for conventional geochemical
modelling of magma evolution and for Th–U disequilibrium studies.
Received: 3 November 1999 / Accepted: 29 February 2000 相似文献