全文获取类型
收费全文 | 26018篇 |
免费 | 456篇 |
国内免费 | 326篇 |
专业分类
测绘学 | 733篇 |
大气科学 | 1831篇 |
地球物理 | 5014篇 |
地质学 | 9025篇 |
海洋学 | 2366篇 |
天文学 | 6397篇 |
综合类 | 59篇 |
自然地理 | 1375篇 |
出版年
2021年 | 210篇 |
2020年 | 249篇 |
2019年 | 317篇 |
2018年 | 626篇 |
2017年 | 610篇 |
2016年 | 752篇 |
2015年 | 430篇 |
2014年 | 716篇 |
2013年 | 1340篇 |
2012年 | 821篇 |
2011年 | 1080篇 |
2010年 | 984篇 |
2009年 | 1282篇 |
2008年 | 1171篇 |
2007年 | 1184篇 |
2006年 | 1153篇 |
2005年 | 868篇 |
2004年 | 858篇 |
2003年 | 780篇 |
2002年 | 728篇 |
2001年 | 633篇 |
2000年 | 643篇 |
1999年 | 573篇 |
1998年 | 563篇 |
1997年 | 529篇 |
1996年 | 404篇 |
1995年 | 403篇 |
1994年 | 420篇 |
1993年 | 321篇 |
1992年 | 317篇 |
1991年 | 263篇 |
1990年 | 312篇 |
1989年 | 274篇 |
1988年 | 258篇 |
1987年 | 287篇 |
1986年 | 242篇 |
1985年 | 327篇 |
1984年 | 347篇 |
1983年 | 332篇 |
1982年 | 321篇 |
1981年 | 256篇 |
1980年 | 270篇 |
1979年 | 224篇 |
1978年 | 214篇 |
1977年 | 218篇 |
1976年 | 189篇 |
1975年 | 193篇 |
1974年 | 186篇 |
1973年 | 169篇 |
1972年 | 114篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
681.
Dissolved trace elements in river water: spatial distribution and the influencing factor, a study for the Pearl River Delta Economic Zone, China 总被引:1,自引:0,他引:1
T. P. Ouyang Z. Y. Zhu Y. Q. Kuang N. S. Huang J. J. Tan G. Z. Guo L. S. Gu B. Sun 《Environmental Geology》2006,49(5):733-742
Twenty-nine water samples were collected from different river channels of the Pearl River Delta Economic Zone, China. An inductively
coupled plasma-mass spectromonitor (ICP-MS) was used to measure concentrations of the trace elements in these samples. The
results suggest that the average concentrations of rare earth elements in river water show an increasing trend from the West
River, the North River, the rivers of the Pearl River Delta, and the Shenzhen River to the East River. Relatively high concentrations
of heavy metals appear in the East River, the rivers of the Pearl River Delta and the Shenzhen River, while the West River
and the North River have relatively low heavy metal concentrations. Trace element concentrations in samples collected near
urban or industrial areas are much higher than those of samples collected from distant areas, away from urban and industrial
areas. After natural conditions, human activities have significant influence on the trace element concentrations in river
water. This trace element concentration’s spatial distribution in the river water from the Pearl River Delta Economic Zone
is actually an integrated effect of natural conditions and human activity. 相似文献
682.
Summary The granulite terrane of the Czech part of the Gf?hl unit includes numerous small bodies of mantle derived peridotite, some
of which contain layers or lenses of eclogite and garnet pyroxenite. These eclogitic rocks have generally been considered
to be high-pressure crystal cumulates formed in the upper mantle. We present new analyses of whole-rock major and trace element
contents for three kynanite-quartz eclogite samples taken from the Nové Dvory garnet peridotite body. Integrating these data
with previously published analyses from the literature on eclogitic rocks from this terrane, we demonstrate that a magnesian
group of eclogites, including these three new samples, were originally formed as cumulus gabbros, which were later transformed
to eclogites in the mantle. A gabbroic origin for some mafic layers (Type II) has been advocated for other orogenic peridotites,
such as Beni Bousera (Morocco), Ronda (Spain), and Horoman (Japan). By comparing these sets of data with those from the Bohemian
Massif, we propose a simple method of identifying groups of metagabbros by utilizing MgO-normalization in oxide ratio plots
for whole-rock major element analyses. 相似文献
683.
684.
The Tyrell catchment lies on the western margin of the Riverine Province in the south-central Murray Basin, one of Australia’s most important groundwater resources. Groundwater from the shallow, unconfined Pliocene Sands aquifer and the underlying Renmark Group aquifer is saline (total dissolved solids up to 150,000 mg/L) and is Na-Cl-Mg type. There is no systematic change in salinity along hydraulic gradients implying that the aquifers are hydraulically connected and mixing during vertical flow is important. Stable isotopes (18O+2H) and Cl/Br ratios indicate that groundwater is entirely of meteoric origin and salts in this system have largely been derived by evapotranspiration of rainfall with only minor halite dissolution, rock weathering (mainly feldspar dissolution), and ion exchange between Na and Mg on clays. Similarity in chemistry of all groundwater in the catchment implies relative consistency in processes over time, independent of any climatic variation. Groundwater in both the Pliocene Sands and Renmark Group aquifers yield ages of up to 25 ka. The Tyrrell Catchment is arid to semi-arid and has low topography. This has resulted in relatively low recharge rates and hydraulic gradients that have resulted in long groundwater residence times. 相似文献
685.
686.
687.
Zr-in-rutile thermometry in blueschists from Sifnos, Greece 总被引:1,自引:0,他引:1
Frank S. Spear David A. Wark John T. Cheney John C. Schumacher E. Bruce Watson 《Contributions to Mineralogy and Petrology》2006,152(3):375-385
Zr-in-rutile thermometry on samples of blueschist from Sifnos, Greece, yields temperatures that reflect progressive crystallization of rutile from ca. 445 to 505°C with an analytical precision of + 18/−27 and ± 10°C using the electron microprobe and ± 1.5–3.5°C using the ion microprobe. Individual grains are generally homogeneous within analytical uncertainty. Different grains within a single sample record temperature differences as large 55°, although in most samples the range of temperatures is on the order of 25°. In several samples, Zr-in-rutile temperatures from grains within garnet are lower than temperatures from matrix grains, reflecting growth of rutile with increasing temperature of metamorphism. Although the specific rutile-producing reactions have not been identified, it is inferred that rutile grows from either continuous reaction involving the breakdown of lower grade phases (possibly ilmenite), or from pseudomorph reactions involving the breakdown of relic igneous precursors at blueschist-facies conditions. No systematic variation in rutile temperatures was observed across the blueschist belt of northern Sifnos, consistent with the belt having behaved as a coherent block during subduction. 相似文献
688.
River Vrishabhavathy, a tributary of Cauvery River was studied for 12 physico-chemical parameters at four sites over a distance
of 50 km for a period of 2 years (1999–2001) at monthly intervals. Water was faintly alkaline, with pH showing negative correlation
with temperature. The dissolved oxygen content increased downstream with negative correlation to biological oxygen demand
(BOD), chemical oxygen demand (COD) and turbidity. Bicarbonate alkalinity was very low compared with carbonate alkalinity.
Carbonate alkalinity, total hardness, total dissolved solids, total suspended solids, electrical conductivity, BOD and COD
decreased downstream, with an upward trend in the middle reaches due to the introduction of raw sewage. The seasonal and yearly
trends are also discussed. The river is a sewer collector undergoing self-purification. 相似文献
689.
N. Takafuji K. Fujino T. Nagai Y. Seto D. Hamane 《Physics and Chemistry of Minerals》2006,33(10):651-654
High-pressure and temperature experiments (28–62 GPa, and 1,490–2,000 K, corresponding to approximately 770–1,500 km depth in the mantle) have been conducted on a MgCO3 + SiO2 mixture using a laser-heated diamond anvil cell combined with analytical transmission electron microscope observation of the product phases to constrain the fate of carbonates carried on the subducting basalt into the lower mantle. At these conditions, the decarbonation reaction MgCO3 (magnesite) + SiO2 (stishovite) → MgSiO3 (perovskite) + CO2 (solid) has been recognized. This indicates that above reaction takes place as a candidate for decarbonation of the carbonated subducting mid ocean ridge basalts in the Earth’s lower mantle. 相似文献
690.
E. M. Chamorro Pérez I. Daniel J.-C. Chervin P. Dumas J. D. Bass T. Inoue 《Physics and Chemistry of Minerals》2006,33(7):502-510
High-pressure synchrotron infrared (IR) absorption spectra were collected between 650 and 4,000 cm−1 at ambient temperature for hydrous Mg-ringwoodite (γ-Mg2SiO4) up to 30 GPa. The main feature in the OH− stretching region is an extremely broad band centred at 3,150 cm−1. The hydrogen bond is strong for most protons and the most probable site for protonation is the tetrahedral edge. With increasing pressure, this band shifts downward while decreasing its integrated intensity until disappearance at a pressure of 25 GPa. Only one band at 2,450 cm−1 and an absorption plateau persist with a maximum wavenumber of 3,800 cm−1. This behaviour is reversible upon pressure release. We interpret this as a second-order phase transition occurring in hydrated Mg-ringwoodite at high pressure (beyond ∼ 25 GPa). This result is compatible with the observation by Kleppe et al. (Phys Chem Miner 29:473–476, 2002a) who suggested the presence of Si–O–Si linkages and/or partial increase in the coordination of Si. Beyond the phase transition, the protons are delocalized and their environment on the ringwoodite structure is probably quite different from that at low pressure. Data obtained in situ at high pressures and temperatures are needed to better understand the effect of protonation on the structure and to better constrain this phase transition. 相似文献